Boundary Events

Boundary events are attached to tasks or subprocesses and trigger when a specific event occurs while that element is active.

Interrupting vs. non-interrupting

Mode	XML	Behavior
Interrupting	`cancelActivity="true"` (default)	Cancels the attached element’s token and continues down the boundary’s outgoing flow
Non-interrupting	`cancelActivity="false"`	Keeps the attached element’s token active and forks a new token down the boundary’s outgoing flow

Timer boundary event


<boundaryEvent id="escalateTimer" attachedToRef="processOrder" cancelActivity="true">
  <timerEventDefinition>
    <timeDuration>PT2H</timeDuration>
  </timerEventDefinition>
</boundaryEvent>

The timer is scheduled when the attached task becomes active. If the task completes before the timer fires, the timer is cancelled.

Message boundary event


<boundaryEvent id="cancelBoundary" attachedToRef="processOrder" cancelActivity="false">
  <messageEventDefinition messageRef="OrderCancelled" />
</boundaryEvent>

A subscription is created when the task becomes active and consumed when the message arrives. Non-interrupting: the original task continues alongside the new path.

Error boundary event


<boundaryEvent id="handleError" attachedToRef="callExternalService" cancelActivity="true">
  <errorEventDefinition errorRef="ServiceError" />
</boundaryEvent>
<error id="ServiceError" errorCode="SERVICE_UNAVAILABLE" />

Triggered when the attached task fails (retries = 0 from a worker) or when a child subprocess throws a matching error.

Error code matching priority:

Specific code match (errorCode equals the thrown code)
Catch-all (no errorCode specified)

Escalation boundary event

Same matching logic as error boundary events but for escalation codes thrown by escalation throw events or escalation end events inside subprocesses.


<boundaryEvent id="handleEscalation" attachedToRef="subprocess" cancelActivity="false">
  <escalationEventDefinition escalationRef="NeedsReview" />
</boundaryEvent>
<escalation id="NeedsReview" escalationCode="NEEDS_REVIEW" />

Signal boundary event


<boundaryEvent id="signalBoundary" attachedToRef="longRunningTask" cancelActivity="false">
  <signalEventDefinition signalRef="PauseSignal" />
</boundaryEvent>

Triggered when a signal broadcast matches the boundary’s signalRef. Useful for monitoring or side-channel notifications while the main task continues running.