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SequenceFlow

Sequence flow behaviour.

Conditional flows

All outbound sequence flows can have conditions. Flows are evaluated in sequence. Default flow will be taken if no other flow was taken.

Example source:

<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <process id="theProcess" isExecutable="true">
    <task id="task1" default="to-task2" />
    <sequenceFlow id="to-task2" sourceRef="task1" targetRef="task2" />
    <sequenceFlow id="to-task3" sourceRef="task1" targetRef="task3" />
    <sequenceFlow id="to-task4" sourceRef="task1" targetRef="task4">
      <conditionExpression xsi:type="tFormalExpression" language="javascript">next(null, this.environment.variables.take4);</conditionExpression>
    </sequenceFlow>
    <sequenceFlow id="to-task5" sourceRef="task1" targetRef="task5">
      <conditionExpression xsi:type="tFormalExpression">${environment.variables.take5}</conditionExpression>
    </sequenceFlow>
    <task id="task2" />
    <task id="task3" />
    <task id="task4" />
    <task id="task5" />
  </process>
</definitions>`;

Sequence flows:

  • to-task2: default flow. If no other flow was taken then default flow is taken
  • to-task3: unconditional. Flow is taken
  • to-task4: script condition. Callback (next) is called with environment variable as result. If result is truthy the flow is taken, otherwise discarded
  • to-task5: expression condition. Expression will be evaluated and passed as result. If result is truthy the flow is taken, otherwise discarded

Service function conditions

An expression condition can resolve to a service function instead of a plain value. The function then produces the take/discard result, which makes per-flow state and asynchronous conditions possible. There are two forms, and they receive different arguments:

  • ${environment.services.takeOnce} (no call) — the expression resolves to the function itself and the sequence flow invokes it with the flow execution scope as the first argument; this is bound to the owning sequence flow. The scope exposes the flow id, the source activity content, and the environment. Return a value synchronously, or declare a second callback parameter to resolve asynchronously:

    // synchronous — return the result
function takeOnce(scope) {
  const taken = (scope.environment.variables.takenFlows ??= new Set());
  if (taken.has(scope.id)) return false; // scope.id is the sequence flow id
  taken.add(scope.id);
  return true;
}

// asynchronous — resolve through the callback
function isAllowed(scope, callback) {
  checkRemotely(scope.content, (err, ok) => callback(err, ok));
}

  • ${environment.services.takeFlow(content.id)} (called) — the expression calls the function and the flow uses the returned value. Arguments are resolved by the expression handler; note that empty parentheses takeFlow() pass the resolution context { environment, ...message } as the single argument (not zero arguments).

The flow id is only reachable through the no-call form, via scope.id. The called form receives the source activity in content (content.id is the activity, not the flow).

When no conditional flow is taken

If every conditional outbound flow evaluates to falsy, the behaviour depends on the element:

  • a diverging exclusive or inclusive gateway takes its default flow when declared, otherwise it raises an <id> no conditional flow taken error — these gateways require exactly one (exclusive) or at least one (inclusive) outbound to be taken;
  • any other activity (task, event, …) simply ends the branch, no error is raised.

The runtime no longer publishes outbound flow discards, so outbound flow discard counters stay at 0.