GraphQL

Current Working Draft

Introduction

This is the specification for GraphQL, a query language and execution engine originally created at Facebook in 2012 for describing the capabilities and requirements of data models for client‐server applications. The development of this open standard started in 2015.

GraphQL has evolved and may continue to evolve in future editions of this specification. Previous editions of the GraphQL specification can be found at permalinks that match their release tag. The latest working draft release can be found at facebook.github.io/graphql/draft/.

Copyright notice

Copyright © 2015‐present, Facebook, Inc.

As of September 26, 2017, the following persons or entities have made this Specification available under the Open Web Foundation Final Specification Agreement (OWFa 1.0), which is available at openwebfoundation.org.

You can review the signed copies of the Open Web Foundation Final Specification Agreement Version 1.0 for this specification at github.com/facebook/graphql, which may also include additional parties to those listed above.

Your use of this Specification may be subject to other third party rights. THIS SPECIFICATION IS PROVIDED “AS IS.” The contributors expressly disclaim any warranties (express, implied, or otherwise), including implied warranties of merchantability, non‐infringement, fitness for a particular purpose, or title, related to the Specification. The entire risk as to implementing or otherwise using the Specification is assumed by the Specification implementer and user. IN NO EVENT WILL ANY PARTY BE LIABLE TO ANY OTHER PARTY FOR LOST PROFITS OR ANY FORM OF INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES OF ANY CHARACTER FROM ANY CAUSES OF ACTION OF ANY KIND WITH RESPECT TO THIS SPECIFICATION OR ITS GOVERNING AGREEMENT, WHETHER BASED ON BREACH OF CONTRACT, TORT (INCLUDING NEGLIGENCE), OR OTHERWISE, AND WHETHER OR NOT THE OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Conformance

A conforming implementation of GraphQL must fulfill all normative requirements. Conformance requirements are described in this document via both descriptive assertions and key words with clearly defined meanings.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative portions of this document are to be interpreted as described in IETF RFC 2119. These key words may appear in lowercase and still retain their meaning unless explicitly declared as non‐normative.

A conforming implementation of GraphQL may provide additional functionality, but must not where explicitly disallowed or would otherwise result in non‐conformance.

Conforming Algorithms

Algorithm steps phrased in imperative grammar (e.g. “Return the result of calling resolver”) are to be interpreted with the same level of requirement as the algorithm it is contained within. Any algorithm referenced within an algorithm step (e.g. “Let completedResult be the result of calling CompleteValue()”) is to be interpreted as having at least the same level of requirement as the algorithm containing that step.

Conformance requirements expressed as algorithms can be fulfilled by an implementation of this specification in any way as long as the perceived result is equivalent. Algorithms described in this document are written to be easy to understand. Implementers are encouraged to include equivalent but optimized implementations.

See Appendix A for more details about the definition of algorithms and other notational conventions used in this document.

Non‐Normative Portions

All contents of this document are normative except portions explicitly declared as non‐normative.

Examples in this document are non‐normative, and are presented to aid understanding of introduced concepts and the behavior of normative portions of the specification. Examples are either introduced explicitly in prose (e.g. “for example”) or are set apart in example or counter‐example blocks, like this:

Example № 1This is an example of a non-normative example.
Counter Example № 2This is an example of a non-normative counter-example.

Notes in this document are non‐normative, and are presented to clarify intent, draw attention to potential edge‐cases and pit‐falls, and answer common questions that arise during implementation. Notes are either introduced explicitly in prose (e.g. “Note: “) or are set apart in a note block, like this:

This is an example of a non‐normative note.

1Overview

GraphQL is a query language designed to build client applications by providing an intuitive and flexible syntax and system for describing their data requirements and interactions.

For example, this GraphQL request will receive the name of the user with id 4 from the Facebook implementation of GraphQL.

Example № 3{
  user(id: 4) {
    name
  }
}

Which produces the resulting data (in JSON):

Example № 4{
  "user": {
    "name": "Mark Zuckerberg"
  }
}

GraphQL is not a programming language capable of arbitrary computation, but is instead a language used to query application servers that have capabilities defined in this specification. GraphQL does not mandate a particular programming language or storage system for application servers that implement it. Instead, application servers take their capabilities and map them to a uniform language, type system, and philosophy that GraphQL encodes. This provides a unified interface friendly to product development and a powerful platform for tool‐building.

GraphQL has a number of design principles:

Because of these principles, GraphQL is a powerful and productive environment for building client applications. Product developers and designers building applications against working GraphQL servers -- supported with quality tools -- can quickly become productive without reading extensive documentation and with little or no formal training. To enable that experience, there must be those that build those servers and tools.

The following formal specification serves as a reference for those builders. It describes the language and its grammar, the type system and the introspection system used to query it, and the execution and validation engines with the algorithms to power them. The goal of this specification is to provide a foundation and framework for an ecosystem of GraphQL tools, client libraries, and server implementations -- spanning both organizations and platforms -- that has yet to be built. We look forward to working with the community in order to do that.

2Language

Clients use the GraphQL query language to make requests to a GraphQL service. We refer to these request sources as documents. A document may contain operations (queries, mutations, and subscriptions) as well as fragments, a common unit of composition allowing for query reuse.

A GraphQL document is defined as a syntactic grammar where terminal symbols are tokens (indivisible lexical units). These tokens are defined in a lexical grammar which matches patterns of source characters (defined by a double‐colon ::).

See Appendix A for more details about the definition of lexical and syntactic grammar and other notational conventions used in this document.

2.1Source Text

SourceCharacter
/[\u0009\u000A\u000D\u0020-\uFFFF]/

GraphQL documents are expressed as a sequence of Unicode characters. However, with few exceptions, most of GraphQL is expressed only in the original non‐control ASCII range so as to be as widely compatible with as many existing tools, languages, and serialization formats as possible and avoid display issues in text editors and source control.

2.1.1Unicode

UnicodeBOM
Byte Order Mark (U+FEFF)

Non‐ASCII Unicode characters may freely appear within StringValue and Comment portions of GraphQL.

The “Byte Order Mark” is a special Unicode character which may appear at the beginning of a file containing Unicode which programs may use to determine the fact that the text stream is Unicode, what endianness the text stream is in, and which of several Unicode encodings to interpret.

2.1.2White Space

WhiteSpace
Horizontal Tab (U+0009)
Space (U+0020)

White space is used to improve legibility of source text and act as separation between tokens, and any amount of white space may appear before or after any token. White space between tokens is not significant to the semantic meaning of a GraphQL Document, however white space characters may appear within a String or Comment token.

GraphQL intentionally does not consider Unicode “Zs” category characters as white‐space, avoiding misinterpretation by text editors and source control tools.

2.1.3Line Terminators

LineTerminator
New Line (U+000A)
Carriage Return (U+000D)New Line (U+000A)
Carriage Return (U+000D)New Line (U+000A)

Like white space, line terminators are used to improve the legibility of source text, any amount may appear before or after any other token and have no significance to the semantic meaning of a GraphQL Document. Line terminators are not found within any other token.

Any error reporting which provide the line number in the source of the offending syntax should use the preceding amount of LineTerminator to produce the line number.

2.1.4Comments

GraphQL source documents may contain single‐line comments, starting with the # marker.

A comment can contain any Unicode code point except LineTerminator so a comment always consists of all code points starting with the # character up to but not including the line terminator.

Comments behave like white space and may appear after any token, or before a line terminator, and have no significance to the semantic meaning of a GraphQL Document.

2.1.5Insignificant Commas

Similar to white space and line terminators, commas (,) are used to improve the legibility of source text and separate lexical tokens but are otherwise syntactically and semantically insignificant within GraphQL Documents.

Non‐significant comma characters ensure that the absence or presence of a comma does not meaningfully alter the interpreted syntax of the document, as this can be a common user‐error in other languages. It also allows for the stylistic use of either trailing commas or line‐terminators as list delimiters which are both often desired for legibility and maintainability of source code.

2.1.6Lexical Tokens

A GraphQL document is comprised of several kinds of indivisible lexical tokens defined here in a lexical grammar by patterns of source Unicode characters.

Tokens are later used as terminal symbols in a GraphQL Document syntactic grammars.

2.1.7Ignored Tokens

Before and after every lexical token may be any amount of ignored tokens including WhiteSpace and Comment. No ignored regions of a source document are significant, however ignored source characters may appear within a lexical token in a significant way, for example a String may contain white space characters.

No characters are ignored while parsing a given token, as an example no white space characters are permitted between the characters defining a FloatValue.

2.1.8Punctuators

Punctuator
!$()...:=@[]{|}

GraphQL documents include punctuation in order to describe structure. GraphQL is a data description language and not a programming language, therefore GraphQL lacks the punctuation often used to describe mathematical expressions.

2.1.9Names

Name
/[_A-Za-z][_0-9A-Za-z]*/

GraphQL Documents are full of named things: operations, fields, arguments, types, directives, fragments, and variables. All names must follow the same grammatical form.

Names in GraphQL are case‐sensitive. That is to say name, Name, and NAME all refer to different names. Underscores are significant, which means other_name and othername are two different names.

Names in GraphQL are limited to this ASCII subset of possible characters to support interoperation with as many other systems as possible.

2.2Document

A GraphQL Document describes a complete file or request string operated on by a GraphQL service or client. A document contains multiple definitions, either executable or representative of a GraphQL type system.

Documents are only executable by a GraphQL service if they contain an OperationDefinition and otherwise only contain ExecutableDefinition. However documents which do not contain OperationDefinition or do contain TypeSystemDefinition or TypeSystemExtension may still be parsed and validated to allow client tools to represent many GraphQL uses which may appear across many individual files.

If a Document contains only one operation, that operation may be unnamed or represented in the shorthand form, which omits both the query keyword and operation name. Otherwise, if a GraphQL Document contains multiple operations, each operation must be named. When submitting a Document with multiple operations to a GraphQL service, the name of the desired operation to be executed must also be provided.

GraphQL services which only seek to provide GraphQL query execution may choose to only include ExecutableDefinition and omit the TypeSystemDefinition and TypeSystemExtension rules from Definition.

2.3Operations

OperationType
querymutationsubscription

There are three types of operations that GraphQL models:

Each operation is represented by an optional operation name and a selection set.

For example, this mutation operation might “like” a story and then retrieve the new number of likes:

Example № 5mutation {
  likeStory(storyID: 12345) {
    story {
      likeCount
    }
  }
}

Query shorthand

If a document contains only one query operation, and that query defines no variables and contains no directives, that operation may be represented in a short‐hand form which omits the query keyword and query name.

For example, this unnamed query operation is written via query shorthand.

Example № 6{
  field
}
many examples below will use the query short‐hand syntax.

2.4Selection Sets

An operation selects the set of information it needs, and will receive exactly that information and nothing more, avoiding over‐fetching and under‐fetching data.

Example № 7{
  id
  firstName
  lastName
}

In this query, the id, firstName, and lastName fields form a selection set. Selection sets may also contain fragment references.

2.5Fields

A selection set is primarily composed of fields. A field describes one discrete piece of information available to request within a selection set.

Some fields describe complex data or relationships to other data. In order to further explore this data, a field may itself contain a selection set, allowing for deeply nested requests. All GraphQL operations must specify their selections down to fields which return scalar values to ensure an unambiguously shaped response.

For example, this operation selects fields of complex data and relationships down to scalar values.

Example № 8{
  me {
    id
    firstName
    lastName
    birthday {
      month
      day
    }
    friends {
      name
    }
  }
}

Fields in the top‐level selection set of an operation often represent some information that is globally accessible to your application and its current viewer. Some typical examples of these top fields include references to a current logged‐in viewer, or accessing certain types of data referenced by a unique identifier.

Example № 9# `me` could represent the currently logged in viewer.
{
  me {
    name
  }
}

# `user` represents one of many users in a graph of data, referred to by a
# unique identifier.
{
  user(id: 4) {
    name
  }
}

2.6Arguments

ArgumentsConst
(ArgumentConstlist)

Fields are conceptually functions which return values, and occasionally accept arguments which alter their behavior. These arguments often map directly to function arguments within a GraphQL server’s implementation.

In this example, we want to query a specific user (requested via the id argument) and their profile picture of a specific size:

Example № 10{
  user(id: 4) {
    id
    name
    profilePic(size: 100)
  }
}

Many arguments can exist for a given field:

Example № 11{
  user(id: 4) {
    id
    name
    profilePic(width: 100, height: 50)
  }
}

Arguments are unordered

Arguments may be provided in any syntactic order and maintain identical semantic meaning.

These two queries are semantically identical:

Example № 12{
  picture(width: 200, height: 100)
}
Example № 13{
  picture(height: 100, width: 200)
}

2.7Field Alias

By default, the key in the response object will use the field name queried. However, you can define a different name by specifying an alias.

In this example, we can fetch two profile pictures of different sizes and ensure the resulting object will not have duplicate keys:

Example № 14{
  user(id: 4) {
    id
    name
    smallPic: profilePic(size: 64)
    bigPic: profilePic(size: 1024)
  }
}

Which returns the result:

Example № 15{
  "user": {
    "id": 4,
    "name": "Mark Zuckerberg",
    "smallPic": "https://cdn.site.io/pic-4-64.jpg",
    "bigPic": "https://cdn.site.io/pic-4-1024.jpg"
  }
}

Since the top level of a query is a field, it also can be given an alias:

Example № 16{
  zuck: user(id: 4) {
    id
    name
  }
}

Returns the result:

Example № 17{
  "zuck": {
    "id": 4,
    "name": "Mark Zuckerberg"
  }
}

A field’s response key is its alias if an alias is provided, and it is otherwise the field’s name.

2.8Fragments

Fragments are the primary unit of composition in GraphQL.

Fragments allow for the reuse of common repeated selections of fields, reducing duplicated text in the document. Inline Fragments can be used directly within a selection to condition upon a type condition when querying against an interface or union.

For example, if we wanted to fetch some common information about mutual friends as well as friends of some user:

Example № 18query noFragments {
  user(id: 4) {
    friends(first: 10) {
      id
      name
      profilePic(size: 50)
    }
    mutualFriends(first: 10) {
      id
      name
      profilePic(size: 50)
    }
  }
}

The repeated fields could be extracted into a fragment and composed by a parent fragment or query.

Example № 19query withFragments {
  user(id: 4) {
    friends(first: 10) {
      ...friendFields
    }
    mutualFriends(first: 10) {
      ...friendFields
    }
  }
}

fragment friendFields on User {
  id
  name
  profilePic(size: 50)
}

Fragments are consumed by using the spread operator (...). All fields selected by the fragment will be added to the query field selection at the same level as the fragment invocation. This happens through multiple levels of fragment spreads.

For example:

Example № 20query withNestedFragments {
  user(id: 4) {
    friends(first: 10) {
      ...friendFields
    }
    mutualFriends(first: 10) {
      ...friendFields
    }
  }
}

fragment friendFields on User {
  id
  name
  ...standardProfilePic
}

fragment standardProfilePic on User {
  profilePic(size: 50)
}

The queries noFragments, withFragments, and withNestedFragments all produce the same response object.

2.8.1Type Conditions

Fragments must specify the type they apply to. In this example, friendFields can be used in the context of querying a User.

Fragments cannot be specified on any input value (scalar, enumeration, or input object).

Fragments can be specified on object types, interfaces, and unions.

Selections within fragments only return values when concrete type of the object it is operating on matches the type of the fragment.

For example in this query on the Facebook data model:

Example № 21query FragmentTyping {
  profiles(handles: ["zuck", "cocacola"]) {
    handle
    ...userFragment
    ...pageFragment
  }
}

fragment userFragment on User {
  friends {
    count
  }
}

fragment pageFragment on Page {
  likers {
    count
  }
}

The profiles root field returns a list where each element could be a Page or a User. When the object in the profiles result is a User, friends will be present and likers will not. Conversely when the result is a Page, likers will be present and friends will not.

Example № 22{
  "profiles": [
    {
      "handle": "zuck",
      "friends": { "count" : 1234 }
    },
    {
      "handle": "cocacola",
      "likers": { "count" : 90234512 }
    }
  ]
}

2.8.2Inline Fragments

Fragments can be defined inline within a selection set. This is done to conditionally include fields based on their runtime type. This feature of standard fragment inclusion was demonstrated in the query FragmentTyping example. We could accomplish the same thing using inline fragments.

Example № 23query inlineFragmentTyping {
  profiles(handles: ["zuck", "cocacola"]) {
    handle
    ... on User {
      friends {
        count
      }
    }
    ... on Page {
      likers {
        count
      }
    }
  }
}

Inline fragments may also be used to apply a directive to a group of fields. If the TypeCondition is omitted, an inline fragment is considered to be of the same type as the enclosing context.

Example № 24query inlineFragmentNoType($expandedInfo: Boolean) {
  user(handle: "zuck") {
    id
    name
    ... @include(if: $expandedInfo) {
      firstName
      lastName
      birthday
    }
  }
}

2.9Input Values

Field and directive arguments accept input values of various literal primitives; input values can be scalars, enumeration values, lists, or input objects.

If not defined as constant (for example, in DefaultValue), input values can be specified as a variable. List and inputs objects may also contain variables (unless defined to be constant).

2.9.1Int Value

Digit
0123456789

An Int number is specified without a decimal point or exponent (ex. 1).

2.9.2Float Value

A Float number includes either a decimal point (ex. 1.0) or an exponent (ex. 1e50) or both (ex. 6.0221413e23).

2.9.3Boolean Value

BooleanValue
truefalse

The two keywords true and false represent the two boolean values.

2.9.4String Value

EscapedUnicode
/[0-9A-Fa-f]{4}/

Strings are sequences of characters wrapped in double‐quotes ("). (ex. "Hello World"). White space and other otherwise‐ignored characters are significant within a string value.

Unicode characters are allowed within String value literals, however SourceCharacter must not contain some ASCII control characters so escape sequences must be used to represent these characters.

Block Strings

Block strings are sequences of characters wrapped in triple‐quotes ("""). White space, line terminators, quote, and backslash characters may all be used unescaped to enable verbatim text. Characters must all be valid SourceCharacter.

Since block strings represent freeform text often used in indented positions, the string value semantics of a block string excludes uniform indentation and blank initial and trailing lines via BlockStringValue().

For example, the following operation containing a block string:

Example № 25mutation {
  sendEmail(message: """
    Hello,
      World!

    Yours,
      GraphQL.
  """)
}

Is identical to the standard quoted string:

Example № 26mutation {
  sendEmail(message: "Hello,\n  World!\n\nYours,\n  GraphQL.")
}

Since block string values strip leading and trailing empty lines, there is no single canonical printed block string for a given value. Because block strings typically represent freeform text, it is considered easier to read if they begin and end with an empty line.

Example № 27"""
This starts with and ends with an empty line,
which makes it easier to read.
"""
Counter Example № 28"""This does not start with or end with any empty lines,
which makes it a little harder to read."""
If non‐printable ASCII characters are needed in a string value, a standard quoted string with appropriate escape sequences must be used instead of a block string.

Semantics

StringValue
  1. Return the Unicode character sequence of all StringCharacter Unicode character values (which may be an empty sequence).
StringCharacter
  1. Return the character whose code unit value in the Unicode Basic Multilingual Plane is the 16‐bit hexadecimal value EscapedUnicode.
StringCharacter
  1. Return the character value of EscapedCharacter according to the table below.
Escaped Character Code Unit Value Character Name
" U+0022 double quote
\ U+005C reverse solidus (back slash)
/ U+002F solidus (forward slash)
b U+0008 backspace
f U+000C form feed
n U+000A line feed (new line)
r U+000D carriage return
t U+0009 horizontal tab
StringValue
  1. Let rawValue be the Unicode character sequence of all BlockStringCharacter Unicode character values (which may be an empty sequence).
  2. Return the result of BlockStringValue(rawValue).
BlockStringCharacter
  1. Return the character value of SourceCharacter.
BlockStringCharacter
\"""
  1. Return the character sequence """.
BlockStringValue(rawValue)
  1. Let lines be the result of splitting rawValue by LineTerminator.
  2. Let commonIndent be null.
  3. For each line in lines:
    1. If line is the first item in lines, continue to the next line.
    2. Let length be the number of characters in line.
    3. Let indent be the number of leading consecutive WhiteSpace characters in line.
    4. If indent is less than length:
      1. If commonIndent is null or indent is less than commonIndent:
        1. Let commonIndent be indent.
  4. If commonIndent is not null:
    1. For each line in lines:
      1. If line is the first item in lines, continue to the next line.
      2. Remove commonIndent characters from the beginning of line.
  5. While the first item line in lines contains only WhiteSpace:
    1. Remove the first item from lines.
  6. While the last item line in lines contains only WhiteSpace:
    1. Remove the last item from lines.
  7. Let formatted be the empty character sequence.
  8. For each line in lines:
    1. If line is the first item in lines:
      1. Append formatted with line.
    2. Otherwise:
      1. Append formatted with a line feed character (U+000A).
      2. Append formatted with line.
  9. Return formatted.

2.9.5Null Value

Null values are represented as the keyword null.

GraphQL has two semantically different ways to represent the lack of a value:

  • Explicitly providing the literal value: null.
  • Implicitly not providing a value at all.

For example, these two field calls are similar, but are not identical:

Example № 29{
  field(arg: null)
  field
}

The first has explicitly provided null to the argument “arg”, while the second has implicitly not provided a value to the argument “arg”. These two forms may be interpreted differently. For example, a mutation representing deleting a field vs not altering a field, respectively. Neither form may be used for an input expecting a Non‐Null type.

The same two methods of representing the lack of a value are possible via variables by either providing the a variable value as null and not providing a variable value at all.

2.9.6Enum Value

EnumValue
Nametruefalsenull

Enum values are represented as unquoted names (ex. MOBILE_WEB). It is recommended that Enum values be “all caps”. Enum values are only used in contexts where the precise enumeration type is known. Therefore it’s not necessary to supply an enumeration type name in the literal.

2.9.7List Value

ListValueConst
[]
[ValueConstlist]

Lists are ordered sequences of values wrapped in square‐brackets [ ]. The values of a List literal may be any value literal or variable (ex. [1, 2, 3]).

Commas are optional throughout GraphQL so trailing commas are allowed and repeated commas do not represent missing values.

Semantics

ListValue
[]
  1. Return a new empty list value.
ListValue
[Valuelist]
  1. Let inputList be a new empty list value.
  2. For each Valuelist
    1. Let value be the result of evaluating Value.
    2. Append value to inputList.
  3. Return inputList

2.9.8Input Object Values

ObjectValueConst
{}
{ObjectFieldConstlist}

Input object literal values are unordered lists of keyed input values wrapped in curly‐braces { }. The values of an object literal may be any input value literal or variable (ex. { name: "Hello world", score: 1.0 }). We refer to literal representation of input objects as “object literals.”

Input object fields are unordered

Input object fields may be provided in any syntactic order and maintain identical semantic meaning.

These two queries are semantically identical:

Example № 30{
  nearestThing(location: { lon: 12.43, lat: -53.211 })
}
Example № 31{
  nearestThing(location: { lat: -53.211, lon: 12.43 })
}

Semantics

ObjectValue
{}
  1. Return a new input object value with no fields.
ObjectValue
  1. Let inputObject be a new input object value with no fields.
  2. For each field in ObjectFieldlist
    1. Let name be Name in field.
    2. Let value be the result of evaluating Value in field.
    3. Add a field to inputObject of name name containing value value.
  3. Return inputObject

2.10Variables

A GraphQL query can be parameterized with variables, maximizing query reuse, and avoiding costly string building in clients at runtime.

If not defined as constant (for example, in DefaultValue), a Variable can be supplied for an input value.

Variables must be defined at the top of an operation and are in scope throughout the execution of that operation.

In this example, we want to fetch a profile picture size based on the size of a particular device:

Example № 32query getZuckProfile($devicePicSize: Int) {
  user(id: 4) {
    id
    name
    profilePic(size: $devicePicSize)
  }
}

Values for those variables are provided to a GraphQL service along with a request so they may be substituted during execution. If providing JSON for the variables’ values, we could run this query and request profilePic of size 60 width:

Example № 33{
  "devicePicSize": 60
}

Variable use within Fragments

Query variables can be used within fragments. Query variables have global scope with a given operation, so a variable used within a fragment must be declared in any top‐level operation that transitively consumes that fragment. If a variable is referenced in a fragment and is included by an operation that does not define that variable, the operation cannot be executed.

2.11Type References

GraphQL describes the types of data expected by query variables. Input types may be lists of another input type, or a non‐null variant of any other input type.

Semantics

Type
  1. Let name be the string value of Name
  2. Let type be the type defined in the Schema named name
  3. type must not be null
  4. Return type
Type
  1. Let itemType be the result of evaluating Type
  2. Let type be a List type where itemType is the contained type.
  3. Return type
Type
  1. Let nullableType be the result of evaluating Type
  2. Let type be a Non‐Null type where nullableType is the contained type.
  3. Return type

2.12Directives

DirectivesConst
DirectiveConstlist

Directives provide a way to describe alternate runtime execution and type validation behavior in a GraphQL document.

In some cases, you need to provide options to alter GraphQL’s execution behavior in ways field arguments will not suffice, such as conditionally including or skipping a field. Directives provide this by describing additional information to the executor.

Directives have a name along with a list of arguments which may accept values of any input type.

Directives can be used to describe additional information for types, fields, fragments and operations.

As future versions of GraphQL adopt new configurable execution capabilities, they may be exposed via directives.

3Type System

The GraphQL Type system describes the capabilities of a GraphQL server and is used to determine if a query is valid. The type system also describes the input types of query variables to determine if values provided at runtime are valid.

TypeSystemDefinition

The GraphQL language includes an IDL used to describe a GraphQL service’s type system. Tools may use this definition language to provide utilities such as client code generation or service boot‐strapping.

GraphQL tools which only seek to provide GraphQL query execution may choose not to parse TypeSystemDefinition.

A GraphQL Document which contains TypeSystemDefinition must not be executed; GraphQL execution services which receive a GraphQL Document containing type system definitions should return a descriptive error.

The type system definition language is used throughout the remainder of this specification document when illustrating example type systems.

3.1Type System Extensions

Type system extensions are used to represent a GraphQL type system which has been extended from some original type system. For example, this might be used by a local service to represent data a GraphQL client only accesses locally, or by a GraphQL service which is itself an extension of another GraphQL service.

3.2Schema

A GraphQL service’s collective type system capabilities are referred to as that service’s “schema”. A schema is defined in terms of the types and directives it supports as well as the root operation types for each kind of operation: query, mutation, and subscription; this determines the place in the type system where those operations begin.

A GraphQL schema must itself be internally valid. This section describes the rules for this validation process where relevant.

All types within a GraphQL schema must have unique names. No two provided types may have the same name. No provided type may have a name which conflicts with any built in types (including Scalar and Introspection types).

All directives within a GraphQL schema must have unique names.

All types and directives defined within a schema must not have a name which begins with "__" (two underscores), as this is used exclusively by GraphQL’s introspection system.

3.2.1Root Operation Types

A schema defines the initial root operation type for each kind of operation it supports: query, mutation, and subscription; this determines the place in the type system where those operations begin.

The query root operation type must be provided and must be an Object type.

The mutation root operation type is optional; if it is not provided, the service does not support mutations. If it is provided, it must be an Object type.

Similarly, the subscription root operation type is also optional; if it is not provided, the service does not support subscriptions. If it is provided, it must be an Object type.

The fields on the query root operation type indicate what fields are available at the top level of a GraphQL query. For example, a basic GraphQL query like:

Example № 34query {
  myName
}

Is valid when the query root operation type has a field named “myName”.

Example № 35type Query {
  myName: String
}

Similarly, the following mutation is valid if a mutation root operation type has a field named “setName”. Note that the query and mutation root types must be different types.

Example № 36mutation {
  setName(name: "Zuck") {
    newName
  }
}

When using the type system definition language, a document must include at most one schema definition.

In this example, a GraphQL schema is defined with both query and mutation root types:

Example № 37schema {
  query: MyQueryRootType
  mutation: MyMutationRootType
}

type MyQueryRootType {
  someField: String
}

type MyMutationRootType {
  setSomeField(to: String): String
}

Default Root Operation Type Names

While any type can be the root operation type for a GraphQL operation, the type system definition language can omit the schema definition when the query, mutation, and subscription root types are named Query, Mutation, and Subscription respectively.

Likewise, when representing a GraphQL schema using the type system definition language, a schema definition should be omitted if it only uses the default root operation type names.

This example describes a valid complete GraphQL schema, despite not explicitly including a schema definition. The Query type is presumed to be the query root operation type of the schema.

Example № 38type Query {
  someField: String
}

3.2.2Schema Extension

SchemaExtension
extendschemaDirectivesConstopt{OperationTypeDefinitionlist}
extendschemaDirectivesConst

Schema extensions are used to represent a schema which has been extended from an original schema. For example, this might be used by a GraphQL service which adds additional operation types, or additional directives to an existing schema.

Schema Validation

Schema extensions have the potential to be invalid if incorrectly defined.

  1. The Schema must already be defined.
  2. Any directives provided must not already apply to the original Schema.

3.3Descriptions

Documentation is first‐class feature of GraphQL type systems. To ensure the documentation of a GraphQL service remains consistent with its capabilities, descriptions of GraphQL definitions are provided alongside their definitions and made available via introspection.

To allow GraphQL service designers to easily publish documentation alongside the capabilities of a GraphQL service, GraphQL descriptions are defined using the Markdown syntax (as specified by CommonMark). In the type system definition language, these description strings (often BlockString) occur immediately before the definition they describe.

All GraphQL types, fields, arguments and other definitions which can be described should provide a Description unless they are considered self descriptive.

As an example, this simple GraphQL schema is well described:

Example № 39"""
A simple GraphQL schema which is well described.
"""
type Query {
  """
  Translates a string from a given language into a different language.
  """
  translate(
    "The original language that `text` is provided in."
    fromLanguage: Language

    "The translated language to be returned."
    toLanguage: Language

    "The text to be translated."
    text: String
  ): String
}

"""
The set of languages supported by `translate`.
"""
enum Language {
  "English"
  EN

  "French"
  FR

  "Chinese"
  CH
}

3.4Types

The fundamental unit of any GraphQL Schema is the type. There are six kinds of named type definitions in GraphQL, and two wrapping types.

The most basic type is a Scalar. A scalar represents a primitive value, like a string or an integer. Oftentimes, the possible responses for a scalar field are enumerable. GraphQL offers an Enum type in those cases, where the type specifies the space of valid responses.

Scalars and Enums form the leaves in response trees; the intermediate levels are Object types, which define a set of fields, where each field is another type in the system, allowing the definition of arbitrary type hierarchies.

GraphQL supports two abstract types: interfaces and unions.

An Interface defines a list of fields; Object types that implement that interface are guaranteed to implement those fields. Whenever the type system claims it will return an interface, it will return a valid implementing type.

A Union defines a list of possible types; similar to interfaces, whenever the type system claims a union will be returned, one of the possible types will be returned.

Finally, oftentimes it is useful to provide complex structs as inputs to GraphQL field arguments or variables; the Input Object type allows the schema to define exactly what data is expected.

3.4.1Wrapping Types

All of the types so far are assumed to be both nullable and singular: e.g. a scalar string returns either null or a singular string.

A GraphQL schema may describe that a field represents list of another types; the List type is provided for this reason, and wraps another type.

Similarly, the Non-Null type wraps another type, and denotes that the resulting value will never be null (and that an error cannot result in a null value).

These two types are referred to as “wrapping types”; non‐wrapping types are referred to as “named types”. A wrapping type has an underlying named type, found by continually unwrapping the type until a named type is found.

3.4.2Input and Output Types

Types are used throughout GraphQL to describe both the values accepted as input to arguments and variables as well as the values output by fields. These two uses categorize types as input types and output types. Some kinds of types, like Scalar and Enum types, can be used as both input types and output types; other kinds types can only be used in one or the other. Input Object types can only be used as input types. Object, Interface, and Union types can only be used as output types. Lists and Non‐Null types may be used as input types or output types depending on how the wrapped type may be used.

IsInputType(type)
  1. If type is a List type or Non‐Null type:
    1. Let unwrappedType be the unwrapped type of type.
    2. Return IsInputType(unwrappedType)
  2. If type is a Scalar, Enum, or Input Object type:
    1. Return true
  3. Return false
IsOutputType(type)
  1. If type is a List type or Non‐Null type:
    1. Let unwrappedType be the unwrapped type of type.
    2. Return IsOutputType(unwrappedType)
  2. If type is a Scalar, Object, Interface, Union, or Enum type:
    1. Return true
  3. Return false

3.4.3Type Extensions

Type extensions are used to represent a GraphQL type which has been extended from some original type. For example, this might be used by a local service to represent additional fields a GraphQL client only accesses locally.

3.5Scalars

Scalar types represent primitive leaf values in a GraphQL type system. GraphQL responses take the form of a hierarchical tree; the leaves on these trees are GraphQL scalars.

All GraphQL scalars are representable as strings, though depending on the response format being used, there may be a more appropriate primitive for the given scalar type, and server should use those types when appropriate.

GraphQL provides a number of built‐in scalars, but type systems can add additional scalars with semantic meaning. For example, a GraphQL system could define a scalar called Time which, while serialized as a string, promises to conform to ISO‐8601. When querying a field of type Time, you can then rely on the ability to parse the result with an ISO‐8601 parser and use a client‐specific primitive for time. Another example of a potentially useful custom scalar is Url, which serializes as a string, but is guaranteed by the server to be a valid URL.

Example № 40scalar Time
scalar Url

A server may omit any of the built‐in scalars from its schema, for example if a schema does not refer to a floating‐point number, then it must not include the Float type. However, if a schema includes a type with the name of one of the types described here, it must adhere to the behavior described. As an example, a server must not include a type called Int and use it to represent 128‐bit numbers, internationalization information, or anything other than what is defined in this document.

When representing a GraphQL schema using the type system definition language, the built‐in scalar types should be omitted for brevity.

Result Coercion

A GraphQL server, when preparing a field of a given scalar type, must uphold the contract the scalar type describes, either by coercing the value or producing a field error if a value cannot be coerced or if coercion may result in data loss.

A GraphQL service may decide to allow coercing different internal types to the expected return type. For example when coercing a field of type Int a boolean true value may produce 1 or a string value "123" may be parsed as base‐10 123. However if internal type coercion cannot be reasonably performed without losing information, then it must raise a field error.

Since this coercion behavior is not observable to clients of the GraphQL server, the precise rules of coercion are left to the implementation. The only requirement is that the server must yield values which adhere to the expected Scalar type.

Input Coercion

If a GraphQL server expects a scalar type as input to an argument, coercion is observable and the rules must be well defined. If an input value does not match a coercion rule, a query error must be raised.

GraphQL has different constant literals to represent integer and floating‐point input values, and coercion rules may apply differently depending on which type of input value is encountered. GraphQL may be parameterized by query variables, the values of which are often serialized when sent over a transport like HTTP. Since some common serializations (ex. JSON) do not discriminate between integer and floating‐point values, they are interpreted as an integer input value if they have an empty fractional part (ex. 1.0) and otherwise as floating‐point input value.

For all types below, with the exception of Non‐Null, if the explicit value null is provided, then the result of input coercion is null.

Built‐in Scalars

GraphQL provides a basic set of well‐defined Scalar types. A GraphQL server should support all of these types, and a GraphQL server which provide a type by these names must adhere to the behavior described below.

3.5.1Int

The Int scalar type represents a signed 32‐bit numeric non‐fractional value. Response formats that support a 32‐bit integer or a number type should use that type to represent this scalar.

Result Coercion

Fields returning the type Int expect to encounter 32‐bit integer internal values.

GraphQL servers may coerce non‐integer internal values to integers when reasonable without losing information, otherwise they must raise a field error. Examples of this may include returning 1 for the floating‐point number 1.0, or returning 123 for the string "123". In scenarios where coercion may lose data, raising a field error is more appropriate. For example, a floating‐point number 1.2 should raise a field error instead of being truncated to 1.

If the integer internal value represents a value less than -231 or greater than or equal to 231, a field error should be raised.

Input Coercion

When expected as an input type, only integer input values are accepted. All other input values, including strings with numeric content, must raise a query error indicating an incorrect type. If the integer input value represents a value less than -231 or greater than or equal to 231, a query error should be raised.

Numeric integer values larger than 32‐bit should either use String or a custom‐defined Scalar type, as not all platforms and transports support encoding integer numbers larger than 32‐bit.

3.5.2Float

The Float scalar type represents signed double‐precision fractional values as specified by IEEE 754. Response formats that support an appropriate double‐precision number type should use that type to represent this scalar.

Result Coercion

Fields returning the type Float expect to encounter double‐precision floating‐point internal values.

GraphQL servers may coerce non‐floating‐point internal values to Float when reasonable without losing information, otherwise they must raise a field error. Examples of this may include returning 1.0 for the integer number 1, or 123.0 for the string "123".

Input Coercion

When expected as an input type, both integer and float input values are accepted. Integer input values are coerced to Float by adding an empty fractional part, for example 1.0 for the integer input value 1. All other input values, including strings with numeric content, must raise a query error indicating an incorrect type. If the integer input value represents a value not representable by IEEE 754, a query error should be raised.

3.5.3String

The String scalar type represents textual data, represented as UTF‐8 character sequences. The String type is most often used by GraphQL to represent free‐form human‐readable text. All response formats must support string representations, and that representation must be used here.

Result Coercion

Fields returning the type String expect to encounter UTF‐8 string internal values.

GraphQL servers may coerce non‐string raw values to String when reasonable without losing information, otherwise they must raise a field error. Examples of this may include returning the string "true" for a boolean true value, or the string "1" for the integer 1.

Input Coercion

When expected as an input type, only valid UTF‐8 string input values are accepted. All other input values must raise a query error indicating an incorrect type.

3.5.4Boolean

The Boolean scalar type represents true or false. Response formats should use a built‐in boolean type if supported; otherwise, they should use their representation of the integers 1 and 0.

Result Coercion

Fields returning the type Boolean expect to encounter boolean internal values.

GraphQL servers may coerce non‐boolean raw values to Boolean when reasonable without losing information, otherwise they must raise a field error. Examples of this may include returning true for non‐zero numbers.

Input Coercion

When expected as an input type, only boolean input values are accepted. All other input values must raise a query error indicating an incorrect type.

3.5.5ID

The ID scalar type represents a unique identifier, often used to refetch an object or as the key for a cache. The ID type is serialized in the same way as a String; however, it is not intended to be human‐readable. While it is often numeric, it should always serialize as a String.

Result Coercion

GraphQL is agnostic to ID format, and serializes to string to ensure consistency across many formats ID could represent, from small auto‐increment numbers, to large 128‐bit random numbers, to base64 encoded values, or string values of a format like GUID.

GraphQL servers should coerce as appropriate given the ID formats they expect. When coercion is not possible they must raise a field error.

Input Coercion

When expected as an input type, any string (such as "4") or integer (such as 4 or -4) input value should be coerced to ID as appropriate for the ID formats a given GraphQL server expects. Any other input value, including float input values (such as 4.0), must raise a query error indicating an incorrect type.

3.5.6Scalar Extensions

Scalar type extensions are used to represent a scalar type which has been extended from some original scalar type. For example, this might be used by a GraphQL tool or service which adds directives to an existing scalar.

Type Validation

Scalar type extensions have the potential to be invalid if incorrectly defined.

  1. The named type must already be defined and must be a Scalar type.
  2. Any directives provided must not already apply to the original Scalar type.

3.6Objects

GraphQL queries are hierarchical and composed, describing a tree of information. While Scalar types describe the leaf values of these hierarchical queries, Objects describe the intermediate levels.

GraphQL Objects represent a list of named fields, each of which yield a value of a specific type. Object values should be serialized as ordered maps, where the queried field names (or aliases) are the keys and the result of evaluating the field is the value, ordered by the order in which they appear in the query.

All fields defined within an Object type must not have a name which begins with "__" (two underscores), as this is used exclusively by GraphQL’s introspection system.

For example, a type Person could be described as:

Example № 41type Person {
  name: String
  age: Int
  picture: Url
}

Where name is a field that will yield a String value, and age is a field that will yield an Int value, and picture is a field that will yield a Url value.

A query of an object value must select at least one field. This selection of fields will yield an ordered map containing exactly the subset of the object queried, which should be represented in the order in which they were queried. Only fields that are declared on the object type may validly be queried on that object.

For example, selecting all the fields of Person:

Example № 42{
  name
  age
  picture
}

Would yield the object:

Example № 43{
  "name": "Mark Zuckerberg",
  "age": 30,
  "picture": "http://some.cdn/picture.jpg"
}

While selecting a subset of fields:

Example № 44{
  age
  name
}

Must only yield exactly that subset:

Example № 45{
  "age": 30,
  "name": "Mark Zuckerberg"
}

A field of an Object type may be a Scalar, Enum, another Object type, an Interface, or a Union. Additionally, it may be any wrapping type whose underlying base type is one of those five.

For example, the Person type might include a relationship:

Example № 46type Person {
  name: String
  age: Int
  picture: Url
  relationship: Person
}

Valid queries must supply a nested field set for a field that returns an object, so this query is not valid:

Counter Example № 47{
  name
  relationship
}

However, this example is valid:

Example № 48{
  name
  relationship {
    name
  }
}

And will yield the subset of each object type queried:

Example № 49{
  "name": "Mark Zuckerberg",
  "relationship": {
    "name": "Priscilla Chan"
  }
}

Field Ordering

When querying an Object, the resulting mapping of fields are conceptually ordered in the same order in which they were encountered during query execution, excluding fragments for which the type does not apply and fields or fragments that are skipped via @skip or @include directives. This ordering is correctly produced when using the CollectFields() algorithm.

Response serialization formats capable of representing ordered maps should maintain this ordering. Serialization formats which can only represent unordered maps (such as JSON) should retain this order textually. That is, if two fields {foo, bar} were queried in that order, the resulting JSON serialization should contain {"foo": "...", "bar": "..."} in the same order.

Producing a response where fields are represented in the same order in which they appear in the request improves human readability during debugging and enables more efficient parsing of responses if the order of properties can be anticipated.

If a fragment is spread before other fields, the fields that fragment specifies occur in the response before the following fields.

Example № 50{
  foo
  ...Frag
  qux
}

fragment Frag on Query {
  bar
  baz
}

Produces the ordered result:

Example № 51{
  "foo": 1,
  "bar": 2,
  "baz": 3,
  "qux": 4
}

If a field is queried multiple times in a selection, it is ordered by the first time it is encountered. However fragments for which the type does not apply does not affect ordering.

Example № 52{
  foo
  ...Ignored
  ...Matching
  bar
}

fragment Ignored on UnknownType {
  qux
  baz
}

fragment Matching on Query {
  bar
  qux
  foo
}

Produces the ordered result:

Example № 53{
  "foo": 1,
  "bar": 2,
  "qux": 3
}

Also, if directives result in fields being excluded, they are not considered in the ordering of fields.

Example № 54{
  foo @skip(if: true)
  bar
  foo
}

Produces the ordered result:

Example № 55{
  "bar": 1,
  "foo": 2
}

Result Coercion

Determining the result of coercing an object is the heart of the GraphQL executor, so this is covered in that section of the spec.

Input Coercion

Objects are never valid inputs.

Type Validation

Object types have the potential to be invalid if incorrectly defined. This set of rules must be adhered to by every Object type in a GraphQL schema.

  1. An Object type must define one or more fields.
  2. For each field of an Object type:
    1. The field must have a unique name within that Object type; no two fields may share the same name.
    2. The field must not have a name which begins with the characters "__" (two underscores).
    3. The field must return a type where IsOutputType(fieldType) returns true.
    4. For each argument of the field:
      1. The argument must not have a name which begins with the characters "__" (two underscores).
      2. The argument must accept a type where IsInputType(argumentType) returns true.
  3. An object type may declare that it implements one or more unique interfaces.
  4. An object type must be a super‐set of all interfaces it implements:
    1. The object type must include a field of the same name for every field defined in an interface.
      1. The object field must be of a type which is equal to or a sub‐type of the interface field (covariant).
        1. An object field type is a valid sub‐type if it is equal to (the same type as) the interface field type.
        2. An object field type is a valid sub‐type if it is an Object type and the interface field type is either an Interface type or a Union type and the object field type is a possible type of the interface field type.
        3. An object field type is a valid sub‐type if it is a List type and the interface field type is also a List type and the list‐item type of the object field type is a valid sub‐type of the list‐item type of the interface field type.
        4. An object field type is a valid sub‐type if it is a Non‐Null variant of a valid sub‐type of the interface field type.
      2. The object field must include an argument of the same name for every argument defined in the interface field.
        1. The object field argument must accept the same type (invariant) as the interface field argument.
      3. The object field may include additional arguments not defined in the interface field, but any additional argument must not be required, e.g. must not be of a non‐nullable type.

3.6.1Field Arguments

Object fields are conceptually functions which yield values. Occasionally object fields can accept arguments to further specify the return value. Object field arguments are defined as a list of all possible argument names and their expected input types.

All arguments defined within a field must not have a name which begins with "__" (two underscores), as this is used exclusively by GraphQL’s introspection system.

For example, a Person type with a picture field could accept an argument to determine what size of an image to return.

Example № 56type Person {
  name: String
  picture(size: Int): Url
}

GraphQL queries can optionally specify arguments to their fields to provide these arguments.

This example query:

Example № 57{
  name
  picture(size: 600)
}

May yield the result:

Example № 58{
  "name": "Mark Zuckerberg",
  "picture": "http://some.cdn/picture_600.jpg"
}

The type of an object field argument must be an input type (any type except an Object, Interface, or Union type).

3.6.2Field Deprecation

Fields in an object may be marked as deprecated as deemed necessary by the application. It is still legal to query for these fields (to ensure existing clients are not broken by the change), but the fields should be appropriately treated in documentation and tooling.

When using the type system definition language, @deprecated directives are used to indicate that a field is deprecated:

Example № 59type ExampleType {
  oldField: String @deprecated
}

3.6.3Object Extensions

Object type extensions are used to represent a type which has been extended from some original type. For example, this might be used to represent local data, or by a GraphQL service which is itself an extension of another GraphQL service.

In this example, a local data field is added to a Story type:

Example № 60extend type Story {
  isHiddenLocally: Boolean
}

Object type extensions may choose not to add additional fields, instead only adding interfaces or directives.

In this example, a directive is added to a User type without adding fields:

Example № 61extend type User @addedDirective

Type Validation

Object type extensions have the potential to be invalid if incorrectly defined.

  1. The named type must already be defined and must be an Object type.
  2. The fields of an Object type extension must have unique names; no two fields may share the same name.
  3. Any fields of an Object type extension must not be already defined on the original Object type.
  4. Any directives provided must not already apply to the original Object type.
  5. Any interfaces provided must not be already implemented by the original Object type.
  6. The resulting extended object type must be a super‐set of all interfaces it implements.

3.7Interfaces

GraphQL interfaces represent a list of named fields and their arguments. GraphQL objects can then implement these interfaces which requires that the object type will define all fields defined by those interfaces.

Fields on a GraphQL interface have the same rules as fields on a GraphQL object; their type can be Scalar, Object, Enum, Interface, or Union, or any wrapping type whose base type is one of those five.

For example, an interface NamedEntity may describe a required field and types such as Person or Business may then implement this interface to guarantee this field will always exist.

Types may also implement multiple interfaces. For example, Business implements both the NamedEntity and ValuedEntity interfaces in the example below.

Example № 62interface NamedEntity {
  name: String
}

interface ValuedEntity {
  value: Int
}

type Person implements NamedEntity {
  name: String
  age: Int
}

type Business implements NamedEntity & ValuedEntity {
  name: String
  value: Int
  employeeCount: Int
}

Fields which yield an interface are useful when one of many Object types are expected, but some fields should be guaranteed.

To continue the example, a Contact might refer to NamedEntity.

Example № 63type Contact {
  entity: NamedEntity
  phoneNumber: String
  address: String
}

This allows us to write a query for a Contact that can select the common fields.

Example № 64{
  entity {
    name
  }
  phoneNumber
}

When querying for fields on an interface type, only those fields declared on the interface may be queried. In the above example, entity returns a NamedEntity, and name is defined on NamedEntity, so it is valid. However, the following would not be a valid query:

Counter Example № 65{
  entity {
    name
    age
  }
  phoneNumber
}

because entity refers to a NamedEntity, and age is not defined on that interface. Querying for age is only valid when the result of entity is a Person; the query can express this using a fragment or an inline fragment:

Example № 66{
  entity {
    name
    ... on Person {
      age
    }
  },
  phoneNumber
}

Result Coercion

The interface type should have some way of determining which object a given result corresponds to. Once it has done so, the result coercion of the interface is the same as the result coercion of the object.

Input Coercion

Interfaces are never valid inputs.

Type Validation

Interface types have the potential to be invalid if incorrectly defined.

  1. An Interface type must define one or more fields.
  2. For each field of an Interface type:
    1. The field must have a unique name within that Interface type; no two fields may share the same name.
    2. The field must not have a name which begins with the characters "__" (two underscores).
    3. The field must return a type where IsOutputType(fieldType) returns true.
    4. For each argument of the field:
      1. The argument must not have a name which begins with the characters "__" (two underscores).
      2. The argument must accept a type where IsInputType(argumentType) returns true.

3.7.1Interface Extensions

Interface type extensions are used to represent an interface which has been extended from some original interface. For example, this might be used to represent common local data on many types, or by a GraphQL service which is itself an extension of another GraphQL service.

In this example, an extended data field is added to a NamedEntity type along with the types which implement it:

Example № 67extend interface NamedEntity {
  nickname: String
}

extend type Person {
  nickname: String
}

extend type Business {
  nickname: String
}

Interface type extensions may choose not to add additional fields, instead only adding directives.

In this example, a directive is added to a NamedEntity type without adding fields:

Example № 68extend interface NamedEntity @addedDirective

Type Validation

Interface type extensions have the potential to be invalid if incorrectly defined.

  1. The named type must already be defined and must be an Interface type.
  2. The fields of an Interface type extension must have unique names; no two fields may share the same name.
  3. Any fields of an Interface type extension must not be already defined on the original Interface type.
  4. Any Object type which implemented the original Interface type must also be a super‐set of the fields of the Interface type extension (which may be due to Object type extension).
  5. Any directives provided must not already apply to the original Interface type.

3.8Unions

GraphQL Unions represent an object that could be one of a list of GraphQL Object types, but provides for no guaranteed fields between those types. They also differ from interfaces in that Object types declare what interfaces they implement, but are not aware of what unions contain them.

With interfaces and objects, only those fields defined on the type can be queried directly; to query other fields on an interface, typed fragments must be used. This is the same as for unions, but unions do not define any fields, so no fields may be queried on this type without the use of type refining fragments or inline fragments.

For example, we might define the following types:

Example № 69union SearchResult = Photo | Person

type Person {
  name: String
  age: Int
}

type Photo {
  height: Int
  width: Int
}

type SearchQuery {
  firstSearchResult: SearchResult
}

When querying the firstSearchResult field of type SearchQuery, the query would ask for all fields inside of a fragment indicating the appropriate type. If the query wanted the name if the result was a Person, and the height if it was a photo, the following query is invalid, because the union itself defines no fields:

Counter Example № 70{
  firstSearchResult {
    name
    height
  }
}

Instead, the query would be:

Example № 71{
  firstSearchResult {
    ... on Person {
      name
    }
    ... on Photo {
      height
    }
  }
}

Union members may be defined with an optional leading | character to aid formatting when representing a longer list of possible types:

Example № 72union SearchResult =
  | Photo
  | Person

Result Coercion

The union type should have some way of determining which object a given result corresponds to. Once it has done so, the result coercion of the union is the same as the result coercion of the object.

Input Coercion

Unions are never valid inputs.

Type Validation

Union types have the potential to be invalid if incorrectly defined.

  1. A Union type must include one or more unique member types.
  2. The member types of a Union type must all be Object base types; Scalar, Interface and Union types must not be member types of a Union. Similarly, wrapping types must not be member types of a Union.

3.8.1Union Extensions

Union type extensions are used to represent a union type which has been extended from some original union type. For example, this might be used to represent additional local data, or by a GraphQL service which is itself an extension of another GraphQL service.

Type Validation

Union type extensions have the potential to be invalid if incorrectly defined.

  1. The named type must already be defined and must be a Union type.
  2. The member types of a Union type extension must all be Object base types; Scalar, Interface and Union types must not be member types of a Union. Similarly, wrapping types must not be member types of a Union.
  3. All member types of a Union type extension must be unique.
  4. All member types of a Union type extension must not already be a member of the original Union type.
  5. Any directives provided must not already apply to the original Union type.

3.9Enums

GraphQL Enum types, like scalar types, also represent leaf values in a GraphQL type system. However Enum types describe the set of possible values.

Enums are not references for a numeric value, but are unique values in their own right. They may serialize as a string: the name of the represented value.

In this example, an Enum type called Direction is defined:

Example № 73enum Direction {
  NORTH
  EAST
  SOUTH
  WEST
}

Result Coercion

GraphQL servers must return one of the defined set of possible values. If a reasonable coercion is not possible they must raise a field error.

Input Coercion

GraphQL has a constant literal to represent enum input values. GraphQL string literals must not be accepted as an enum input and instead raise a query error.

Query variable transport serializations which have a different representation for non‐string symbolic values (for example, EDN) should only allow such values as enum input values. Otherwise, for most transport serializations that do not, strings may be interpreted as the enum input value with the same name.

Type Validation

Enum types have the potential to be invalid if incorrectly defined.

  1. An Enum type must define one or more unique enum values.

3.9.1Enum Extensions

Enum type extensions are used to represent an enum type which has been extended from some original enum type. For example, this might be used to represent additional local data, or by a GraphQL service which is itself an extension of another GraphQL service.

Type Validation

Enum type extensions have the potential to be invalid if incorrectly defined.

  1. The named type must already be defined and must be an Enum type.
  2. All values of an Enum type extension must be unique.
  3. All values of an Enum type extension must not already be a value of the original Enum.
  4. Any directives provided must not already apply to the original Enum type.

3.10Input Objects

Fields may accept arguments to configure their behavior. These inputs are often scalars or enums, but they sometimes need to represent more complex values.

A GraphQL Input Object defines a set of input fields; the input fields are either scalars, enums, or other input objects. This allows arguments to accept arbitrarily complex structs.

In this example, an Input Object called Point2D describes x and y inputs:

Example № 74input Point2D {
  x: Float
  y: Float
}
The GraphQL Object type (ObjectTypeDefinition) defined above is inappropriate for re‐use here, because Object types can contain fields that define arguments or contain references to interfaces and unions, neither of which is appropriate for use as an input argument. For this reason, input objects have a separate type in the system.

Result Coercion

An input object is never a valid result. Input Object types cannot be the return type of an Object or Interface field.

Input Coercion

The value for an input object should be an input object literal or an unordered map supplied by a variable, otherwise a query error must be thrown. In either case, the input object literal or unordered map must not contain any entries with names not defined by a field of this input object type, otherwise an error must be thrown.

The result of coercion is an unordered map with an entry for each field both defined by the input object type and for which a value exists. The resulting map is constructed with the following rules:

Following are examples of input coercion for an input object type with a String field a and a required (non‐null) Int! field b:

Example № 75input ExampleInputObject {
  a: String
  b: Int!
}
Literal Value Variables Coerced Value
{ a: "abc", b: 123 } {} { a: "abc", b: 123 }
{ a: null, b: 123 } {} { a: null, b: 123 }
{ b: 123 } {} { b: 123 }
{ a: $var, b: 123 } { var: null } { a: null, b: 123 }
{ a: $var, b: 123 } {} { b: 123 }
{ b: $var } { var: 123 } { b: 123 }
$var { var: { b: 123 } } { b: 123 }
"abc123" {} Error: Incorrect value
$var { var: "abc123" } } Error: Incorrect value
{ a: "abc", b: "123" } {} Error: Incorrect value for field b
{ a: "abc" } {} Error: Missing required field b
{ b: $var } {} Error: Missing required field b.
$var { var: { a: "abc" } } Error: Missing required field b
{ a: "abc", b: null } {} Error: b must be non‐null.
{ b: $var } { var: null } Error: b must be non‐null.
{ b: 123, c: "xyz" } {} Error: Unexpected field c

Type Validation

  1. An Input Object type must define one or more input fields.
  2. For each input field of an Input Object type:
    1. The input field must have a unique name within that Input Object type; no two input fields may share the same name.
    2. The input field must not have a name which begins with the characters "__" (two underscores).
    3. The input field must accept a type where IsInputType(inputFieldType) returns true.

3.10.1Input Object Extensions

Input object type extensions are used to represent an input object type which has been extended from some original input object type. For example, this might be used by a GraphQL service which is itself an extension of another GraphQL service.

Type Validation

Input object type extensions have the potential to be invalid if incorrectly defined.

  1. The named type must already be defined and must be a Input Object type.
  2. All fields of an Input Object type extension must have unique names.
  3. All fields of an Input Object type extension must not already be a field of the original Input Object.
  4. Any directives provided must not already apply to the original Input Object type.

3.11List

A GraphQL list is a special collection type which declares the type of each item in the List (referred to as the item type of the list). List values are serialized as ordered lists, where each item in the list is serialized as per the item type. To denote that a field uses a List type the item type is wrapped in square brackets like this: pets: [Pet].

Result Coercion

GraphQL servers must return an ordered list as the result of a list type. Each item in the list must be the result of a result coercion of the item type. If a reasonable coercion is not possible it must raise a field error. In particular, if a non‐list is returned, the coercion should fail, as this indicates a mismatch in expectations between the type system and the implementation.

If a list’s item type is nullable, then errors occurring during preparation or coercion of an individual item in the list must result in a the value null at that position in the list along with an error added to the response. If a list’s item type is non‐null, an error occurring at an individual item in the list must result in a field error for the entire list.

For more information on the error handling process, see “Errors and Non‐Nullability” within the Execution section.

Input Coercion

When expected as an input, list values are accepted only when each item in the list can be accepted by the list’s item type.

If the value passed as an input to a list type is not a list and not the null value, then the result of input coercion is a list of size one, where the single item value is the result of input coercion for the list’s item type on the provided value (note this may apply recursively for nested lists).

This allow inputs which accept one or many arguments (sometimes referred to as “var args”) to declare their input type as a list while for the common case of a single value, a client can just pass that value directly rather than constructing the list.

Following are examples of input coercion with various list types and values:

Expected Type Provided Value Coerced Value
[Int] [1, 2, 3] [1, 2, 3]
[Int] [1, "b", true] Error: Incorrect item value
[Int] 1 [1]
[Int] null null
[[Int]] [[1], [2, 3]] [[1], [2, 3]]
[[Int]] [1, 2, 3] Error: Incorrect item value
[[Int]] 1 [[1]]
[[Int]] null null

3.12Non-Null

By default, all types in GraphQL are nullable; the null value is a valid response for all of the above types. To declare a type that disallows null, the GraphQL Non‐Null type can be used. This type wraps an underlying type, and this type acts identically to that wrapped type, with the exception that null is not a valid response for the wrapping type. A trailing exclamation mark is used to denote a field that uses a Non‐Null type like this: name: String!.

Nullable vs. Optional

Fields are always optional within the context of a query, a field may be omitted and the query is still valid. However fields that return Non‐Null types will never return the value null if queried.

Inputs (such as field arguments), are always optional by default. However a non‐null input type is required. In addition to not accepting the value null, it also does not accept omission. For the sake of simplicity nullable types are always optional and non‐null types are always required.

Result Coercion

In all of the above result coercions, null was considered a valid value. To coerce the result of a Non‐Null type, the coercion of the wrapped type should be performed. If that result was not null, then the result of coercing the Non‐Null type is that result. If that result was null, then a field error must be raised.

When a field error is raised on a non‐null value, the error propagates to the parent field. For more information on this process, see “Errors and Non‐Nullability” within the Execution section.

Input Coercion

If an argument or input‐object field of a Non‐Null type is not provided, is provided with the literal value null, or is provided with a variable that was either not provided a value at runtime, or was provided the value null, then a query error must be raised.

If the value provided to the Non‐Null type is provided with a literal value other than null, or a Non‐Null variable value, it is coerced using the input coercion for the wrapped type.

A non‐null argument cannot be omitted:

Counter Example № 76{
  fieldWithNonNullArg
}

The value null cannot be provided to a non‐null argument:

Counter Example № 77{
  fieldWithNonNullArg(nonNullArg: null)
}

A variable of a nullable type cannot be provided to a non‐null argument:

Example № 78query withNullableVariable($var: String) {
  fieldWithNonNullArg(nonNullArg: $var)
}
The Validation section defines providing a nullable variable type to a non‐null input type as invalid.

Type Validation

  1. A Non‐Null type must not wrap another Non‐Null type.

3.12.1Combining List and Non-Null

The List and Non‐Null wrapping types can compose, representing more complex types. The rules for result coercion and input coercion of Lists and Non‐Null types apply in a recursive fashion.

For example if the inner item type of a List is Non‐Null (e.g. [T!]), then that List may not contain any null items. However if the inner type of a Non‐Null is a List (e.g. [T]!), then null is not accepted however an empty list is accepted.

Following are examples of result coercion with various types and values:

Expected Type Internal Value Coerced Result
[Int] [1, 2, 3] [1, 2, 3]
[Int] null null
[Int] [1, 2, null] [1, 2, null]
[Int] [1, 2, Error] [1, 2, null] (With logged error)
[Int]! [1, 2, 3] [1, 2, 3]
[Int]! null Error: Value cannot be null
[Int]! [1, 2, null] [1, 2, null]
[Int]! [1, 2, Error] [1, 2, null] (With logged error)
[Int!] [1, 2, 3] [1, 2, 3]
[Int!] null null
[Int!] [1, 2, null] null (With logged coercion error)
[Int!] [1, 2, Error] null (With logged error)
[Int!]! [1, 2, 3] [1, 2, 3]
[Int!]! null Error: Value cannot be null
[Int!]! [1, 2, null] Error: Item cannot be null
[Int!]! [1, 2, Error] Error: Error occurred in item

3.13Directives

ExecutableDirectiveLocation
QUERY
MUTATION
SUBSCRIPTION
FIELD
FRAGMENT_DEFINITION
FRAGMENT_SPREAD
INLINE_FRAGMENT
VARIABLE_DEFINITION
TypeSystemDirectiveLocation
SCHEMA
SCALAR
OBJECT
FIELD_DEFINITION
ARGUMENT_DEFINITION
INTERFACE
UNION
ENUM
ENUM_VALUE
INPUT_OBJECT
INPUT_FIELD_DEFINITION

A GraphQL schema describes directives which are used to annotate various parts of a GraphQL document as an indicator that they should be evaluated differently by a validator, executor, or client tool such as a code generator.

GraphQL implementations should provide the @skip and @include directives.

GraphQL implementations that support the type system definition language must provide the @deprecated directive if representing deprecated portions of the schema.

Directives must only be used in the locations they are declared to belong in. In this example, a directive is defined which can be used to annotate a field:

Example № 79directive @example on FIELD

fragment SomeFragment on SomeType {
  field @example
}

Directive locations may be defined with an optional leading | character to aid formatting when representing a longer list of possible locations:

Example № 80directive @example on
  | FIELD
  | FRAGMENT_SPREAD
  | INLINE_FRAGMENT

Directives can also be used to annotate the type system definition language as well, which can be a useful tool for supplying additional metadata in order to generate GraphQL execution services, produce client generated runtime code, or many other useful extensions of the GraphQL semantics.

In this example, the directive @example annotates field and argument definitions:

Example № 81directive @example on FIELD_DEFINITION | ARGUMENT_DEFINITION

type SomeType {
  field(arg: Int @example): String @example
}

While defining a directive, it must not reference itself directly or indirectly:

Counter Example № 82directive @invalidExample(arg: String @invalidExample) on ARGUMENT_DEFINITION

Validation

  1. A directive definition must not contain the use of a directive which references itself directly.
  2. A directive definition must not contain the use of a directive which references itself indirectly by referencing a Type or Directive which transitively includes a reference to this directive.
  3. The directive must not have a name which begins with the characters "__" (two underscores).
  4. For each argument of the directive:
    1. The argument must not have a name which begins with the characters "__" (two underscores).
    2. The argument must accept a type where IsInputType(argumentType) returns true.

3.13.1@skip

directive @skip(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT

The @skip directive may be provided for fields, fragment spreads, and inline fragments, and allows for conditional exclusion during execution as described by the if argument.

In this example experimentalField will only be queried if the variable $someTest has the value false.

Example № 83query myQuery($someTest: Boolean) {
  experimentalField @skip(if: $someTest)
}

3.13.2@include

directive @include(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT

The @include directive may be provided for fields, fragment spreads, and inline fragments, and allows for conditional inclusion during execution as described by the if argument.

In this example experimentalField will only be queried if the variable $someTest has the value true

Example № 84query myQuery($someTest: Boolean) {
  experimentalField @include(if: $someTest)
}
Neither @skip nor @include has precedence over the other. In the case that both the @skip and @include directives are provided on the same field or fragment, it must be queried only if the @skip condition is false and the @include condition is true. Stated conversely, the field or fragment must not be queried if either the @skip condition is true or the @include condition is false.

3.13.3@deprecated

directive @deprecated(
  reason: String = "No longer supported"
) on FIELD_DEFINITION | ENUM_VALUE

The @deprecated directive is used within the type system definition language to indicate deprecated portions of a GraphQL service’s schema, such as deprecated fields on a type or deprecated enum values.

Deprecations include a reason for why it is deprecated, which is formatted using Markdown syntax (as specified by CommonMark).

In this example type definition, oldField is deprecated in favor of using newField.

Example № 85type ExampleType {
  newField: String
  oldField: String @deprecated(reason: "Use `newField`.")
}

4Introspection

A GraphQL server supports introspection over its schema. This schema is queried using GraphQL itself, creating a powerful platform for tool‐building.

Take an example query for a trivial app. In this case there is a User type with three fields: id, name, and birthday.

For example, given a server with the following type definition:

Example № 86type User {
  id: String
  name: String
  birthday: Date
}

The query

Example № 87{
  __type(name: "User") {
    name
    fields {
      name
      type {
        name
      }
    }
  }
}

would return

Example № 88{
  "__type": {
    "name": "User",
    "fields": [
      {
        "name": "id",
        "type": { "name": "String" }
      },
      {
        "name": "name",
        "type": { "name": "String" }
      },
      {
        "name": "birthday",
        "type": { "name": "Date" }
      },
    ]
  }
}

4.1Reserved Names

Types and fields required by the GraphQL introspection system that are used in the same context as user‐defined types and fields are prefixed with "__" two underscores. This in order to avoid naming collisions with user‐defined GraphQL types. Conversely, GraphQL type system authors must not define any types, fields, arguments, or any other type system artifact with two leading underscores.

4.2Documentation

All types in the introspection system provide a description field of type String to allow type designers to publish documentation in addition to capabilities. A GraphQL server may return the description field using Markdown syntax (as specified by CommonMark). Therefore it is recommended that any tool that displays description use a CommonMark‐compliant Markdown renderer.

4.3Deprecation

To support the management of backwards compatibility, GraphQL fields and enum values can indicate whether or not they are deprecated (isDeprecated: Boolean) and a description of why it is deprecated (deprecationReason: String).

Tools built using GraphQL introspection should respect deprecation by discouraging deprecated use through information hiding or developer‐facing warnings.

4.4Type Name Introspection

GraphQL supports type name introspection at any point within a query by the meta‐field __typename: String! when querying against any Object, Interface, or Union. It returns the name of the object type currently being queried.

This is most often used when querying against Interface or Union types to identify which actual type of the possible types has been returned.

This field is implicit and does not appear in the fields list in any defined type.

4.5Schema Introspection

The schema introspection system is accessible from the meta‐fields __schema and __type which are accessible from the type of the root of a query operation.

__schema: __Schema!
__type(name: String!): __Type

These fields are implicit and do not appear in the fields list in the root type of the query operation.

The schema of the GraphQL schema introspection system:

type __Schema {
  types: [__Type!]!
  queryType: __Type!
  mutationType: __Type
  subscriptionType: __Type
  directives: [__Directive!]!
}

type __Type {
  kind: __TypeKind!
  name: String
  description: String

  # OBJECT and INTERFACE only
  fields(includeDeprecated: Boolean = false): [__Field!]

  # OBJECT only
  interfaces: [__Type!]

  # INTERFACE and UNION only
  possibleTypes: [__Type!]

  # ENUM only
  enumValues(includeDeprecated: Boolean = false): [__EnumValue!]

  # INPUT_OBJECT only
  inputFields: [__InputValue!]

  # NON_NULL and LIST only
  ofType: __Type
}

type __Field {
  name: String!
  description: String
  args: [__InputValue!]!
  type: __Type!
  isDeprecated: Boolean!
  deprecationReason: String
}

type __InputValue {
  name: String!
  description: String
  type: __Type!
  defaultValue: String
}

type __EnumValue {
  name: String!
  description: String
  isDeprecated: Boolean!
  deprecationReason: String
}

enum __TypeKind {
  SCALAR
  OBJECT
  INTERFACE
  UNION
  ENUM
  INPUT_OBJECT
  LIST
  NON_NULL
}

type __Directive {
  name: String!
  description: String
  locations: [__DirectiveLocation!]!
  args: [__InputValue!]!
}

enum __DirectiveLocation {
  QUERY
  MUTATION
  SUBSCRIPTION
  FIELD
  FRAGMENT_DEFINITION
  FRAGMENT_SPREAD
  INLINE_FRAGMENT
  SCHEMA
  SCALAR
  OBJECT
  FIELD_DEFINITION
  ARGUMENT_DEFINITION
  INTERFACE
  UNION
  ENUM
  ENUM_VALUE
  INPUT_OBJECT
  INPUT_FIELD_DEFINITION
}

4.5.1The __Type Type

__Type is at the core of the type introspection system. It represents scalars, interfaces, object types, unions, enums in the system.

__Type also represents type modifiers, which are used to modify a type that it refers to (ofType: __Type). This is how we represent lists, non‐nullable types, and the combinations thereof.

4.5.2Type Kinds

There are several different kinds of type. In each kind, different fields are actually valid. These kinds are listed in the __TypeKind enumeration.

4.5.2.1Scalar

Represents scalar types such as Int, String, and Boolean. Scalars cannot have fields.

A GraphQL type designer should describe the data format and scalar coercion rules in the description field of any scalar.

Fields

  • kind must return __TypeKind.SCALAR.
  • name must return a String.
  • description may return a String or null.
  • All other fields must return null.
4.5.2.2Object

Object types represent concrete instantiations of sets of fields. The introspection types (e.g. __Type, __Field, etc) are examples of objects.

Fields

  • kind must return __TypeKind.OBJECT.
  • name must return a String.
  • description may return a String or null.
  • fields: The set of fields query‐able on this type.
    • Accepts the argument includeDeprecated which defaults to false. If true, deprecated fields are also returned.
  • interfaces: The set of interfaces that an object implements.
  • All other fields must return null.
4.5.2.3Union

Unions are an abstract type where no common fields are declared. The possible types of a union are explicitly listed out in possibleTypes. Types can be made parts of unions without modification of that type.

Fields

  • kind must return __TypeKind.UNION.
  • name must return a String.
  • description may return a String or null.
  • possibleTypes returns the list of types that can be represented within this union. They must be object types.
  • All other fields must return null.
4.5.2.4Interface

Interfaces are an abstract type where there are common fields declared. Any type that implements an interface must define all the fields with names and types exactly matching. The implementations of this interface are explicitly listed out in possibleTypes.

Fields

  • kind must return __TypeKind.INTERFACE.
  • name must return a String.
  • description may return a String or null.
  • fields: The set of fields required by this interface.
    • Accepts the argument includeDeprecated which defaults to false. If true, deprecated fields are also returned.
  • possibleTypes returns the list of types that implement this interface. They must be object types.
  • All other fields must return null.
4.5.2.5Enum

Enums are special scalars that can only have a defined set of values.

Fields

  • kind must return __TypeKind.ENUM.
  • name must return a String.
  • description may return a String or null.
  • enumValues: The list of EnumValue. There must be at least one and they must have unique names.
    • Accepts the argument includeDeprecated which defaults to false. If true, deprecated enum values are also returned.
  • All other fields must return null.
4.5.2.6Input Object

Input objects are composite types used as inputs into queries defined as a list of named input values.

For example the input object Point could be defined as:

Example № 89input Point {
  x: Int
  y: Int
}

Fields

  • kind must return __TypeKind.INPUT_OBJECT.
  • name must return a String.
  • description may return a String or null.
  • inputFields: a list of InputValue.
  • All other fields must return null.
4.5.2.7List

Lists represent sequences of values in GraphQL. A List type is a type modifier: it wraps another type instance in the ofType field, which defines the type of each item in the list.

Fields

  • kind must return __TypeKind.LIST.
  • ofType: Any type.
  • All other fields must return null.
4.5.2.8Non-Null

GraphQL types are nullable. The value null is a valid response for field type.

A Non‐null type is a type modifier: it wraps another type instance in the ofType field. Non‐null types do not allow null as a response, and indicate required inputs for arguments and input object fields.

  • kind must return __TypeKind.NON_NULL.
  • ofType: Any type except Non‐null.
  • All other fields must return null.

4.5.3The __Field Type

The __Field type represents each field in an Object or Interface type.

Fields

  • name must return a String
  • description may return a String or null
  • args returns a List of __InputValue representing the arguments this field accepts.
  • type must return a __Type that represents the type of value returned by this field.
  • isDeprecated returns true if this field should no longer be used, otherwise false.
  • deprecationReason optionally provides a reason why this field is deprecated.

4.5.4The __InputValue Type

The __InputValue type represents field and directive arguments as well as the inputFields of an input object.

Fields

  • name must return a String
  • description may return a String or null
  • type must return a __Type that represents the type this input value expects.
  • defaultValue may return a String encoding (using the GraphQL language) of the default value used by this input value in the condition a value is not provided at runtime. If this input value has no default value, returns null.

4.5.5The __EnumValue Type

The __EnumValue type represents one of possible values of an enum.

Fields

  • name must return a String
  • description may return a String or null
  • isDeprecated returns true if this field should no longer be used, otherwise false.
  • deprecationReason optionally provides a reason why this field is deprecated.

4.5.6The __Directive Type

The __Directive type represents a Directive that a server supports.

Fields

  • name must return a String
  • description may return a String or null
  • locations returns a List of __DirectiveLocation representing the valid locations this directive may be placed.
  • args returns a List of __InputValue representing the arguments this directive accepts.

5Validation

GraphQL does not just verify if a request is syntactically correct, but also ensures that it is unambiguous and mistake‐free in the context of a given GraphQL schema.

An invalid request is still technically executable, and will always produce a stable result as defined by the algorithms in the Execution section, however that result may be ambiguous, surprising, or unexpected relative to a request containing validation errors, so execution should only occur for valid requests.

Typically validation is performed in the context of a request immediately before execution, however a GraphQL service may execute a request without explicitly validating it if that exact same request is known to have been validated before. For example: the request may be validated during development, provided it does not later change, or a service may validate a request once and memoize the result to avoid validating the same request again in the future. Any client‐side or development‐time tool should report validation errors and not allow the formulation or execution of requests known to be invalid at that given point in time.

Type system evolution

As GraphQL type system schema evolve over time by adding new types and new fields, it is possible that a request which was previously valid could later become invalid. Any change that can cause a previously valid request to become invalid is considered a breaking change. GraphQL services and schema maintainers are encouraged to avoid breaking changes, however in order to be more resilient to these breaking changes, sophisticated GraphQL systems may still allow for the execution of requests which at some point were known to be free of any validation errors, and have not changed since.

Examples

For this section of this schema, we will assume the following type system in order to demonstrate examples:

Example № 90type Query {
  dog: Dog
}

enum DogCommand { SIT, DOWN, HEEL }

type Dog implements Pet {
  name: String!
  nickname: String
  barkVolume: Int
  doesKnowCommand(dogCommand: DogCommand!): Boolean!
  isHousetrained(atOtherHomes: Boolean): Boolean!
  owner: Human
}

interface Sentient {
  name: String!
}

interface Pet {
  name: String!
}

type Alien implements Sentient {
  name: String!
  homePlanet: String
}

type Human implements Sentient {
  name: String!
}

enum CatCommand { JUMP }

type Cat implements Pet {
  name: String!
  nickname: String
  doesKnowCommand(catCommand: CatCommand!): Boolean!
  meowVolume: Int
}

union CatOrDog = Cat | Dog
union DogOrHuman = Dog | Human
union HumanOrAlien = Human | Alien

5.1Documents

5.1.1Executable Definitions

Formal Specification

Explanatory Text

GraphQL execution will only consider the executable definitions Operation and Fragment. Type system definitions and extensions are not executable, and are not considered during execution.

To avoid ambiguity, a document containing TypeSystemDefinition is invalid for execution.

GraphQL documents not intended to be directly executed may include TypeSystemDefinition.

For example, the following document is invalid for execution since the original executing schema may not know about the provided type extension:

Counter Example № 91query getDogName {
  dog {
    name
    color
  }
}

extend type Dog {
  color: String
}

5.2Operations

5.2.1Named Operation Definitions

5.2.1.1Operation Name Uniqueness

Formal Specification

  • For each operation definition operation in the document.
  • Let operationName be the name of operation.
  • If operationName exists
    • Let operations be all operation definitions in the document named operationName.
    • operations must be a set of one.

Explanatory Text

Each named operation definition must be unique within a document when referred to by its name.

For example the following document is valid:

Example № 92query getDogName {
  dog {
    name
  }
}

query getOwnerName {
  dog {
    owner {
      name
    }
  }
}

While this document is invalid:

Counter Example № 93query getName {
  dog {
    name
  }
}

query getName {
  dog {
    owner {
      name
    }
  }
}

It is invalid even if the type of each operation is different:

Counter Example № 94query dogOperation {
  dog {
    name
  }
}

mutation dogOperation {
  mutateDog {
    id
  }
}

5.2.2Anonymous Operation Definitions

5.2.2.1Lone Anonymous Operation

Formal Specification

  • Let operations be all operation definitions in the document.
  • Let anonymous be all anonymous operation definitions in the document.
  • If operations is a set of more than 1:
    • anonymous must be empty.

Explanatory Text

GraphQL allows a short‐hand form for defining query operations when only that one operation exists in the document.

For example the following document is valid:

Example № 95{
  dog {
    name
  }
}

While this document is invalid:

Counter Example № 96{
  dog {
    name
  }
}

query getName {
  dog {
    owner {
      name
    }
  }
}

5.2.3Subscription Operation Definitions

5.2.3.1Single root field

Formal Specification

  • For each subscription operation definition subscription in the document
  • Let subscriptionType be the root Subscription type in schema.
  • Let selectionSet be the top level selection set on subscription.
  • Let variableValues be the empty set.
  • Let groupedFieldSet be the result of CollectFields(subscriptionType, selectionSet, variableValues).
  • groupedFieldSet must have exactly one entry.

Explanatory Text

Subscription operations must have exactly one root field.

Valid examples:

Example № 97subscription sub {
  newMessage {
    body
    sender
  }
}
Example № 98subscription sub {
  ...newMessageFields
}

fragment newMessageFields on Subscription {
  newMessage {
    body
    sender
  }
}

Invalid:

Counter Example № 99subscription sub {
  newMessage {
    body
    sender
  }
  disallowedSecondRootField
}
Counter Example № 100subscription sub {
  ...multipleSubscriptions
}

fragment multipleSubscriptions on Subscription {
  newMessage {
    body
    sender
  }
  disallowedSecondRootField
}

Introspection fields are counted. The following example is also invalid:

Counter Example № 101subscription sub {
  newMessage {
    body
    sender
  }
  __typename
}
While each subscription must have exactly one root field, a document may contain any number of operations, each of which may contain different root fields. When executed, a document containing multiple subscription operations must provide the operation name as described in GetOperation().

5.3Fields

5.3.1Field Selections on Objects, Interfaces, and Unions Types

Formal Specification

  • For each selection in the document.
  • Let fieldName be the target field of selection
  • fieldName must be defined on type in scope

Explanatory Text

The target field of a field selection must be defined on the scoped type of the selection set. There are no limitations on alias names.

For example the following fragment would not pass validation:

Counter Example № 102fragment fieldNotDefined on Dog {
  meowVolume
}

fragment aliasedLyingFieldTargetNotDefined on Dog {
  barkVolume: kawVolume
}

For interfaces, direct field selection can only be done on fields. Fields of concrete implementors are not relevant to the validity of the given interface‐typed selection set.

For example, the following is valid:

Example № 103fragment interfaceFieldSelection on Pet {
  name
}

and the following is invalid:

Counter Example № 104fragment definedOnImplementorsButNotInterface on Pet {
  nickname
}

Because unions do not define fields, fields may not be directly selected from a union‐typed selection set, with the exception of the meta‐field __typename. Fields from a union‐typed selection set must only be queried indirectly via a fragment.

For example the following is valid:

Example № 105fragment inDirectFieldSelectionOnUnion on CatOrDog {
  __typename
  ... on Pet {
    name
  }
  ... on Dog {
    barkVolume
  }
}

But the following is invalid:

Counter Example № 106fragment directFieldSelectionOnUnion on CatOrDog {
  name
  barkVolume
}

5.3.2Field Selection Merging

Formal Specification

  • Let set be any selection set defined in the GraphQL document.
  • FieldsInSetCanMerge(set) must be true.
FieldsInSetCanMerge(set)
  1. Let fieldsForName be the set of selections with a given response name in set including visiting fragments and inline fragments.
  2. Given each pair of members fieldA and fieldB in fieldsForName:
    1. SameResponseShape(fieldA, fieldB) must be true.
    2. If the parent types of fieldA and fieldB are equal or if either is not an Object Type:
      1. fieldA and fieldB must have identical field names.
      2. fieldA and fieldB must have identical sets of arguments.
      3. Let mergedSet be the result of adding the selection set of fieldA and the selection set of fieldB.
      4. FieldsInSetCanMerge(mergedSet) must be true.
SameResponseShape(fieldA, fieldB)
  1. Let typeA be the return type of fieldA.
  2. Let typeB be the return type of fieldB.
  3. If typeA or typeB is Non‐Null.
    1. If typeA or typeB is nullable, return false.
    2. Let typeA be the nullable type of typeA
    3. Let typeB be the nullable type of typeB
  4. If typeA or typeB is List.
    1. If typeA or typeB is not List, return false.
    2. Let typeA be the item type of typeA
    3. Let typeB be the item type of typeB
    4. Repeat from step 3.
  5. If typeA or typeB is Scalar or Enum.
    1. If typeA and typeB are the same type return true, otherwise return false.
  6. If typeA or typeB is not a composite type, return false.
  7. Let mergedSet be the result of adding the selection set of fieldA and the selection set of fieldB.
  8. Let fieldsForName be the set of selections with a given response name in mergedSet including visiting fragments and inline fragments.
  9. Given each pair of members subfieldA and subfieldB in fieldsForName:
    1. If SameResponseShape(subfieldA, subfieldB) is false, return false.
  10. Return true.

Explanatory Text

If multiple field selections with the same response names are encountered during execution, the field and arguments to execute and the resulting value should be unambiguous. Therefore any two field selections which might both be encountered for the same object are only valid if they are equivalent.

During execution, the simultaneous execution of fields with the same response name is accomplished by MergeSelectionSets() and CollectFields().

For simple hand‐written GraphQL, this rule is obviously a clear developer error, however nested fragments can make this difficult to detect manually.

The following selections correctly merge:

Example № 107fragment mergeIdenticalFields on Dog {
  name
  name
}

fragment mergeIdenticalAliasesAndFields on Dog {
  otherName: name
  otherName: name
}

The following is not able to merge:

Counter Example № 108fragment conflictingBecauseAlias on Dog {
  name: nickname
  name
}

Identical arguments are also merged if they have identical arguments. Both values and variables can be correctly merged.

For example the following correctly merge:

Example № 109fragment mergeIdenticalFieldsWithIdenticalArgs on Dog {
  doesKnowCommand(dogCommand: SIT)
  doesKnowCommand(dogCommand: SIT)
}

fragment mergeIdenticalFieldsWithIdenticalValues on Dog {
  doesKnowCommand(dogCommand: $dogCommand)
  doesKnowCommand(dogCommand: $dogCommand)
}

The following do not correctly merge:

Counter Example № 110fragment conflictingArgsOnValues on Dog {
  doesKnowCommand(dogCommand: SIT)
  doesKnowCommand(dogCommand: HEEL)
}

fragment conflictingArgsValueAndVar on Dog {
  doesKnowCommand(dogCommand: SIT)
  doesKnowCommand(dogCommand: $dogCommand)
}

fragment conflictingArgsWithVars on Dog {
  doesKnowCommand(dogCommand: $varOne)
  doesKnowCommand(dogCommand: $varTwo)
}

fragment differingArgs on Dog {
  doesKnowCommand(dogCommand: SIT)
  doesKnowCommand
}

The following fields would not merge together, however both cannot be encountered against the same object, so they are safe:

Example № 111fragment safeDifferingFields on Pet {
  ... on Dog {
    volume: barkVolume
  }
  ... on Cat {
    volume: meowVolume
  }
}

fragment safeDifferingArgs on Pet {
  ... on Dog {
    doesKnowCommand(dogCommand: SIT)
  }
  ... on Cat {
    doesKnowCommand(catCommand: JUMP)
  }
}

However, the field responses must be shapes which can be merged. For example, scalar values must not differ. In this example, someValue might be a String or an Int:

Counter Example № 112fragment conflictingDifferingResponses on Pet {
  ... on Dog {
    someValue: nickname
  }
  ... on Cat {
    someValue: meowVolume
  }
}

5.3.3Leaf Field Selections

Formal Specification

  • For each selection in the document
  • Let selectionType be the result type of selection
  • If selectionType is a scalar or enum:
    • The subselection set of that selection must be empty
  • If selectionType is an interface, union, or object
    • The subselection set of that selection must NOT BE empty

Explanatory Text

Field selections on scalars or enums are never allowed, because they are the leaf nodes of any GraphQL query.

The following is valid.

Example № 113fragment scalarSelection on Dog {
  barkVolume
}

The following is invalid.

Counter Example № 114fragment scalarSelectionsNotAllowedOnInt on Dog {
  barkVolume {
    sinceWhen
  }
}

Conversely the leaf field selections of GraphQL queries must be of type scalar or enum. Leaf selections on objects, interfaces, and unions without subfields are disallowed.

Let’s assume the following additions to the query root type of the schema:

Example № 115extend type Query {
  human: Human
  pet: Pet
  catOrDog: CatOrDog
}

The following examples are invalid

Counter Example № 116query directQueryOnObjectWithoutSubFields {
  human
}

query directQueryOnInterfaceWithoutSubFields {
  pet
}

query directQueryOnUnionWithoutSubFields {
  catOrDog
}

5.4Arguments

Arguments are provided to both fields and directives. The following validation rules apply in both cases.

5.4.1Argument Names

Formal Specification

  • For each argument in the document
  • Let argumentName be the Name of argument.
  • Let argumentDefinition be the argument definition provided by the parent field or definition named argumentName.
  • argumentDefinition must exist.

Explanatory Text

Every argument provided to a field or directive must be defined in the set of possible arguments of that field or directive.

For example the following are valid:

Example № 117fragment argOnRequiredArg on Dog {
  doesKnowCommand(dogCommand: SIT)
}

fragment argOnOptional on Dog {
  isHousetrained(atOtherHomes: true) @include(if: true)
}

the following is invalid since command is not defined on DogCommand.

Counter Example № 118fragment invalidArgName on Dog {
  doesKnowCommand(command: CLEAN_UP_HOUSE)
}

and this is also invalid as unless is not defined on @include.

Counter Example № 119fragment invalidArgName on Dog {
  isHousetrained(atOtherHomes: true) @include(unless: false)
}

In order to explore more complicated argument examples, let’s add the following to our type system:

Example № 120type Arguments {
  multipleReqs(x: Int!, y: Int!): Int!
  booleanArgField(booleanArg: Boolean): Boolean
  floatArgField(floatArg: Float): Float
  intArgField(intArg: Int): Int
  nonNullBooleanArgField(nonNullBooleanArg: Boolean!): Boolean!
  booleanListArgField(booleanListArg: [Boolean]!): [Boolean]
  optionalNonNullBooleanArgField(optionalBooleanArg: Boolean! = false): Boolean!
}

extend type Query {
  arguments: Arguments
}

Order does not matter in arguments. Therefore both the following example are valid.

Example № 121fragment multipleArgs on Arguments {
  multipleReqs(x: 1, y: 2)
}

fragment multipleArgsReverseOrder on Arguments {
  multipleReqs(y: 2, x: 1)
}

5.4.2Argument Uniqueness

Fields and directives treat arguments as a mapping of argument name to value. More than one argument with the same name in an argument set is ambiguous and invalid.

Formal Specification

  • For each argument in the Document.
  • Let argumentName be the Name of argument.
  • Let arguments be all Arguments named argumentName in the Argument Set which contains argument.
  • arguments must be the set containing only argument.
5.4.2.1Required Arguments
  • For each Field or Directive in the document.
  • Let arguments be the arguments provided by the Field or Directive.
  • Let argumentDefinitions be the set of argument definitions of that Field or Directive.
  • For each argumentDefinition in argumentDefinitions:
    • Let type be the expected type of argumentDefinition.
    • Let defaultValue be the default value of argumentDefinition.
    • If type is Non‐Null and defaultValue does not exist:
      • Let argumentName be the name of argumentDefinition.
      • Let argument be the argument in arguments named argumentName
      • argument must exist.
      • Let value be the value of argument.
      • value must not be the null literal.

Explanatory Text

Arguments can be required. An argument is required if the argument type is non‐null and does not have a default value. Otherwise, the argument is optional.

For example the following are valid:

Example № 122fragment goodBooleanArg on Arguments {
  booleanArgField(booleanArg: true)
}

fragment goodNonNullArg on Arguments {
  nonNullBooleanArgField(nonNullBooleanArg: true)
}

The argument can be omitted from a field with a nullable argument.

Therefore the following query is valid:

Example № 123fragment goodBooleanArgDefault on Arguments {
  booleanArgField
}

but this is not valid on a required argument.

Counter Example № 124fragment missingRequiredArg on Arguments {
  nonNullBooleanArgField
}

Providing the explicit value null is also not valid since required arguments always have a non‐null type.

Counter Example № 125fragment missingRequiredArg on Arguments {
  nonNullBooleanArgField(nonNullBooleanArg: null)
}

5.5Fragments

5.5.1Fragment Declarations

5.5.1.1Fragment Name Uniqueness

Formal Specification

  • For each fragment definition fragment in the document
  • Let fragmentName be the name of fragment.
  • Let fragments be all fragment definitions in the document named fragmentName.
  • fragments must be a set of one.

Explanatory Text

Fragment definitions are referenced in fragment spreads by name. To avoid ambiguity, each fragment’s name must be unique within a document.

Inline fragments are not considered fragment definitions, and are unaffected by this validation rule.

For example the following document is valid:

Example № 126{
  dog {
    ...fragmentOne
    ...fragmentTwo
  }
}

fragment fragmentOne on Dog {
  name
}

fragment fragmentTwo on Dog {
  owner {
    name
  }
}

While this document is invalid:

Counter Example № 127{
  dog {
    ...fragmentOne
  }
}

fragment fragmentOne on Dog {
  name
}

fragment fragmentOne on Dog {
  owner {
    name
  }
}
5.5.1.2Fragment Spread Type Existence

Formal Specification

  • For each named spread namedSpread in the document
  • Let fragment be the target of namedSpread
  • The target type of fragment must be defined in the schema

Explanatory Text

Fragments must be specified on types that exist in the schema. This applies for both named and inline fragments. If they are not defined in the schema, the query does not validate.

For example the following fragments are valid:

Example № 128fragment correctType on Dog {
  name
}

fragment inlineFragment on Dog {
  ... on Dog {
    name
  }
}

fragment inlineFragment2 on Dog {
  ... @include(if: true) {
    name
  }
}

and the following do not validate:

Counter Example № 129fragment notOnExistingType on NotInSchema {
  name
}

fragment inlineNotExistingType on Dog {
  ... on NotInSchema {
    name
  }
}
5.5.1.3Fragments On Composite Types

Formal Specification

  • For each fragment defined in the document.
  • The target type of fragment must have kind UNION, INTERFACE, or OBJECT.

Explanatory Text

Fragments can only be declared on unions, interfaces, and objects. They are invalid on scalars. They can only be applied on non‐leaf fields. This rule applies to both inline and named fragments.

The following fragment declarations are valid:

Example № 130fragment fragOnObject on Dog {
  name
}

fragment fragOnInterface on Pet {
  name
}

fragment fragOnUnion on CatOrDog {
  ... on Dog {
    name
  }
}

and the following are invalid:

Counter Example № 131fragment fragOnScalar on Int {
  something
}

fragment inlineFragOnScalar on Dog {
  ... on Boolean {
    somethingElse
  }
}
5.5.1.4Fragments Must Be Used

Formal Specification

  • For each fragment defined in the document.
  • fragment must be the target of at least one spread in the document

Explanatory Text

Defined fragments must be used within a document.

For example the following is an invalid document:

Counter Example № 132fragment nameFragment on Dog { # unused
  name
}

{
  dog {
    name
  }
}

5.5.2Fragment Spreads

Field selection is also determined by spreading fragments into one another. The selection set of the target fragment is unioned with the selection set at the level at which the target fragment is referenced.

5.5.2.1Fragment spread target defined

Formal Specification

  • For every namedSpread in the document.
  • Let fragment be the target of namedSpread
  • fragment must be defined in the document

Explanatory Text

Named fragment spreads must refer to fragments defined within the document. It is a validation error if the target of a spread is not defined.

Counter Example № 133{
  dog {
    ...undefinedFragment
  }
}
5.5.2.2Fragment spreads must not form cycles

Formal Specification

  • For each fragmentDefinition in the document
  • Let visited be the empty set.
  • DetectCycles(fragmentDefinition, visited)

DetectCycles(fragmentDefinition, visited) :

  • Let spreads be all fragment spread descendants of fragmentDefinition
  • For each spread in spreads
    • visited must not contain spread
    • Let nextVisited be the set including spread and members of visited
    • Let nextFragmentDefinition be the target of spread
    • DetectCycles(nextFragmentDefinition, nextVisited)

Explanatory Text

The graph of fragment spreads must not form any cycles including spreading itself. Otherwise an operation could infinitely spread or infinitely execute on cycles in the underlying data.

This invalidates fragments that would result in an infinite spread:

Counter Example № 134{
  dog {
    ...nameFragment
  }
}

fragment nameFragment on Dog {
  name
  ...barkVolumeFragment
}

fragment barkVolumeFragment on Dog {
  barkVolume
  ...nameFragment
}

If the above fragments were inlined, this would result in the infinitely large:

Example № 135{
  dog {
    name
    barkVolume
    name
    barkVolume
    name
    barkVolume
    name
    # forever...
  }
}

This also invalidates fragments that would result in an infinite recursion when executed against cyclic data:

Counter Example № 136{
  dog {
    ...dogFragment
  }
}

fragment dogFragment on Dog {
  name
  owner {
    ...ownerFragment
  }
}

fragment ownerFragment on Dog {
  name
  pets {
    ...dogFragment
  }
}
5.5.2.3Fragment spread is possible

Formal Specification

  • For each spread (named or inline) defined in the document.
  • Let fragment be the target of spread
  • Let fragmentType be the type condition of fragment
  • Let parentType be the type of the selection set containing spread
  • Let applicableTypes be the intersection of GetPossibleTypes(fragmentType) and GetPossibleTypes(parentType)
  • applicableTypes must not be empty.
GetPossibleTypes(type)
  1. If type is an object type, return a set containing type
  2. If type is an interface type, return the set of types implementing type
  3. If type is a union type, return the set of possible types of type

Explanatory Text

Fragments are declared on a type and will only apply when the runtime object type matches the type condition. They also are spread within the context of a parent type. A fragment spread is only valid if its type condition could ever apply within the parent type.

5.5.2.3.1Object Spreads In Object Scope

In the scope of an object type, the only valid object type fragment spread is one that applies to the same type that is in scope.

For example

Example № 137fragment dogFragment on Dog {
  ... on Dog {
    barkVolume
  }
}

and the following is invalid

Counter Example № 138fragment catInDogFragmentInvalid on Dog {
  ... on Cat {
    meowVolume
  }
}
5.5.2.3.2Abstract Spreads in Object Scope

In scope of an object type, unions or interface spreads can be used if the object type implements the interface or is a member of the union.

For example

Example № 139fragment petNameFragment on Pet {
  name
}

fragment interfaceWithinObjectFragment on Dog {
  ...petNameFragment
}

is valid because Dog implements Pet.

Likewise

Example № 140fragment catOrDogNameFragment on CatOrDog {
  ... on Cat {
    meowVolume
  }
}

fragment unionWithObjectFragment on Dog {
  ...catOrDogNameFragment
}

is valid because Dog is a member of the CatOrDog union. It is worth noting that if one inspected the contents of the CatOrDogNameFragment you could note that no valid results would ever be returned. However we do not specify this as invalid because we only consider the fragment declaration, not its body.

5.5.2.3.3Object Spreads In Abstract Scope

Union or interface spreads can be used within the context of an object type fragment, but only if the object type is one of the possible types of that interface or union.

For example, the following fragments are valid:

Example № 141fragment petFragment on Pet {
  name
  ... on Dog {
    barkVolume
  }
}

fragment catOrDogFragment on CatOrDog {
  ... on Cat {
    meowVolume
  }
}

petFragment is valid because Dog implements the interface Pet. catOrDogFragment is valid because Cat is a member of the CatOrDog union.

By contrast the following fragments are invalid:

Counter Example № 142fragment sentientFragment on Sentient {
  ... on Dog {
    barkVolume
  }
}

fragment humanOrAlienFragment on HumanOrAlien {
  ... on Cat {
    meowVolume
  }
}

Dog does not implement the interface Sentient and therefore sentientFragment can never return meaningful results. Therefore the fragment is invalid. Likewise Cat is not a member of the union HumanOrAlien, and it can also never return meaningful results, making it invalid.

5.5.2.3.4Abstract Spreads in Abstract Scope

Union or interfaces fragments can be used within each other. As long as there exists at least one object type that exists in the intersection of the possible types of the scope and the spread, the spread is considered valid.

So for example

Example № 143fragment unionWithInterface on Pet {
  ...dogOrHumanFragment
}

fragment dogOrHumanFragment on DogOrHuman {
  ... on Dog {
    barkVolume
  }
}

is consider valid because Dog implements interface Pet and is a member of DogOrHuman.

However

Counter Example № 144fragment nonIntersectingInterfaces on Pet {
  ...sentientFragment
}

fragment sentientFragment on Sentient {
  name
}

is not valid because there exists no type that implements both Pet and Sentient.

5.6Values

5.6.1Values of Correct Type

Format Specification

  • For each input Value value in the document.
    • Let type be the type expected in the position value is found.
    • value must be coercible to type.

Explanatory Text

Literal values must be compatible with the type expected in the position they are found as per the coercion rules defined in the Type System chapter.

The type expected in a position include the type defined by the argument a value is provided for, the type defined by an input object field a value is provided for, and the type of a variable definition a default value is provided for.

The following examples are valid use of value literals:

Example № 145fragment goodBooleanArg on Arguments {
  booleanArgField(booleanArg: true)
}

fragment coercedIntIntoFloatArg on Arguments {
  # Note: The input coercion rules for Float allow Int literals.
  floatArgField(floatArg: 123)
}

query goodComplexDefaultValue($search: ComplexInput = { name: "Fido" }) {
  findDog(complex: $search)
}

Non‐coercible values (such as a String into an Int) are invalid. The following examples are invalid:

Counter Example № 146fragment stringIntoInt on Arguments {
  intArgField(intArg: "123")
}

query badComplexValue {
  findDog(complex: { name: 123 })
}

5.6.2Input Object Field Names

Formal Specification

  • For each Input Object Field inputField in the document
  • Let inputFieldName be the Name of inputField.
  • Let inputFieldDefinition be the input field definition provided by the parent input object type named inputFieldName.
  • inputFieldDefinition must exist.

Explanatory Text

Every input field provided in an input object value must be defined in the set of possible fields of that input object’s expected type.

For example the following example input object is valid:

Example № 147{
  findDog(complex: { name: "Fido" })
}

While the following example input‐object uses a field “favoriteCookieFlavor” which is not defined on the expected type:

Counter Example № 148{
  findDog(complex: { favoriteCookieFlavor: "Bacon" })
}

5.6.3Input Object Field Uniqueness

Formal Specification

  • For each input object value inputObject in the document.
  • For every inputField in inputObject
    • Let name be the Name of inputField.
    • Let fields be all Input Object Fields named name in inputObject.
    • fields must be the set containing only inputField.

Explanatory Text

Input objects must not contain more than one field of the same name, otherwise an ambiguity would exist which includes an ignored portion of syntax.

For example the following query will not pass validation.

Counter Example № 149{
  field(arg: { field: true, field: false })
}

5.6.4Input Object Required Fields

Formal Specification

  • For each Input Object in the document.
    • Let fields be the fields provided by that Input Object.
    • Let fieldDefinitions be the set of input field definitions of that Input Object.
  • For each fieldDefinition in fieldDefinitions:
    • Let type be the expected type of fieldDefinition.
    • Let defaultValue be the default value of fieldDefinition.
    • If type is Non‐Null and defaultValue does not exist:
      • Let fieldName be the name of fieldDefinition.
      • Let field be the input field in fields named fieldName
      • field must exist.
      • Let value be the value of field.
      • value must not be the null literal.

Explanatory Text

Input object fields may be required. Much like a field may have required arguments, an input object may have required fields. An input field is required if it has a non‐null type and does not have a default value. Otherwise, the input object field is optional.

5.7Directives

5.7.1Directives Are Defined

Formal Specification

  • For every directive in a document.
  • Let directiveName be the name of directive.
  • Let directiveDefinition be the directive named directiveName.
  • directiveDefinition must exist.

Explanatory Text

GraphQL servers define what directives they support. For each usage of a directive, the directive must be available on that server.

5.7.2Directives Are In Valid Locations

Formal Specification

  • For every directive in a document.
  • Let directiveName be the name of directive.
  • Let directiveDefinition be the directive named directiveName.
  • Let locations be the valid locations for directiveDefinition.
  • Let adjacent be the AST node the directive affects.
  • adjacent must be represented by an item within locations.

Explanatory Text

GraphQL servers define what directives they support and where they support them. For each usage of a directive, the directive must be used in a location that the server has declared support for.

For example the following query will not pass validation because @skip does not provide QUERY as a valid location.

Counter Example № 150query @skip(if: $foo) {
  field
}

5.7.3Directives Are Unique Per Location

Formal Specification

  • For every location in the document for which Directives can apply:
    • Let directives be the set of Directives which apply to location.
    • For each directive in directives:
      • Let directiveName be the name of directive.
      • Let namedDirectives be the set of all Directives named directiveName in directives.
      • namedDirectives must be a set of one.

Explanatory Text

Directives are used to describe some metadata or behavioral change on the definition they apply to. When more than one directive of the same name is used, the expected metadata or behavior becomes ambiguous, therefore only one of each directive is allowed per location.

For example, the following query will not pass validation because @skip has been used twice for the same field:

Counter Example № 151query ($foo: Boolean = true, $bar: Boolean = false) {
  field @skip(if: $foo) @skip(if: $bar)
}

However the following example is valid because @skip has been used only once per location, despite being used twice in the query and on the same named field:

Example № 152query ($foo: Boolean = true, $bar: Boolean = false) {
  field @skip(if: $foo) {
    subfieldA
  }
  field @skip(if: $bar) {
    subfieldB
  }
}

5.8Variables

5.8.1Variable Uniqueness

Formal Specification

  • For every operation in the document
    • For every variable defined on operation
      • Let variableName be the name of variable
      • Let variables be the set of all variables named variableName on operation
      • variables must be a set of one

Explanatory Text

If any operation defines more than one variable with the same name, it is ambiguous and invalid. It is invalid even if the type of the duplicate variable is the same.

Counter Example № 153query houseTrainedQuery($atOtherHomes: Boolean, $atOtherHomes: Boolean) {
  dog {
    isHousetrained(atOtherHomes: $atOtherHomes)
  }
}

It is valid for multiple operations to define a variable with the same name. If two operations reference the same fragment, it might actually be necessary:

Example № 154query A($atOtherHomes: Boolean) {
  ...HouseTrainedFragment
}

query B($atOtherHomes: Boolean) {
  ...HouseTrainedFragment
}

fragment HouseTrainedFragment {
  dog {
    isHousetrained(atOtherHomes: $atOtherHomes)
  }
}

5.8.2Variables Are Input Types

Formal Specification

  • For every operation in a document
  • For every variable on each operation
    • Let variableType be the type of variable
    • IsInputType(variableType) must be true

Explanatory Text

Variables can only be input types. Objects, unions, and interfaces cannot be used as inputs.

For these examples, consider the following typesystem additions:

Example № 155input ComplexInput { name: String, owner: String }

extend type Query {
  findDog(complex: ComplexInput): Dog
  booleanList(booleanListArg: [Boolean!]): Boolean
}

The following queries are valid:

Example № 156query takesBoolean($atOtherHomes: Boolean) {
  dog {
    isHousetrained(atOtherHomes: $atOtherHomes)
  }
}

query takesComplexInput($complexInput: ComplexInput) {
  findDog(complex: $complexInput) {
    name
  }
}

query TakesListOfBooleanBang($booleans: [Boolean!]) {
  booleanList(booleanListArg: $booleans)
}

The following queries are invalid:

Counter Example № 157query takesCat($cat: Cat) {
  # ...
}

query takesDogBang($dog: Dog!) {
  # ...
}

query takesListOfPet($pets: [Pet]) {
  # ...
}

query takesCatOrDog($catOrDog: CatOrDog) {
  # ...
}

5.8.3All Variable Uses Defined

Formal Specification

  • For each operation in a document
    • For each variableUsage in scope, variable must be in operation‘s variable list.
    • Let fragments be every fragment referenced by that operation transitively
    • For each fragment in fragments
      • For each variableUsage in scope of fragment, variable must be in operation‘s variable list.

Explanatory Text

Variables are scoped on a per‐operation basis. That means that any variable used within the context of an operation must be defined at the top level of that operation

For example:

Example № 158query variableIsDefined($atOtherHomes: Boolean) {
  dog {
    isHousetrained(atOtherHomes: $atOtherHomes)
  }
}

is valid. $atOtherHomes is defined by the operation.

By contrast the following query is invalid:

Counter Example № 159query variableIsNotDefined {
  dog {
    isHousetrained(atOtherHomes: $atOtherHomes)
  }
}

$atOtherHomes is not defined by the operation.

Fragments complicate this rule. Any fragment transitively included by an operation has access to the variables defined by that operation. Fragments can appear within multiple operations and therefore variable usages must correspond to variable definitions in all of those operations.

For example the following is valid:

Example № 160query variableIsDefinedUsedInSingleFragment($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedFragment
  }
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

since isHousetrainedFragment is used within the context of the operation variableIsDefinedUsedInSingleFragment and the variable is defined by that operation.

On the other hand, if a fragment is included within an operation that does not define a referenced variable, the query is invalid.

Counter Example № 161query variableIsNotDefinedUsedInSingleFragment {
  dog {
    ...isHousetrainedFragment
  }
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

This applies transitively as well, so the following also fails:

Counter Example № 162query variableIsNotDefinedUsedInNestedFragment {
  dog {
    ...outerHousetrainedFragment
  }
}

fragment outerHousetrainedFragment on Dog {
  ...isHousetrainedFragment
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

Variables must be defined in all operations in which a fragment is used.

Example № 163query housetrainedQueryOne($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedFragment
  }
}

query housetrainedQueryTwo($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedFragment
  }
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

However the following does not validate:

Counter Example № 164query housetrainedQueryOne($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedFragment
  }
}

query housetrainedQueryTwoNotDefined {
  dog {
    ...isHousetrainedFragment
  }
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

This is because housetrainedQueryTwoNotDefined does not define a variable $atOtherHomes but that variable is used by isHousetrainedFragment which is included in that operation.

5.8.4All Variables Used

Formal Specification

  • For every operation in the document.
  • Let variables be the variables defined by that operation
  • Each variable in variables must be used at least once in either the operation scope itself or any fragment transitively referenced by that operation.

Explanatory Text

All variables defined by an operation must be used in that operation or a fragment transitively included by that operation. Unused variables cause a validation error.

For example the following is invalid:

Counter Example № 165query variableUnused($atOtherHomes: Boolean) {
  dog {
    isHousetrained
  }
}

because $atOtherHomes is not referenced.

These rules apply to transitive fragment spreads as well:

Example № 166query variableUsedInFragment($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedFragment
  }
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

The above is valid since $atOtherHomes is used in isHousetrainedFragment which is included by variableUsedInFragment.

If that fragment did not have a reference to $atOtherHomes it would be not valid:

Counter Example № 167query variableNotUsedWithinFragment($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedWithoutVariableFragment
  }
}

fragment isHousetrainedWithoutVariableFragment on Dog {
  isHousetrained
}

All operations in a document must use all of their variables.

As a result, the following document does not validate.

Counter Example № 168query queryWithUsedVar($atOtherHomes: Boolean) {
  dog {
    ...isHousetrainedFragment
  }
}

query queryWithExtraVar($atOtherHomes: Boolean, $extra: Int) {
  dog {
    ...isHousetrainedFragment
  }
}

fragment isHousetrainedFragment on Dog {
  isHousetrained(atOtherHomes: $atOtherHomes)
}

This document is not valid because queryWithExtraVar defines an extraneous variable.

5.8.5All Variable Usages are Allowed

Formal Specification

  • For each operation in document:
  • Let variableUsages be all usages transitively included in the operation.
  • For each variableUsage in variableUsages:
    • Let variableName be the name of variableUsage.
    • Let variableDefinition be the VariableDefinition named variableName defined within operation.
    • IsVariableUsageAllowed(variableDefinition, variableUsage) must be true.
IsVariableUsageAllowed(variableDefinition, variableUsage)
  1. Let variableType be the expected type of variableDefinition.
  2. Let locationType be the expected type of the Argument, ObjectField, or ListValue entry where variableUsage is located.
  3. If locationType is a non‐null type AND variableType is NOT a non‐null type:
    1. Let hasNonNullVariableDefaultValue be true if a default value exists for variableDefinition and is not the value null.
    2. Let hasLocationDefaultValue be true if a default value exists for the Argument or ObjectField where variableUsage is located.
    3. If hasNonNullVariableDefaultValue is NOT true AND hasLocationDefaultValue is NOT true, return false.
    4. Let nullableLocationType be the unwrapped nullable type of locationType.
    5. Return AreTypesCompatible(variableType, nullableLocationType).
  4. Return AreTypesCompatible(variableType, locationType).
AreTypesCompatible(variableType, locationType)
  1. If locationType is a non‐null type:
    1. If variableType is NOT a non‐null type, return false.
    2. Let nullableLocationType be the unwrapped nullable type of locationType.
    3. Let nullableVariableType be the unwrapped nullable type of variableType.
    4. Return AreTypesCompatible(nullableVariableType, nullableLocationType).
  2. Otherwise, if variableType is a non‐null type:
    1. Let nullableVariableType be the nullable type of variableType.
    2. Return AreTypesCompatible(nullableVariableType, locationType).
  3. Otherwise, if locationType is a list type:
    1. If variableType is NOT a list type, return false.
    2. Let itemLocationType be the unwrapped item type of locationType.
    3. Let itemVariableType be the unwrapped item type of variableType.
    4. Return AreTypesCompatible(itemVariableType, itemLocationType).
  4. Otherwise, if variableType is a list type, return false.
  5. Return true if variableType and locationType are identical, otherwise false.

Explanatory Text

Variable usages must be compatible with the arguments they are passed to.

Validation failures occur when variables are used in the context of types that are complete mismatches, or if a nullable type in a variable is passed to a non‐null argument type.

Types must match:

Counter Example № 169query intCannotGoIntoBoolean($intArg: Int) {
  arguments {
    booleanArgField(booleanArg: $intArg)
  }
}

$intArg typed as Int cannot be used as a argument to booleanArg, typed as Boolean.

List cardinality must also be the same. For example, lists cannot be passed into singular values.

Counter Example № 170query booleanListCannotGoIntoBoolean($booleanListArg: [Boolean]) {
  arguments {
    booleanArgField(booleanArg: $booleanListArg)
  }
}

Nullability must also be respected. In general a nullable variable cannot be passed to a non‐null argument.

Counter Example № 171query booleanArgQuery($booleanArg: Boolean) {
  arguments {
    nonNullBooleanArgField(nonNullBooleanArg: $booleanArg)
  }
}

For list types, the same rules around nullability apply to both outer types and inner types. A nullable list cannot be passed to a non‐null list, and a list of nullable values cannot be passed to a list of non‐null values. The following is valid:

Example № 172query nonNullListToList($nonNullBooleanList: [Boolean]!) {
  arguments {
    booleanListArgField(booleanListArg: $nonNullBooleanList)
  }
}

However, a nullable list cannot be passed to a non‐null list:

Counter Example № 173query listToNonNullList($booleanList: [Boolean]) {
  arguments {
    nonNullBooleanListField(nonNullBooleanListArg: $booleanList)
  }
}

This would fail validation because a [T] cannot be passed to a [T]!. Similarly a [T] cannot be passed to a [T!].

Allowing optional variables when default values exist

A notable exception to typical variable type compatibility is allowing a variable definition with a nullable type to be provided to a non‐null location as long as either that variable or that location provides a default value.

Example № 174query booleanArgQueryWithDefault($booleanArg: Boolean) {
  arguments {
    optionalNonNullBooleanArgField(optionalBooleanArg: $booleanArg)
  }
}

In the example above, an optional variable is allowed to be used in an non‐null argument which provides a default value.

Example № 175query booleanArgQueryWithDefault($booleanArg: Boolean = true) {
  arguments {
    nonNullBooleanArgField(nonNullBooleanArg: $booleanArg)
  }
}

In the example above, a variable provides a default value and can be used in a non‐null argument. This behavior is explicitly supported for compatibility with earlier editions of this specification. GraphQL authoring tools may wish to report this is a warning with the suggestion to replace Boolean with Boolean!.

The value null could still be provided to a such a variable at runtime. A non‐null argument must produce a field error if provided a null value.

6Execution

GraphQL generates a response from a request via execution.

A request for execution consists of a few pieces of information:

Given this information, the result of ExecuteRequest() produces the response, to be formatted according to the Response section below.

6.1Executing Requests

To execute a request, the executor must have a parsed Document and a selected operation name to run if the document defines multiple operations, otherwise the document is expected to only contain a single operation. The result of the request is determined by the result of executing this operation according to the “Executing Operations” section below.

ExecuteRequest(schema, document, operationName, variableValues, initialValue)
  1. Let operation be the result of GetOperation(document, operationName).
  2. Let coercedVariableValues be the result of CoerceVariableValues(schema, operation, variableValues).
  3. If operation is a query operation:
    1. Return ExecuteQuery(operation, schema, coercedVariableValues, initialValue).
  4. Otherwise if operation is a mutation operation:
    1. Return ExecuteMutation(operation, schema, coercedVariableValues, initialValue).
  5. Otherwise if operation is a subscription operation:
    1. Return Subscribe(operation, schema, coercedVariableValues, initialValue).
GetOperation(document, operationName)
  1. If operationName is null:
    1. If document contains exactly one operation.
      1. Return the Operation contained in the document.
    2. Otherwise produce a query error requiring operationName.
  2. Otherwise:
    1. Let operation be the Operation named operationName in document.
    2. If operation was not found, produce a query error.
    3. Return operation.

6.1.1Validating Requests

As explained in the Validation section, only requests which pass all validation rules should be executed. If validation errors are known, they should be reported in the list of “errors” in the response and the request must fail without execution.

Typically validation is performed in the context of a request immediately before execution, however a GraphQL service may execute a request without immediately validating it if that exact same request is known to have been validated before. A GraphQL service should only execute requests which at some point were known to be free of any validation errors, and have since not changed.

For example: the request may be validated during development, provided it does not later change, or a service may validate a request once and memoize the result to avoid validating the same request again in the future.

6.1.2Coercing Variable Values

If the operation has defined any variables, then the values for those variables need to be coerced using the input coercion rules of variable’s declared type. If a query error is encountered during input coercion of variable values, then the operation fails without execution.

CoerceVariableValues(schema, operation, variableValues)
  1. Let coercedValues be an empty unordered Map.
  2. Let variableDefinitions be the variables defined by operation.
  3. For each variableDefinition in variableDefinitions:
    1. Let variableName be the name of variableDefinition.
    2. Let variableType be the expected type of variableDefinition.
    3. Assert: IsInputType(variableType) must be true.
    4. Let defaultValue be the default value for variableDefinition.
    5. Let hasValue be true if variableValues provides a value for the name variableName.
    6. Let value be the value provided in variableValues for the name variableName.
    7. If hasValue is not true and defaultValue exists (including null):
      1. Add an entry to coercedValues named variableName with the value defaultValue.
    8. Otherwise if variableType is a Non‐Nullable type, and either hasValue is not true or value is null, throw a query error.
    9. Otherwise if hasValue is true:
      1. If value is null:
        1. Add an entry to coercedValues named variableName with the value null.
      2. Otherwise:
        1. If value cannot be coerced according to the input coercion rules of variableType, throw a query error.
        2. Let coercedValue be the result of coercing value according to the input coercion rules of variableType.
        3. Add an entry to coercedValues named variableName with the value coercedValue.
  4. Return coercedValues.
This algorithm is very similar to CoerceArgumentValues().

6.2Executing Operations

The type system, as described in the “Type System” section of the spec, must provide a query root object type. If mutations or subscriptions are supported, it must also provide a mutation or subscription root object type, respectively.

6.2.1Query

If the operation is a query, the result of the operation is the result of executing the query’s top level selection set with the query root object type.

An initial value may be provided when executing a query.

ExecuteQuery(query, schema, variableValues, initialValue)
  1. Let queryType be the root Query type in schema.
  2. Assert: queryType is an Object type.
  3. Let selectionSet be the top level Selection Set in query.
  4. Let data be the result of running ExecuteSelectionSet(selectionSet, queryType, initialValue, variableValues) normally (allowing parallelization).
  5. Let errors be any field errors produced while executing the selection set.
  6. Return an unordered map containing data and errors.

6.2.2Mutation

If the operation is a mutation, the result of the operation is the result of executing the mutation’s top level selection set on the mutation root object type. This selection set should be executed serially.

It is expected that the top level fields in a mutation operation perform side‐effects on the underlying data system. Serial execution of the provided mutations ensures against race conditions during these side‐effects.

ExecuteMutation(mutation, schema, variableValues, initialValue)
  1. Let mutationType be the root Mutation type in schema.
  2. Assert: mutationType is an Object type.
  3. Let selectionSet be the top level Selection Set in mutation.
  4. Let data be the result of running ExecuteSelectionSet(selectionSet, mutationType, initialValue, variableValues) serially.
  5. Let errors be any field errors produced while executing the selection set.
  6. Return an unordered map containing data and errors.

6.2.3Subscription

If the operation is a subscription, the result is an event stream called the “Response Stream” where each event in the event stream is the result of executing the operation for each new event on an underlying “Source Stream”.

Executing a subscription creates a persistent function on the server that maps an underlying Source Stream to a returned Response Stream.

Subscribe(subscription, schema, variableValues, initialValue)
  1. Let sourceStream be the result of running CreateSourceEventStream(subscription, schema, variableValues, initialValue).
  2. Let responseStream be the result of running MapSourceToResponseEvent(sourceStream, subscription, schema, variableValues)
  3. Return responseStream.
In large scale subscription systems, the Subscribe() and ExecuteSubscriptionEvent() algorithms may be run on separate services to maintain predictable scaling properties. See the section below on Supporting Subscriptions at Scale.

As an example, consider a chat application. To subscribe to new messages posted to the chat room, the client sends a request like so:

Example № 176subscription NewMessages {
  newMessage(roomId: 123) {
    sender
    text
  }
}

While the client is subscribed, whenever new messages are posted to chat room with ID “123”, the selection for “sender” and “text” will be evaluated and published to the client, for example:

Example № 177{
  "data": {
    "newMessage": {
      "sender": "Hagrid",
      "text": "You're a wizard!"
    }
  }
}

The “new message posted to chat room” could use a “Pub‐Sub” system where the chat room ID is the “topic” and each “publish” contains the sender and text.

Event Streams

An event stream represents a sequence of discrete events over time which can be observed. As an example, a “Pub‐Sub” system may produce an event stream when “subscribing to a topic”, with an event occurring on that event stream for each “publish” to that topic. Event streams may produce an infinite sequence of events or may complete at any point. Event streams may complete in response to an error or simply because no more events will occur. An observer may at any point decide to stop observing an event stream by cancelling it, after which it must receive no more events from that event stream.

Supporting Subscriptions at Scale

Supporting subscriptions is a significant change for any GraphQL service. Query and mutation operations are stateless, allowing scaling via cloning of GraphQL server instances. Subscriptions, by contrast, are stateful and require maintaining the GraphQL document, variables, and other context over the lifetime of the subscription.

Consider the behavior of your system when state is lost due to the failure of a single machine in a service. Durability and availability may be improved by having separate dedicated services for managing subscription state and client connectivity.

Delivery Agnostic

GraphQL subscriptions do not require any specific serialization format or transport mechanism. Subscriptions specifies algorithms for the creation of a stream, the content of each payload on that stream, and the closing of that stream. There are intentionally no specifications for message acknowledgement, buffering, resend requests, or any other quality of service (QoS) details. Message serialization, transport mechanisms, and quality of service details should be chosen by the implementing service.

6.2.3.1Source Stream

A Source Stream represents the sequence of events, each of which will trigger a GraphQL execution corresponding to that event. Like field value resolution, the logic to create a Source Stream is application‐specific.

CreateSourceEventStream(subscription, schema, variableValues, initialValue)
  1. Let subscriptionType be the root Subscription type in schema.
  2. Assert: subscriptionType is an Object type.
  3. Let groupedFieldSet be the result of CollectFields(subscriptionType, selectionSet, variableValues).
  4. If groupedFieldSet does not have exactly one entry, throw a query error.
  5. Let fields be the value of the first entry in groupedFieldSet.
  6. Let fieldName be the name of the first entry in fields. Note: This value is unaffected if an alias is used.
  7. Let field be the first entry in fields.
  8. Let argumentValues be the result of CoerceArgumentValues(subscriptionType, field, variableValues)
  9. Let fieldStream be the result of running ResolveFieldEventStream(subscriptionType, initialValue, fieldName, argumentValues).
  10. Return fieldStream.
ResolveFieldEventStream(subscriptionType, rootValue, fieldName, argumentValues)
  1. Let resolver be the internal function provided by subscriptionType for determining the resolved event stream of a subscription field named fieldName.
  2. Return the result of calling resolver, providing rootValue and argumentValues.
This ResolveFieldEventStream() algorithm is intentionally similar to ResolveFieldValue() to enable consistency when defining resolvers on any operation type.
6.2.3.2Response Stream

Each event in the underlying Source Stream triggers execution of the subscription selection set using that event as a root value.

MapSourceToResponseEvent(sourceStream, subscription, schema, variableValues)
  1. Return a new event stream responseStream which yields events as follows:
  2. For each event on sourceStream:
    1. Let response be the result of running ExecuteSubscriptionEvent(subscription, schema, variableValues, event).
    2. Yield an event containing response.
  3. When responseStream completes: complete this event stream.
ExecuteSubscriptionEvent(subscription, schema, variableValues, initialValue)
  1. Let subscriptionType be the root Subscription type in schema.
  2. Assert: subscriptionType is an Object type.
  3. Let selectionSet be the top level Selection Set in subscription.
  4. Let data be the result of running ExecuteSelectionSet(selectionSet, subscriptionType, initialValue, variableValues) normally (allowing parallelization).
  5. Let errors be any field errors produced while executing the selection set.
  6. Return an unordered map containing data and errors.
The ExecuteSubscriptionEvent() algorithm is intentionally similar to ExecuteQuery() since this is how each event result is produced.
6.2.3.3Unsubscribe

Unsubscribe cancels the Response Stream when a client no longer wishes to receive payloads for a subscription. This may in turn also cancel the Source Stream. This is also a good opportunity to clean up any other resources used by the subscription.

Unsubscribe(responseStream)
  1. Cancel responseStream

6.3Executing Selection Sets

To execute a selection set, the object value being evaluated and the object type need to be known, as well as whether it must be executed serially, or may be executed in parallel.

First, the selection set is turned into a grouped field set; then, each represented field in the grouped field set produces an entry into a response map.

ExecuteSelectionSet(selectionSet, objectType, objectValue, variableValues)
  1. Let groupedFieldSet be the result of CollectFields(objectType, selectionSet, variableValues).
  2. Initialize resultMap to an empty ordered map.
  3. For each groupedFieldSet as responseKey and fields:
    1. Let fieldName be the name of the first entry in fields. Note: This value is unaffected if an alias is used.
    2. Let fieldType be the return type defined for the field fieldName of objectType.
    3. If fieldType is defined:
      1. Let responseValue be ExecuteField(objectType, objectValue, fields, fieldType, variableValues).
      2. Set responseValue as the value for responseKey in resultMap.
  4. Return resultMap.
resultMap is ordered by which fields appear first in the query. This is explained in greater detail in the Field Collection section below.

Errors and Non‐Null Fields

If during ExecuteSelectionSet() a field with a non‐null fieldType throws a field error then that error must propagate to this entire selection set, either resolving to null if allowed or further propagated to a parent field.

If this occurs, any sibling fields which have not yet executed or have not yet yielded a value may be cancelled to avoid unnecessary work.

See the Errors and Non‐Nullability section of Field Execution for more about this behavior.

6.3.1Normal and Serial Execution

Normally the executor can execute the entries in a grouped field set in whatever order it chooses (normally in parallel). Because the resolution of fields other than top‐level mutation fields must always be side effect‐free and idempotent, the execution order must not affect the result, and hence the server has the freedom to execute the field entries in whatever order it deems optimal.

For example, given the following grouped field set to be executed normally:

Example № 178{
  birthday {
    month
  }
  address {
    street
  }
}

A valid GraphQL executor can resolve the four fields in whatever order it chose (however of course birthday must be resolved before month, and address before street).

When executing a mutation, the selections in the top most selection set will be executed in serial order, starting with the first appearing field textually.

When executing a grouped field set serially, the executor must consider each entry from the grouped field set in the order provided in the grouped field set. It must determine the corresponding entry in the result map for each item to completion before it continues on to the next item in the grouped field set:

For example, given the following selection set to be executed serially:

Example № 179{
  changeBirthday(birthday: $newBirthday) {
    month
  }
  changeAddress(address: $newAddress) {
    street
  }
}

The executor must, in serial:

As an illustrative example, let’s assume we have a mutation field changeTheNumber that returns an object containing one field, theNumber. If we execute the following selection set serially:

Example № 180{
  first: changeTheNumber(newNumber: 1) {
    theNumber
  }
  second: changeTheNumber(newNumber: 3) {
    theNumber
  }
  third: changeTheNumber(newNumber: 2) {
    theNumber
  }
}

The executor will execute the following serially:

  • Resolve the changeTheNumber(newNumber: 1) field
  • Execute the { theNumber } sub‐selection set of first normally
  • Resolve the changeTheNumber(newNumber: 3) field
  • Execute the { theNumber } sub‐selection set of second normally
  • Resolve the changeTheNumber(newNumber: 2) field
  • Execute the { theNumber } sub‐selection set of third normally

A correct executor must generate the following result for that selection set:

Example № 181{
  "first": {
    "theNumber": 1
  },
  "second": {
    "theNumber": 3
  },
  "third": {
    "theNumber": 2
  }
}

6.3.2Field Collection

Before execution, the selection set is converted to a grouped field set by calling CollectFields(). Each entry in the grouped field set is a list of fields that share a response key (the alias if defined, otherwise the field name). This ensures all fields with the same response key included via referenced fragments are executed at the same time.

As an example, collecting the fields of this selection set would collect two instances of the field a and one of field b:

Example № 182{
  a {
    subfield1
  }
  ...ExampleFragment
}

fragment ExampleFragment on Query {
  a {
    subfield2
  }
  b
}

The depth‐first‐search order of the field groups produced by CollectFields() is maintained through execution, ensuring that fields appear in the executed response in a stable and predictable order.

CollectFields(objectType, selectionSet, variableValues, visitedFragments)
  1. If visitedFragments if not provided, initialize it to the empty set.
  2. Initialize groupedFields to an empty ordered map of lists.
  3. For each selection in selectionSet:
    1. If selection provides the directive @skip, let skipDirective be that directive.
      1. If skipDirective‘s if argument is true or is a variable in variableValues with the value true, continue with the next selection in selectionSet.
    2. If selection provides the directive @include, let includeDirective be that directive.
      1. If includeDirective‘s if argument is not true and is not a variable in variableValues with the value true, continue with the next selection in selectionSet.
    3. If selection is a Field:
      1. Let responseKey be the response key of selection (the alias if defined, otherwise the field name).
      2. Let groupForResponseKey be the list in groupedFields for responseKey; if no such list exists, create it as an empty list.
      3. Append selection to the groupForResponseKey.
    4. If selection is a FragmentSpread:
      1. Let fragmentSpreadName be the name of selection.
      2. If fragmentSpreadName is in visitedFragments, continue with the next selection in selectionSet.
      3. Add fragmentSpreadName to visitedFragments.
      4. Let fragment be the Fragment in the current Document whose name is fragmentSpreadName.
      5. If no such fragment exists, continue with the next selection in selectionSet.
      6. Let fragmentType be the type condition on fragment.
      7. If DoesFragmentTypeApply(objectType, fragmentType) is false, continue with the next selection in selectionSet.
      8. Let fragmentSelectionSet be the top‐level selection set of fragment.
      9. Let fragmentGroupedFieldSet be the result of calling CollectFields(objectType, fragmentSelectionSet, visitedFragments).
      10. For each fragmentGroup in fragmentGroupedFieldSet:
        1. Let responseKey be the response key shared by all fields in fragmentGroup.
        2. Let groupForResponseKey be the list in groupedFields for responseKey; if no such list exists, create it as an empty list.
        3. Append all items in fragmentGroup to groupForResponseKey.
    5. If selection is an InlineFragment:
      1. Let fragmentType be the type condition on selection.
      2. If fragmentType is not null and DoesFragmentTypeApply(objectType, fragmentType) is false, continue with the next selection in selectionSet.
      3. Let fragmentSelectionSet be the top‐level selection set of selection.
      4. Let fragmentGroupedFieldSet be the result of calling CollectFields(objectType, fragmentSelectionSet, variableValues, visitedFragments).
      5. For each fragmentGroup in fragmentGroupedFieldSet:
        1. Let responseKey be the response key shared by all fields in fragmentGroup.
        2. Let groupForResponseKey be the list in groupedFields for responseKey; if no such list exists, create it as an empty list.
        3. Append all items in fragmentGroup to groupForResponseKey.
  4. Return groupedFields.
DoesFragmentTypeApply(objectType, fragmentType)
  1. If fragmentType is an Object Type:
    1. if objectType and fragmentType are the same type, return true, otherwise return false.
  2. If fragmentType is an Interface Type:
    1. if objectType is an implementation of fragmentType, return true otherwise return false.
  3. If fragmentType is a Union:
    1. if objectType is a possible type of fragmentType, return true otherwise return false.

6.4Executing Fields

Each field requested in the grouped field set that is defined on the selected objectType will result in an entry in the response map. Field execution first coerces any provided argument values, then resolves a value for the field, and finally completes that value either by recursively executing another selection set or coercing a scalar value.

ExecuteField(objectType, objectValue, fieldType, fields, variableValues)
  1. Let field be the first entry in fields.
  2. Let fieldName be the field name of field.
  3. Let argumentValues be the result of CoerceArgumentValues(objectType, field, variableValues)
  4. Let resolvedValue be ResolveFieldValue(objectType, objectValue, fieldName, argumentValues).
  5. Return the result of CompleteValue(fieldType, fields, resolvedValue, variableValues).

6.4.1Coercing Field Arguments

Fields may include arguments which are provided to the underlying runtime in order to correctly produce a value. These arguments are defined by the field in the type system to have a specific input type.

At each argument position in a query may be a literal Value, or a Variable to be provided at runtime.

CoerceArgumentValues(objectType, field, variableValues)
  1. Let coercedValues be an empty unordered Map.
  2. Let argumentValues be the argument values provided in field.
  3. Let fieldName be the name of field.
  4. Let argumentDefinitions be the arguments defined by objectType for the field named fieldName.
  5. For each argumentDefinition in argumentDefinitions:
    1. Let argumentName be the name of argumentDefinition.
    2. Let argumentType be the expected type of argumentDefinition.
    3. Let defaultValue be the default value for argumentDefinition.
    4. Let hasValue be true if argumentValues provides a value for the name argumentName.
    5. Let argumentValue be the value provided in argumentValues for the name argumentName.
    6. If argumentValue is a Variable:
      1. Let variableName be the name of argumentValue.
      2. Let hasValue be true if variableValues provides a value for the name variableName.
      3. Let value be the value provided in variableValues for the name variableName.
    7. Otherwise, let value be argumentValue.
    8. If hasValue is not true and defaultValue exists (including null):
      1. Add an entry to coercedValues named argumentName with the value defaultValue.
    9. Otherwise if argumentType is a Non‐Nullable type, and either hasValue is not true or value is null, throw a field error.
    10. Otherwise if hasValue is true:
      1. If value is null:
        1. Add an entry to coercedValues named argumentName with the value null.
      2. Otherwise, if argumentValue is a Variable:
        1. Add an entry to coercedValues named argumentName with the value value.
      3. Otherwise:
        1. If value cannot be coerced according to the input coercion rules of variableType, throw a field error.
        2. Let coercedValue be the result of coercing value according to the input coercion rules of variableType.
        3. Add an entry to coercedValues named argumentName with the value coercedValue.
  6. Return coercedValues.
Variable values are not coerced because they are expected to be coerced before executing the operation in CoerceVariableValues(), and valid queries must only allow usage of variables of appropriate types.

6.4.2Value Resolution

While nearly all of GraphQL execution can be described generically, ultimately the internal system exposing the GraphQL interface must provide values. This is exposed via ResolveFieldValue, which produces a value for a given field on a type for a real value.

As an example, this might accept the objectType Person, the field "soulMate", and the objectValue representing John Lennon. It would be expected to yield the value representing Yoko Ono.

ResolveFieldValue(objectType, objectValue, fieldName, argumentValues)
  1. Let resolver be the internal function provided by objectType for determining the resolved value of a field named fieldName.
  2. Return the result of calling resolver, providing objectValue and argumentValues.
It is common for resolver to be asynchronous due to relying on reading an underlying database or networked service to produce a value. This necessitates the rest of a GraphQL executor to handle an asynchronous execution flow.

6.4.3Value Completion

After resolving the value for a field, it is completed by ensuring it adheres to the expected return type. If the return type is another Object type, then the field execution process continues recursively.

CompleteValue(fieldType, fields, result, variableValues)
  1. If the fieldType is a Non‐Null type:
    1. Let innerType be the inner type of fieldType.
    2. Let completedResult be the result of calling CompleteValue(innerType, fields, result, variableValues).
    3. If completedResult is null, throw a field error.
    4. Return completedResult.
  2. If result is null (or another internal value similar to null such as undefined or NaN), return null.
  3. If fieldType is a List type:
    1. If result is not a collection of values, throw a field error.
    2. Let innerType be the inner type of fieldType.
    3. Return a list where each list item is the result of calling CompleteValue(innerType, fields, resultItem, variableValues), where resultItem is each item in result.
  4. If fieldType is a Scalar or Enum type:
    1. Return the result of “coercing” result, ensuring it is a legal value of fieldType, otherwise null.
  5. If fieldType is an Object, Interface, or Union type:
    1. If fieldType is an Object type.
      1. Let objectType be fieldType.
    2. Otherwise if fieldType is an Interface or Union type.
      1. Let objectType be ResolveAbstractType(fieldType, result).
    3. Let subSelectionSet be the result of calling MergeSelectionSets(fields).
    4. Return the result of evaluating ExecuteSelectionSet(subSelectionSet, objectType, result, variableValues) normally (allowing for parallelization).

Resolving Abstract Types

When completing a field with an abstract return type, that is an Interface or Union return type, first the abstract type must be resolved to a relevant Object type. This determination is made by the internal system using whatever means appropriate.

A common method of determining the Object type for an objectValue in object‐oriented environments, such as Java or C#, is to use the class name of the objectValue.
ResolveAbstractType(abstractType, objectValue)
  1. Return the result of calling the internal method provided by the type system for determining the Object type of abstractType given the value objectValue.

Merging Selection Sets

When more than one fields of the same name are executed in parallel, their selection sets are merged together when completing the value in order to continue execution of the sub‐selection sets.

An example query illustrating parallel fields with the same name with sub‐selections.

Example № 183{
  me {
    firstName
  }
  me {
    lastName
  }
}

After resolving the value for me, the selection sets are merged together so firstName and lastName can be resolved for one value.

MergeSelectionSets(fields)
  1. Let selectionSet be an empty list.
  2. For each field in fields:
    1. Let fieldSelectionSet be the selection set of field.
    2. If fieldSelectionSet is null or empty, continue to the next field.
    3. Append all selections in fieldSelectionSet to selectionSet.
  3. Return selectionSet.

6.4.4Errors and Non-Nullability

If an error is thrown while resolving a field, it should be treated as though the field returned null, and an error must be added to the "errors" list in the response.

If the result of resolving a field is null (either because the function to resolve the field returned null or because an error occurred), and that field is of a Non-Null type, then a field error is thrown. The error must be added to the "errors" list in the response.

If the field returns null because of an error which has already been added to the "errors" list in the response, the "errors" list must not be further affected. That is, only one error should be added to the errors list per field.

Since Non-Null type fields cannot be null, field errors are propagated to be handled by the parent field. If the parent field may be null then it resolves to null, otherwise if it is a Non-Null type, the field error is further propagated to it’s parent field.

If a List type wraps a Non-Null type, and one of the elements of that list resolves to null, then the entire list must resolve to null. If the List type is also wrapped in a Non-Null, the field error continues to propagate upwards.

If all fields from the root of the request to the source of the field error return Non-Null types, then the "data" entry in the response should be null.

7Response

When a GraphQL server receives a request, it must return a well‐formed response. The server’s response describes the result of executing the requested operation if successful, and describes any errors encountered during the request.

A response may contain both a partial response as well as encountered errors in the case that a field error occurred on a field which was replaced with null.

7.1Response Format

A response to a GraphQL operation must be a map.

If the operation encountered any errors, the response map must contain an entry with key errors. The value of this entry is described in the “Errors” section. If the operation completed without encountering any errors, this entry must not be present.

If the operation included execution, the response map must contain an entry with key data. The value of this entry is described in the “Data” section. If the operation failed before execution, due to a syntax error, missing information, or validation error, this entry must not be present.

The response map may also contain an entry with key extensions. This entry, if set, must have a map as its value. This entry is reserved for implementors to extend the protocol however they see fit, and hence there are no additional restrictions on its contents.

To ensure future changes to the protocol do not break existing servers and clients, the top level response map must not contain any entries other than the three described above.

When errors is present in the response, it may be helpful for it to appear first when serialized to make it more clear when errors are present in a response during debugging.

7.1.1Data

The data entry in the response will be the result of the execution of the requested operation. If the operation was a query, this output will be an object of the schema’s query root type; if the operation was a mutation, this output will be an object of the schema’s mutation root type.

If an error was encountered before execution begins, the data entry should not be present in the result.

If an error was encountered during the execution that prevented a valid response, the data entry in the response should be null.

7.1.2Errors

The errors entry in the response is a non‐empty list of errors, where each error is a map.

If no errors were encountered during the requested operation, the errors entry should not be present in the result.

If the data entry in the response is not present, the errors entry in the response must not be empty. It must contain at least one error. The errors it contains should indicate why no data was able to be returned.

If the data entry in the response is present (including if it is the value null), the errors entry in the response may contain any errors that occurred during execution. If errors occurred during execution, it should contain those errors.

Error result format

Every error must contain an entry with the key message with a string description of the error intended for the developer as a guide to understand and correct the error.

If an error can be associated to a particular point in the requested GraphQL document, it should contain an entry with the key locations with a list of locations, where each location is a map with the keys line and column, both positive numbers starting from 1 which describe the beginning of an associated syntax element.

If an error can be associated to a particular field in the GraphQL result, it must contain an entry with the key path that details the path of the response field which experienced the error. This allows clients to identify whether a null result is intentional or caused by a runtime error.

This field should be a list of path segments starting at the root of the response and ending with the field associated with the error. Path segments that represent fields should be strings, and path segments that represent list indices should be 0‐indexed integers. If the error happens in an aliased field, the path to the error should use the aliased name, since it represents a path in the response, not in the query.

For example, if fetching one of the friends’ names fails in the following query:

Example № 184{
  hero(episode: $episode) {
    name
    heroFriends: friends {
      id
      name
    }
  }
}

The response might look like:

Example № 185{
  "errors": [
    {
      "message": "Name for character with ID 1002 could not be fetched.",
      "locations": [ { "line": 6, "column": 7 } ],
      "path": [ "hero", "heroFriends", 1, "name" ]
    }
  ],
  "data": {
    "hero": {
      "name": "R2-D2",
      "heroFriends": [
        {
          "id": "1000",
          "name": "Luke Skywalker"
        },
        {
          "id": "1002",
          "name": null
        },
        {
          "id": "1003",
          "name": "Leia Organa"
        }
      ]
    }
  }
}

If the field which experienced an error was declared as Non-Null, the null result will bubble up to the next nullable field. In that case, the path for the error should include the full path to the result field where the error occurred, even if that field is not present in the response.

For example, if the name field from above had declared a Non-Null return type in the schema, the result would look different but the error reported would be the same:

Example № 186{
  "errors": [
    {
      "message": "Name for character with ID 1002 could not be fetched.",
      "locations": [ { "line": 6, "column": 7 } ],
      "path": [ "hero", "heroFriends", 1, "name" ]
    }
  ],
  "data": {
    "hero": {
      "name": "R2-D2",
      "heroFriends": [
        {
          "id": "1000",
          "name": "Luke Skywalker"
        },
        null,
        {
          "id": "1003",
          "name": "Leia Organa"
        }
      ]
    }
  }
}

GraphQL services may provide an additional entry to errors with key extensions. This entry, if set, must have a map as its value. This entry is reserved for implementors to add additional information to errors however they see fit, and there are no additional restrictions on its contents.

Example № 187{
  "errors": [
    {
      "message": "Name for character with ID 1002 could not be fetched.",
      "locations": [ { "line": 6, "column": 7 } ],
      "path": [ "hero", "heroFriends", 1, "name" ],
      "extensions": {
        "code": "CAN_NOT_FETCH_BY_ID",
        "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
      }
    }
  ]
}

GraphQL services should not provide any additional entries to the error format since they could conflict with additional entries that may be added in future versions of this specification.

Previous versions of this spec did not describe the extensions entry for error formatting. While non‐specified entries are not violations, they are still discouraged.
Counter Example № 188{
  "errors": [
    {
      "message": "Name for character with ID 1002 could not be fetched.",
      "locations": [ { "line": 6, "column": 7 } ],
      "path": [ "hero", "heroFriends", 1, "name" ],
      "code": "CAN_NOT_FETCH_BY_ID",
      "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
    }
  ]
}

7.2Serialization Format

GraphQL does not require a specific serialization format. However, clients should use a serialization format that supports the major primitives in the GraphQL response. In particular, the serialization format must at least support representations of the following four primitives:

A serialization format should also support the following primitives, each representing one of the common GraphQL scalar types, however a string or simpler primitive may be used as a substitute if any are not directly supported:

This is not meant to be an exhaustive list of what a serialization format may encode. For example custom scalars representing a Date, Time, URI, or number with a different precision may be represented in whichever relevant format a given serialization format may support.

7.2.1JSON Serialization

JSON is the most common serialization format for GraphQL. Though as mentioned above, GraphQL does not require a specific serialization format.

When using JSON as a serialization of GraphQL responses, the following JSON values should be used to encode the related GraphQL values:

GraphQL Value JSON Value
Map Object
List Array
Null null
String String
Boolean true or false
Int Number
Float Number
Enum Value String
For consistency and ease of notation, examples of responses are given in JSON format throughout this document.

7.2.2Serialized Map Ordering

Since the result of evaluating a selection set is ordered, the serialized Map of results should preserve this order by writing the map entries in the same order as those fields were requested as defined by query execution. Producing a serialized response where fields are represented in the same order in which they appear in the request improves human readability during debugging and enables more efficient parsing of responses if the order of properties can be anticipated.

Serialization formats which represent an ordered map should preserve the order of requested fields as defined by CollectFields() in the Execution section. Serialization formats which only represent unordered maps but where order is still implicit in the serialization’s textual order (such as JSON) should preserve the order of requested fields textually.

For example, if the request was { name, age }, a GraphQL service responding in JSON should respond with { "name": "Mark", "age": 30 } and should not respond with { "age": 30, "name": "Mark" }.

While JSON Objects are specified as an unordered collection of key‐value pairs the pairs are represented in an ordered manner. In other words, while the JSON strings { "name": "Mark", "age": 30 } and { "age": 30, "name": "Mark" } encode the same value, they also have observably different property orderings.

This does not violate the JSON spec, as clients may still interpret objects in the response as unordered Maps and arrive at a valid value.

AAppendix: Notation Conventions

This specification document contains a number of notation conventions used to describe technical concepts such as language grammar and semantics as well as runtime algorithms.

This appendix seeks to explain these notations in greater detail to avoid ambiguity.

A.1Context-Free Grammar

A context‐free grammar consists of a number of productions. Each production has an abstract symbol called a “non‐terminal” as its left‐hand side, and zero or more possible sequences of non‐terminal symbols and or terminal characters as its right‐hand side.

Starting from a single goal non‐terminal symbol, a context‐free grammar describes a language: the set of possible sequences of characters that can be described by repeatedly replacing any non‐terminal in the goal sequence with one of the sequences it is defined by, until all non‐terminal symbols have been replaced by terminal characters.

Terminals are represented in this document in a monospace font in two forms: a specific Unicode character or sequence of Unicode characters (ex. = or terminal), and a pattern of Unicode characters defined by a regular expression (ex /[0-9]+/).

Non‐terminal production rules are represented in this document using the following notation for a non‐terminal with a single definition:

While using the following notation for a production with a list of definitions:

NonTerminalWithManyDefinitions
OtherNonTerminalterminal
terminal

A definition may refer to itself, which describes repetitive sequences, for example:

A.2Lexical and Syntactical Grammar

The GraphQL language is defined in a syntactic grammar where terminal symbols are tokens. Tokens are defined in a lexical grammar which matches patterns of source characters. The result of parsing a sequence of source Unicode characters produces a GraphQL AST.

A Lexical grammar production describes non‐terminal “tokens” by patterns of terminal Unicode characters. No “whitespace” or other ignored characters may appear between any terminal Unicode characters in the lexical grammar production. A lexical grammar production is distinguished by a two colon :: definition.

Word
/[A-Za-z]+/

A Syntactical grammar production describes non‐terminal “rules” by patterns of terminal Tokens. Whitespace and other ignored characters may appear before or after any terminal Token. A syntactical grammar production is distinguished by a one colon : definition.

Sentence
NounVerb

A.3Grammar Notation

This specification uses some additional notation to describe common patterns, such as optional or repeated patterns, or parameterized alterations of the definition of a non‐terminal. This section explains these short‐hand notations and their expanded definitions in the context‐free grammar.

Constraints

A grammar production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded.

For example, the production:

SafeName
NameSevenCarlinWords

means that the nonterminal SafeName may be replaced by any sequence of characters that could replace Name provided that the same sequence of characters could not replace SevenCarlinWords.

A grammar may also list a number of restrictions after “but not” separated by “or”.

For example:

Optionality and Lists

A subscript suffix “Symbolopt” is shorthand for two possible sequences, one including that symbol and one excluding it.

As an example:

Sentence
NounVerbAdverbopt

is shorthand for

Sentence
NounVerb
NounVerbAdverb

A subscript suffix “Symbollist” is shorthand for a list of one or more of that symbol.

As an example:

Book
CoverPagelistCover

is shorthand for

Book
CoverPage_listCover

Parameterized Grammar Productions

A symbol definition subscript suffix parameter in braces “SymbolParam” is shorthand for two symbol definitions, one appended with that parameter name, the other without. The same subscript suffix on a symbol is shorthand for that variant of the definition. If the parameter starts with “?”, that form of the symbol is used if in a symbol definition with the same parameter. Some possible sequences can be included or excluded conditionally when respectively prefixed with “[+Param]” and “[~Param]”.

As an example:

ExampleParam
A
BParam
CParam
ParamD
ParamE

is shorthand for

Example
A
B_param
C
E
Example_param
A
B_param
C_param
D

A.4Grammar Semantics

This specification describes the semantic value of many grammar productions in the form of a list of algorithmic steps.

For example, this describes how a parser should interpret a string literal:

StringValue
""
  1. Return an empty Unicode character sequence.
StringValue
  1. Return the Unicode character sequence of all StringCharacter Unicode character values.

A.5Algorithms

This specification describes some algorithms used by the static and runtime semantics, they’re defined in the form of a function‐like syntax with the algorithm’s name and the arguments it accepts along with a list of algorithmic steps to take in the order listed. Each step may establish references to other values, check various conditions, call other algorithms, and eventually return a value representing the outcome of the algorithm for the provided arguments.

For example, the following example describes an algorithm named Fibonacci which accepts a single argument number. The algoritm’s steps produce the next number in the Fibonacci sequence:

Fibonacci(number)
  1. If number is 0:
    1. Return 1.
  2. If number is 1:
    1. Return 2.
  3. Let previousNumber be number - 1.
  4. Let previousPreviousNumber be number - 2.
  5. Return Fibonacci(previousNumber) + Fibonacci(previousPreviousNumber).
Algorithms described in this document are written to be easy to understand. Implementers are encouraged to include equivalent but optimized implementations.

BAppendix: Grammar Summary

SourceCharacter
/[\u0009\u000A\u000D\u0020-\uFFFF]/

B.1Ignored Tokens

UnicodeBOM
Byte Order Mark (U+FEFF)
WhiteSpace
Horizontal Tab (U+0009)
Space (U+0020)
LineTerminator
New Line (U+000A)
Carriage Return (U+000D)New Line (U+000A)
Carriage Return (U+000D)New Line (U+000A)

B.2Lexical Tokens

Punctuator
!$()...:=@[]{|}
Name
/[_A-Za-z][_0-9A-Za-z]*/
Digit
0123456789
EscapedUnicode
/[0-9A-Fa-f]{4}/
Block string values are interpreted to exclude blank initial and trailing lines and uniform indentation with BlockStringValue().

B.3Document

OperationType
querymutationsubscription
ArgumentsConst
(ArgumentConstlist)
BooleanValue
truefalse
EnumValue
Nametruefalsenull
ListValueConst
[]
[ValueConstlist]
ObjectValueConst
{}
{ObjectFieldConstlist}
DirectivesConst
DirectiveConstlist
SchemaExtension
extendschemaDirectivesConstopt{OperationTypeDefinitionlist}
extendschemaDirectivesConst
ExecutableDirectiveLocation
QUERY
MUTATION
SUBSCRIPTION
FIELD
FRAGMENT_DEFINITION
FRAGMENT_SPREAD
INLINE_FRAGMENT
VARIABLE_DEFINITION
TypeSystemDirectiveLocation
SCHEMA
SCALAR
OBJECT
FIELD_DEFINITION
ARGUMENT_DEFINITION
INTERFACE
UNION
ENUM
ENUM_VALUE
INPUT_OBJECT
INPUT_FIELD_DEFINITION

§Index

  1. Alias
  2. AreTypesCompatible
  3. Argument
  4. Arguments
  5. ArgumentsDefinition
  6. BlockStringCharacter
  7. BlockStringValue
  8. BooleanValue
  9. CoerceArgumentValues
  10. CoerceVariableValues
  11. CollectFields
  12. Comma
  13. Comment
  14. CommentChar
  15. CompleteValue
  16. CreateSourceEventStream
  17. DefaultValue
  18. Definition
  19. Description
  20. Digit
  21. Directive
  22. DirectiveDefinition
  23. DirectiveLocation
  24. DirectiveLocations
  25. Directives
  26. Document
  27. DoesFragmentTypeApply
  28. EnumTypeDefinition
  29. EnumTypeExtension
  30. EnumValue
  31. EnumValueDefinition
  32. EnumValuesDefinition
  33. EscapedCharacter
  34. EscapedUnicode
  35. ExecutableDefinition
  36. ExecutableDirectiveLocation
  37. ExecuteField
  38. ExecuteMutation
  39. ExecuteQuery
  40. ExecuteRequest
  41. ExecuteSelectionSet
  42. ExecuteSubscriptionEvent
  43. ExponentIndicator
  44. ExponentPart
  45. Field
  46. FieldDefinition
  47. FieldsDefinition
  48. FieldsInSetCanMerge
  49. FloatValue
  50. FractionalPart
  51. FragmentDefinition
  52. FragmentName
  53. FragmentSpread
  54. GetOperation
  55. GetPossibleTypes
  56. Ignored
  57. ImplementsInterfaces
  58. InlineFragment
  59. InputFieldsDefinition
  60. InputObjectTypeDefinition
  61. InputObjectTypeExtension
  62. InputValueDefinition
  63. IntValue
  64. IntegerPart
  65. InterfaceTypeDefinition
  66. InterfaceTypeExtension
  67. IsInputType
  68. IsOutputType
  69. IsVariableUsageAllowed
  70. LineTerminator
  71. ListType
  72. ListValue
  73. MapSourceToResponseEvent
  74. MergeSelectionSets
  75. Name
  76. NamedType
  77. NegativeSign
  78. NonNullType
  79. NonZeroDigit
  80. NullValue
  81. ObjectField
  82. ObjectTypeDefinition
  83. ObjectTypeExtension
  84. ObjectValue
  85. OperationDefinition
  86. OperationType
  87. Punctuator
  88. ResolveAbstractType
  89. ResolveFieldEventStream
  90. ResolveFieldValue
  91. RootOperationTypeDefinition
  92. SameResponseShape
  93. ScalarTypeDefinition
  94. ScalarTypeExtension
  95. SchemaDefinition
  96. SchemaExtension
  97. Selection
  98. SelectionSet
  99. Sign
  100. SourceCharacter
  101. StringCharacter
  102. StringValue
  103. Subscribe
  104. Token
  105. Type
  106. TypeCondition
  107. TypeDefinition
  108. TypeExtension
  109. TypeSystemDefinition
  110. TypeSystemDirectiveLocation
  111. TypeSystemExtension
  112. UnicodeBOM
  113. UnionMemberTypes
  114. UnionTypeDefinition
  115. UnionTypeExtension
  116. Unsubscribe
  117. Value
  118. Variable
  119. VariableDefinition
  120. VariableDefinitions
  121. WhiteSpace
  1. 1Overview
  2. 2Language
    1. 2.1Source Text
      1. 2.1.1Unicode
      2. 2.1.2White Space
      3. 2.1.3Line Terminators
      4. 2.1.4Comments
      5. 2.1.5Insignificant Commas
      6. 2.1.6Lexical Tokens
      7. 2.1.7Ignored Tokens
      8. 2.1.8Punctuators
      9. 2.1.9Names
    2. 2.2Document
    3. 2.3Operations
    4. 2.4Selection Sets
    5. 2.5Fields
    6. 2.6Arguments
    7. 2.7Field Alias
    8. 2.8Fragments
      1. 2.8.1Type Conditions
      2. 2.8.2Inline Fragments
    9. 2.9Input Values
      1. 2.9.1Int Value
      2. 2.9.2Float Value
      3. 2.9.3Boolean Value
      4. 2.9.4String Value
      5. 2.9.5Null Value
      6. 2.9.6Enum Value
      7. 2.9.7List Value
      8. 2.9.8Input Object Values
    10. 2.10Variables
    11. 2.11Type References
    12. 2.12Directives
  3. 3Type System
    1. 3.1Type System Extensions
    2. 3.2Schema
      1. 3.2.1Root Operation Types
      2. 3.2.2Schema Extension
    3. 3.3Descriptions
    4. 3.4Types
      1. 3.4.1Wrapping Types
      2. 3.4.2Input and Output Types
      3. 3.4.3Type Extensions
    5. 3.5Scalars
      1. 3.5.1Int
      2. 3.5.2Float
      3. 3.5.3String
      4. 3.5.4Boolean
      5. 3.5.5ID
      6. 3.5.6Scalar Extensions
    6. 3.6Objects
      1. 3.6.1Field Arguments
      2. 3.6.2Field Deprecation
      3. 3.6.3Object Extensions
    7. 3.7Interfaces
      1. 3.7.1Interface Extensions
    8. 3.8Unions
      1. 3.8.1Union Extensions
    9. 3.9Enums
      1. 3.9.1Enum Extensions
    10. 3.10Input Objects
      1. 3.10.1Input Object Extensions
    11. 3.11List
    12. 3.12Non-Null
      1. 3.12.1Combining List and Non-Null
    13. 3.13Directives
      1. 3.13.1@skip
      2. 3.13.2@include
      3. 3.13.3@deprecated
  4. 4Introspection
    1. 4.1Reserved Names
    2. 4.2Documentation
    3. 4.3Deprecation
    4. 4.4Type Name Introspection
    5. 4.5Schema Introspection
      1. 4.5.1The __Type Type
      2. 4.5.2Type Kinds
        1. 4.5.2.1Scalar
        2. 4.5.2.2Object
        3. 4.5.2.3Union
        4. 4.5.2.4Interface
        5. 4.5.2.5Enum
        6. 4.5.2.6Input Object
        7. 4.5.2.7List
        8. 4.5.2.8Non-Null
      3. 4.5.3The __Field Type
      4. 4.5.4The __InputValue Type
      5. 4.5.5The __EnumValue Type
      6. 4.5.6The __Directive Type
  5. 5Validation
    1. 5.1Documents
      1. 5.1.1Executable Definitions
    2. 5.2Operations
      1. 5.2.1Named Operation Definitions
        1. 5.2.1.1Operation Name Uniqueness
      2. 5.2.2Anonymous Operation Definitions
        1. 5.2.2.1Lone Anonymous Operation
      3. 5.2.3Subscription Operation Definitions
        1. 5.2.3.1Single root field
    3. 5.3Fields
      1. 5.3.1Field Selections on Objects, Interfaces, and Unions Types
      2. 5.3.2Field Selection Merging
      3. 5.3.3Leaf Field Selections
    4. 5.4Arguments
      1. 5.4.1Argument Names
      2. 5.4.2Argument Uniqueness
        1. 5.4.2.1Required Arguments
    5. 5.5Fragments
      1. 5.5.1Fragment Declarations
        1. 5.5.1.1Fragment Name Uniqueness
        2. 5.5.1.2Fragment Spread Type Existence
        3. 5.5.1.3Fragments On Composite Types
        4. 5.5.1.4Fragments Must Be Used
      2. 5.5.2Fragment Spreads
        1. 5.5.2.1Fragment spread target defined
        2. 5.5.2.2Fragment spreads must not form cycles
        3. 5.5.2.3Fragment spread is possible
          1. 5.5.2.3.1Object Spreads In Object Scope
          2. 5.5.2.3.2Abstract Spreads in Object Scope
          3. 5.5.2.3.3Object Spreads In Abstract Scope
          4. 5.5.2.3.4Abstract Spreads in Abstract Scope
    6. 5.6Values
      1. 5.6.1Values of Correct Type
      2. 5.6.2Input Object Field Names
      3. 5.6.3Input Object Field Uniqueness
      4. 5.6.4Input Object Required Fields
    7. 5.7Directives
      1. 5.7.1Directives Are Defined
      2. 5.7.2Directives Are In Valid Locations
      3. 5.7.3Directives Are Unique Per Location
    8. 5.8Variables
      1. 5.8.1Variable Uniqueness
      2. 5.8.2Variables Are Input Types
      3. 5.8.3All Variable Uses Defined
      4. 5.8.4All Variables Used
      5. 5.8.5All Variable Usages are Allowed
  6. 6Execution
    1. 6.1Executing Requests
      1. 6.1.1Validating Requests
      2. 6.1.2Coercing Variable Values
    2. 6.2Executing Operations
      1. 6.2.1Query
      2. 6.2.2Mutation
      3. 6.2.3Subscription
        1. 6.2.3.1Source Stream
        2. 6.2.3.2Response Stream
        3. 6.2.3.3Unsubscribe
    3. 6.3Executing Selection Sets
      1. 6.3.1Normal and Serial Execution
      2. 6.3.2Field Collection
    4. 6.4Executing Fields
      1. 6.4.1Coercing Field Arguments
      2. 6.4.2Value Resolution
      3. 6.4.3Value Completion
      4. 6.4.4Errors and Non-Nullability
  7. 7Response
    1. 7.1Response Format
      1. 7.1.1Data
      2. 7.1.2Errors
    2. 7.2Serialization Format
      1. 7.2.1JSON Serialization
      2. 7.2.2Serialized Map Ordering
  8. AAppendix: Notation Conventions
    1. A.1Context-Free Grammar
    2. A.2Lexical and Syntactical Grammar
    3. A.3Grammar Notation
    4. A.4Grammar Semantics
    5. A.5Algorithms
  9. BAppendix: Grammar Summary
    1. B.1Ignored Tokens
    2. B.2Lexical Tokens
    3. B.3Document
  10. §Index