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Event-based gateway

An Event-based gateway routes the flow based on which event occurs first. Instead of evaluating data conditions, it subscribes to all the catch events on its outgoing paths and takes the path of the winner — the other branches are cancelled.

Event-based gateway

Rendered as a diamond with a pentagon-in-double-circle marker.

Use cases​

  • Reply or timeout — after sending a quote, wait for the customer's reply message, but escalate if a 48-hour timer fires first.
  • Race competing messages — continue differently depending on whether a payment confirmation or a cancellation arrives first.
  • Periodic re-check while waiting — combine a message catch with a timer to poll or remind until the awaited message arrives.

Usage in BPMN​

The gateway itself carries no configuration; its outgoing sequence flows must each lead directly to an intermediate catch event — in ZenBPM a message catch event or a timer catch event. No conditions, and no other element types (receive tasks or other events after the gateway are not supported).

Execution flow:

  1. A token arrives at the gateway and the engine subscribes to all target events at once — message subscriptions are created and timers armed, while the token waits at the gateway.
  2. The first event to occur wins: for a message, its payload is propagated according to the catch event's output mappings (see Variables).
  3. All other subscriptions are terminated and timers cancelled — the losing branches can no longer trigger.
  4. The token continues along the outgoing flow of the winning catch event, not of the gateway.
  • Message events — message definitions, subscriptions, and correlation keys for the message branches.
  • Timer events — timer formats for the timeout branches.
  • Exclusive gateway — branching on data instead of on events.

XML example​

After sending a quote, the process waits for the customer's reply; if none arrives within 48 hours, the timer branch escalates:

<bpmn:eventBasedGateway id="Gateway_ReplyOrTimeout" name="Reply or timeout?">
  <bpmn:incoming>Flow_In</bpmn:incoming>
  <bpmn:outgoing>Flow_Reply</bpmn:outgoing>
  <bpmn:outgoing>Flow_Timeout</bpmn:outgoing>
</bpmn:eventBasedGateway>

<bpmn:intermediateCatchEvent id="Catch_Reply" name="Reply received">
  <bpmn:incoming>Flow_Reply</bpmn:incoming>
  <bpmn:outgoing>Flow_ProcessReply</bpmn:outgoing>
  <bpmn:messageEventDefinition messageRef="Message_QuoteReply" />
</bpmn:intermediateCatchEvent>

<bpmn:intermediateCatchEvent id="Catch_Timeout" name="48h passed">
  <bpmn:incoming>Flow_Timeout</bpmn:incoming>
  <bpmn:outgoing>Flow_Escalate</bpmn:outgoing>
  <bpmn:timerEventDefinition>
    <bpmn:timeDuration>PT48H</bpmn:timeDuration>
  </bpmn:timerEventDefinition>
</bpmn:intermediateCatchEvent>

<bpmn:message id="Message_QuoteReply" name="QuoteReply">
  <bpmn:extensionElements>
    <zenbpm:subscription correlationKey="=quote.id" />
  </bpmn:extensionElements>
</bpmn:message>