Skip to main content

Non Accepting

Type: Constraint

Prevent a given set of scope items from receiving any new objects. For example, stop a host that is being drained from accepting more tasks while a rebalance runs.

Parameters​

ParameterTypeRequiredDefaultDescription
namestringYes-Descriptive name for logging/debugging
scopestringYes-Scope the listed scope items belong to (e.g. "host", "rack")
itemslist<string>Yes-The scope items that may not receive any new objects

A listed scope item may keep the objects already assigned to it, but no object that is not already there may move in.

Example​

An example use: pin a host so nothing new lands on it. There are 3 hosts and 2 tasks, both initially on host0. A NonAccepting constraint marks host1 as non-accepting. An AssignmentAffinities goal then expresses that task0 prefers host1 and task1 prefers host2. (source)

Initial assignment:

Initial assignment: task0 and task1 both on host0, host1 and host2 empty 
solver.setObjectName("task");
solver.setContainerName("host");

solver.setAssignment(std::map<std::string, std::vector<std::string>>{
    {"host0", {"task0", "task1"}},
    {"host1", {}},
    {"host2", {}},
});

// host1 may not accept any new objects.
NonAcceptingSpec nonAccepting;
nonAccepting.scope() = "host";
nonAccepting.items() = {"host1"};
solver.addConstraint(nonAccepting);

// task0 prefers host1, task1 prefers host2.
auto pref = [](std::string task, std::string host, double affinity) {
  AssignmentAffinity entry;
  entry.objectName() = std::move(task);
  entry.scopeItemName() = std::move(host);
  entry.affinity() = affinity;
  return entry;
};

AssignmentAffinitiesSpec affinities;
affinities.scope() = "host";
affinities.affinities() = {
    pref("task0", "host1", 1),
    pref("task1", "host2", 1),
};
solver.addGoal(affinities);

task1 moves to host2, which it prefers. task0 would prefer host1, but it cannot move there because that would violate the NonAccepting constraint, so it stays on host0.

Final assignment:

Final assignment: task1 moved to host2, task0 stayed on host0 because host1 is non-accepting

Constraint only​

NonAccepting can only be used as a constraint; there is no goal form. It only forbids new placements into the listed scope items, so it always holds in the initial assignment and, under the default constraint policy, Rebalancer is guaranteed to not move an object into a listed scope item.

Source​