Implementing Exclusive Choice with Amazon Simple Workflow Service - Amazon Simple Workflow Service
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Implementing Exclusive Choice with Amazon Simple Workflow Service

In some scenarios, you might want to schedule a different set of activities based on the outcome of a previous activity. The exclusive choice pattern enables you to create flexible workflows that meet the complex requirements of your application.

The Amazon Simple Workflow Service (Amazon SWF) doesn't have a specific exclusive choice action. To use exclusive choice, you simply write your decider logic to make different decisions based on the results of the previous activity. Some applications for exclusive choice include the following:

  • Performing cleanup activities if the results of a previous activity were unsuccessful

  • Scheduling different activities based on whether the customer purchased a basic or advanced plan

  • Performing different customer authentication activities based on the customer's ordering history

In the e-commerce example, you might use exclusive choice to either ship or cancel an order based on the outcome of charging the credit card. In the following figure, the decider schedules the Ship Order and Record Completion activity tasks if the credit card is successfully charged. Otherwise, it schedules the Cancel Order and Email Customer activity tasks.

Diagram of customer order workflow

The decider schedules the ShipOrder activity if the credit card is successfully charged. Otherwise, the decider schedules the CancelOrder activity.

In this case, program the decider to interpret the history and determine whether the credit card was successfully charged. To do this, you might have logic similar to the following

IF lastEvent = "WorkflowExecutionStarted"
	addToDecisions ScheduleActivityTask(ActivityType = "VerifyOrderActivity")

ELSIF lastEvent = "ActivityTaskCompleted"
	    AND ActivityType = "VerifyOrderActivity"
	addToDecisions ScheduleActivityTask(ActivityType = "ChargeCreditCardActivity")

#Successful Credit Card Charge Activities
ELSIF lastEvent = "ActivityTaskCompleted"
      AND ActivityType = "ChargeCreditCardActivity"
  addToDecisions ScheduleActivityTask(ActivityType = "ShipOrderActivity")

ELSIF lastEvent = "ActivityTaskCompleted"
	    AND ActivityType = "ShipOrderActivity"
  addToDecisions ScheduleActivityTask(ActivityType = "RecordOrderCompletionActivity")

ELSIF lastEvent = "ActivityTaskCompleted"
	    AND ActivityType = "RecordOrderCompletionActivity"
	addToDecisions CompleteWorkflowExecution

#Unsuccessful Credit Card Charge Activities
ELSIF lastEvent = "ActivityTaskFailed"
      AND ActivityType = "ChargeCreditCardActivity"
  addToDecisions ScheduleActivityTask(ActivityType = "CancelOrderActivity")

ELSIF lastEvent = "ActivityTaskCompleted"
	    AND ActivityType = "CancelOrderActivity"
	addToDecisions ScheduleActivityTask(ActivityType = "EmailCustomerActivity")

ELSIF lastEvent = "ActivityTaskCompleted"
	    AND ActivityType = "EmailCustomerActivity"
	addToDecisions CompleteWorkflowExecution

ENDIF

If the credit card was successfully charged, the decider should respond with RespondDecisionTaskCompleted to schedule the ShipOrder activity.

https://swf.us-east-1.amazonaws.com.cn
RespondDecisionTaskCompleted
{
  "taskToken": "12342e17-80f6-FAKE-TASK-TOKEN32f0223",
  "decisions":[
      {
          "decisionType":"ScheduleActivityTask",
          "scheduleActivityTaskDecisionAttributes":{
              "control":"OPTIONAL_DATA_FOR_DECIDER",
              "activityType":{
                  "name":"ShipOrder",
                  "version":"2.4"
              },
              "activityId":"3e2e6e55-e7c4-fee-deed-aa815722b7be",
              "scheduleToCloseTimeout":"3600",
              "taskList":{
                  "name":"SHIPPING"
              },
              "scheduleToStartTimeout":"600",
              "startToCloseTimeout":"3600",
              "heartbeatTimeout":"300",
              "input": "123 Main Street, Anytown, United States"
          }
      }
  ]
}

If the credit card was not successfully charged, the decider should respond with RespondDecisionTaskCompleted to schedule the CancelOrder activity.

https://swf.us-east-1.amazonaws.com.cn
RespondDecisionTaskCompleted
{
  "taskToken": "12342e17-80f6-FAKE-TASK-TOKEN32f0223",
  "decisions":[
      {
          "decisionType":"ScheduleActivityTask",
          "scheduleActivityTaskDecisionAttributes":{
              "control":"OPTIONAL_DATA_FOR_DECIDER",
              "activityType":{
                  "name":"CancelOrder",
                  "version":"2.4"
              },
              "activityId":"3e2e6e55-e7c4-fee-deed-aa815722b7be",
              "scheduleToCloseTimeout":"3600",
              "taskList":{
                  "name":"CANCELLATIONS"
              },
              "scheduleToStartTimeout":"600",
              "startToCloseTimeout":"3600",
              "heartbeatTimeout":"300",
              "input": "Out of Stock"
          }
      }
  ]
}

If Amazon SWF is able to validate the data in the RespondDecisionTaskCompleted action, Amazon SWF returns a successful HTTP response similar to the following.

HTTP/1.1 200 OK
Content-Length: 11
Content-Type: application/json
x-amzn-RequestId: 93cec6f7-0747-11e1-b533-79b402604df1