Amazon Simple Workflow Service Activity Task Cancellation

Activity task cancellation enables the decider to end activities that no longer need to be performed. Amazon SWF uses a cooperative cancellation mechanism and doesn't forcibly interrupt running activity tasks. You must program your activity workers to handle cancellation requests.

The decider can decide to cancel an activity task while it is processing a decision task. To cancel an activity task, the decider uses the RespondDecisionTaskCompleted action with the RequestCancelActivityTask decision.

If the activity task has not yet been acquired by an activity worker, the service will cancel the task. Note that there is a potential race condition in that an activity worker could acquire the task at any time. If the task has already been assigned to an activity worker, then the activity worker will be requested to cancel the task.

In this example, the workflow execution is sent a signal to cancel the order.

https://swf.us-east-1.amazonaws.com
SignalWorkflowExecution
{"domain": "867530901",
 "workflowId": "20110927-T-1",
 "runId": "9ba33198-4b18-4792-9c15-7181fb3a8852",
 "signalName": "CancelOrder",
 "input": "order 3553"}

If the workflow execution receives the signal, Amazon SWF returns a successful HTTP response similar to the following. Amazon SWF will generate a decision task to inform the decider to process the signal.

HTTP/1.1 200 OK
Content-Length: 0
Content-Type: application/json
x-amzn-RequestId: 6c0373ce-074c-11e1-9083-8318c48dee96

When the decider processes the decision task and sees the signal in the history, the decider attempts to cancel the outstanding activity that has the ShipOrderActivity0001 activity ID. The activity ID is provided in the workflow history from the schedule activity task event.

https://swf.us-east-1.amazonaws.com
RespondDecisionTaskCompleted
{
  "taskToken":"12342e17-80f6-FAKE-TASK-TOKEN32f0223",
  "decisions":[{
          "decisionType":"RequestCancelActivityTask",
          "RequestCancelActivityTaskDecisionAttributes":{
              "ActivityID":"ShipOrderActivity0001"
          }
      }
  ]
}

If Amazon SWF successfully receives the cancellation request, it returns a successful HTTP response similar to the following:

HTTP/1.1 200 OK
Content-Length: 0
Content-Type: application/json
x-amzn-RequestId: 6c0373ce-074c-11e1-9083-8318c48dee96

The cancellation attempt is recorded in the history as the ActivityTaskCancelRequested event.

If the task is successfully canceled—as indicated by an ActivityTaskCanceled event—program your decider to take the appropriate steps that should follow task cancellation such as closing the workflow execution.

If the activity task could not be canceled—for example, if the task completes, fails, or times out instead of canceling—your decider should accept the results of the activity or perform any cleanup or mitigation necessitated by your use case.

If the activity task has already been acquired by an activity worker, then the request to cancel is transmitted through the task-heartbeat mechanism. Activity workers can periodically use RecordActivityTaskHeartbeat to report to Amazon SWF that the task is still in progress.

Note that activity workers are not required to heartbeat, although it is recommended for long-running tasks. Task cancellation requires periodic heartbeat to be recorded; if the worker doesn't heartbeat, the task can't be canceled.

If the decider requests a cancellation of the task, Amazon SWF sets the value of the cancelRequest object to true. The cancelRequest object is part of the ActivityTaskStatus object which is returned by the service in response to RecordActivityTaskHeartbeat.

Amazon SWF doesn't prevent the successful completion of an activity task whose cancellation has been requested; it is up to the activity to determine how to handle the cancellation request. Depending on your requirements, program the activity worker to either cancel the activity task or ignore the cancellation request.

If you want the activity worker to indicate that the work for the activity task was canceled, program it to respond with a RespondActivityTaskCanceled. If you want the activity worker to complete the task, program it to respond with a standard RespondActivityTaskCompleted.

When Amazon SWF receives the RespondActivityTaskCompleted or RespondActivityTaskCanceled request, it updates the workflow execution history and schedules a decision task to inform the decider.

Program the decider to process the decision task and return any additional decisions. If the activity task is successfully canceled, program the decider to perform the tasks needed to continue or close the workflow execution. If the activity task isn't successfully canceled, program the decider to accept the results, ignore the results, or schedule any required cleanup.