Handling Errors in Amazon SWF

There are a number of different types of errors that can occur during the course of a workflow execution.

Validation Errors

Validation errors occur when a request to Amazon SWF fails because it isn't properly formed or it contains invalid data. In this context, a request could be an action such as DescribeDomain or it could be a decision such as StartTimer. If the request is an action, Amazon SWF returns an error code in the response. Check this error code as it may provide information about what aspect of the request caused the failure. For example, one or more of the arguments passed with the request might be invalid. For a list of common error codes, go to the topic for the action in the Amazon Simple Workflow Service API Reference.

If the request that failed is a decision, an appropriate event will be listed in the workflow execution history. For example, if the StartTimer decision failed, you would see a StartTimerFailed event in the history. The decider should check for these events when it receives the history in response to PollForDecisionTask or GetWorkflowExecutionHistory. Below is a list of possible decision failure events that can occur when the decision isn't correctly formed or contains invalid data.

Errors in Enacting Actions or Decisions

Even if the request is properly formed, errors may occur when Amazon SWF attempts to carry out the request. In these cases, one of the following events in the history will indicate that an error occurred. Look at the reason field of the event to determine the cause of failure.

• CancelTimerFailed

• RequestCancelActivityTaskFailed

• RequestCancelExternalWorkflowExecutionFailed

• ScheduleActivityTaskFailed

• SignalExternalWorkflowExecutionFailed

• StartChildWorkflowExecutionFailed

• StartTimerFailed

Timeouts

Deciders, activity workers, and workflow executions all operate within the constraints of timeout periods. In this type of error, a task or a child workflow times out. An event will appear in the history that describes the timeout. The decider should handle this event by, for example, rescheduling the task or restarting the child workflow. For more information about timeouts, see Amazon SWF Timeout Types

• ActivityTaskTimedOut

• ChildWorkflowExecutionTimedOut

• DecisionTaskTimedOut

• WorkflowExecutionTimedOut

Errors raised by user code

Examples of this type of error condition are activity task failures and child-workflow failures. As with timeout errors, Amazon SWF adds an appropriate event to the workflow execution history. The decider should handle this event, possibly by rescheduling the task or restarting the child workflow.

• ActivityTaskFailed

• ChildWorkflowExecutionFailed

Errors related to closing a workflow execution

Deciders may also see the following events if they attempt to close a workflow that has a pending decision task.

• FailWorkflowExecutionFailed

• CompleteWorkFlowExecutionFailed

• ContinueAsNewWorkflowExecutionFailed

• CancelWorkflowExecutionFailed

For more information about any of the events listed above, see History Event in the Amazon SWF API Reference.