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BSER Binary Protocol

The basic JSON protocol in watchman allows quick and easy integration. Applications with higher performance requirements may want to consider the binary protocol instead.

The binary protocol is enabled by the client sending the byte sequence "\x00x\x01".


A PDU is prefixed by its length expressed as an encoded integer. This allows the peer to determine how much storage is required to read and decode it.


Arrays are indicated by a 0x00 byte value followed by an integer value to indicate how many items follow. Then each item is encoded one after the other.


Objects are indicated by a 0x01 byte value followed by an integer value to indicate the number of properties in the object. Then each key/value pair is encoded one after the other.


Strings are indicated by a 0x02 byte value followed by an integer value to indicate the number of bytes in the string, followed by the bytes of the string.


Unlike JSON, strings are not defined as having any particular encoding; they are transmitted as binary strings. This is because the underlying filesystem APIs don't define any particular encoding for names.

Exception: Keys in objects that are defined by watchman commands are always ASCII. In general, keys in objects are always UTF-8.

Rationale: Several programming languages like Python 3 expect all text to be in a particular encoding and make it inconvenient to pass in bytestrings or other encodings. Also, the primary purpose of not defining an encoding is that filenames don't always have one, and filenames are unlikely to show up as keys.


All integers are signed and transmitted in the host byte order of the system running the watchman daemon.

  • 0x03 indicates an int8_t. It is followed by the int8_t value.
  • 0x04 indicates an int16_t. It is followed by the int16_t value.
  • 0x05 indicates an int32_t. It is followed by the int32_t value.
  • 0x06 indicates an int64_t. It is followed by the int64_t value.


A real number is indicated by a 0x07 byte followed by 8 bytes of double value.


  • 0x08 indicates boolean true
  • 0x09 indicates boolean false


0x0a indicates the null value

Array of Templated Objects

0x0b indicates a compact array of objects follows. Some of the bigger datastructures returned by watchman are tabular data expressed as an array of objects. This serialization type factors out the repeated object keys into a header array listing the keys, followed by an array containing all the values of the objects.

To represent missing keys in templated arrays, the 0x0c encoding value may be present. If encountered it is interpreted as meaning that there is no value for the key that would have been decoded in this position. This is distinct from the null value.

For example:

{"name": "fred", "age": 20},
{"name": "pete", "age": 30},
{"age": 25 },

is represented similar to:

["name", "age"],
"fred", 20,
"pete", 30,
0x0c, 25

The precise sequence is:

0b          template
00 array -- start prop names
0302 int, 2 -- two prop names
02 string -- first prop "name"
0304 int, 4
6e616d65 "name"
02 string -- 2nd prop "age"
0303 int, 3
616765 "age"
0303 int, 3 -- there are 3 objects
02 string -- object 1, prop 1 name=fred
0304 int, 4
66726564 "fred"
0314 int 0x14 -- object 1, prop 2 age=20
02 string -- object 2, prop 1 name=pete
0304 int 4
70657465 "pete"
031e int, 0x1e -- object 2, prop 2 age=30
0c skip -- object 3, prop 1, not set
0319 int, 0x19 -- object 3, prop 2 age=25

Note: to avoid hostile "decompression bombs", Watchman will reject parsing template objects that have an empty set of keys.