Sub-Process

An embedded sub-process is a collapsible container that holds a complete inner flow (start event, tasks, gateways, end event). The outer token enters the sub-process and waits until the inner flow completes, then continues along the outgoing sequence flow.

Sub-processes are useful for grouping related steps, applying boundary events to a whole block of work, or managing a scope of variables.

Properties

Scoping

Variables defined inside a sub-process are in the same scope as the parent instance — they are visible to the outer process and persist after the sub-process completes. There is no variable isolation between the sub-process and its parent.

Boundary Events on Sub-Processes

Boundary events can be attached to a sub-process just like a task. A Boundary Timer Event attached to a sub-process will fire if the entire inner flow takes too long, cancelling all active inner tokens (when interrupting).

XML Example

<bpmn:subProcess id="approval_flow" name="Approval Flow">
  <bpmn:incoming>flow_to_approval</bpmn:incoming>
  <bpmn:outgoing>flow_after_approval</bpmn:outgoing>

  <!-- Inner start event -->
  <bpmn:startEvent id="inner_start">
    <bpmn:outgoing>inner_flow_1</bpmn:outgoing>
  </bpmn:startEvent>

  <!-- Inner task -->
  <bpmn:userTask id="manager_review" name="Manager Review">
    <bpmn:incoming>inner_flow_1</bpmn:incoming>
    <bpmn:outgoing>inner_flow_2</bpmn:outgoing>
  </bpmn:userTask>

  <!-- Inner end event -->
  <bpmn:endEvent id="inner_end">
    <bpmn:incoming>inner_flow_2</bpmn:incoming>
  </bpmn:endEvent>

  <bpmn:sequenceFlow id="inner_flow_1" sourceRef="inner_start" targetRef="manager_review"/>
  <bpmn:sequenceFlow id="inner_flow_2" sourceRef="manager_review" targetRef="inner_end"/>
</bpmn:subProcess>

Nesting

Sub-processes can be nested — a sub-process can contain another sub-process. The engine resolves element references recursively.

Notes

• Each sub-process must contain exactly one start event (no timer/message variants) and at least one end event
• Parallel and inclusive gateways inside a sub-process work normally; all inner tokens must reach an end event before the outer token is released
• For independently-running child processes, use a separate process definition started via POST /api/v1/instances instead