An ad hoc subprocess is a subprocess in which the contained activities do not have to be executed in a predefined order. Activities can be activated dynamically at runtime in any order and multiple times. An ad hoc subprocess is useful for modeling case-like scenarios where the flow of work is determined at runtime rather than at design time. In Fluxnova, optional sequence flows between activities are permitted to route work through specific activity sequences.

An ad hoc subprocess has one ordering mode:

• Parallel (default and only supported mode): Multiple activities within the subprocess may be active simultaneously. This allows any activity to be started at any time, regardless of whether other activities have already been started or completed.

To enforce sequential execution between activities, use optional sequence flows to connect them within the ad hoc subprocess.

An ad hoc subprocess completes when its completionCondition expression evaluates to true. If no completionCondition is specified, the subprocess completes when all activities have been completed at least once. The optional attribute cancelRemainingInstances (default: true) controls whether still-active activities are cancelled when the completion condition is met.

Constraints for an ad hoc subprocess:

• An ad hoc subprocess must not have start or end events (BPMN 2.0 standard).
• Boundary events on the ad hoc subprocess itself behave the same as for an embedded subprocess.
• Activities within an ad hoc subprocess may optionally be connected by sequence flows to define activity routing (Fluxnova extension).

An ad hoc subprocess is visualized as a subprocess with a tilde (~) marker at the bottom center:

When expanded, the internal activities are shown without connecting sequence flows:

An ad hoc subprocess is represented in XML using the adHocSubProcess element. The ordering attribute must be set to "Parallel" (Fluxnova does not support sequential ordering). The optional completionCondition child element defines the expression that, when evaluated as true, causes the subprocess to complete:

<bpmn:adHocSubProcess id="Activity_11nc5qc" 
                      ordering="Parallel"
                      cancelRemainingInstances="false"
                      camunda:asyncBefore="true">
  <bpmn:extensionElements>
    <fluxnova:properties>
      <fluxnova:property name="activeTasksCollection" value="Task_A,Task_B" />
    </fluxnova:properties>
  </bpmn:extensionElements>
  <bpmn:completionCondition xsi:type="bpmn:tFormalExpression">${approved==true}</bpmn:completionCondition>
  
  <bpmn:userTask id="Task_A" name="Task A" />
  <bpmn:userTask id="Task_B" name="Task B">
    <bpmn:extensionElements>
      <camunda:inputOutput>
        <camunda:inputParameter name="Input_1">value</camunda:inputParameter>
      </camunda:inputOutput>
    </bpmn:extensionElements>
  </bpmn:userTask>
  <bpmn:userTask id="Task_C" name="Task C" />
  
  <!-- Optional sequence flows for routing -->
  <bpmn:sequenceFlow id="Flow_1" sourceRef="Task_A" targetRef="Task_C" />
</bpmn:adHocSubProcess>

Fluxnova Extensions

Fluxnova Implementation Details

Core Components

• AdHocSubProcessActivityBehavior — Main behavior implementation
  • Evaluates completionCondition after each child activity completes
  • Auto-completes when no completion condition is set AND no active children remain
  • Re-evaluates condition periodically (on child completion, final completion)
  • If condition is false, subprocess stays open indefinitely
• TriggerAdHocActivitiesCmd & RuntimeService.triggerAdHocActivities() — Dynamic activity triggering
  • Allows activating starter activities after subprocess entry (not BPMN 2.0 standard)
  • Supports per-activity variable mapping
  • Validates activities are startable and not already active
• CompleteAdHocSubProcessCmd & RuntimeService.completeAdHocSubProcess() — Manual completion API
  • Bypasses completionCondition entirely (forced/manual override)
  • Supports optional variable setting at completion
  • Requires all inner activities to be idle

REST Endpoints

• POST /engine-rest/execution/{id}/ad-hoc-activities/trigger — trigger activities with per-activity variables
• POST /engine-rest/execution/{id}/ad-hoc-activities/complete — complete with optional variables

Fluxnova-Specific Deviations from BPMN 2.0

Key Behaviors

• Idle state possible — If no activeTasksCollection defined or is empty, subprocess enters idle state awaiting triggerAdHocActivities()
• Condition holds open — If completionCondition evaluates to false after any child finishes, subprocess remains open
• Manual override — The complete API is a forced exit mechanism independent of condition evaluation
• Active tasks collection — The activeTasksCollection property specifies which activities auto-start when the ad hoc subprocess enters. Multiple activities can be specified as a comma-separated list (e.g., Task_A,Task_B)
• Multi-instance support — Ad hoc subprocesses support multiInstanceLoopCharacteristics for creating multiple instances of the entire ad hoc subprocess

Additional Resources

• BPMN 2.0 Specification: Ad Hoc Sub-Process (Section 10.6)