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# 使用 Amazon SWF 实现独家选择
<a name="swf-dg-exclusive-choice"></a>

在某些应用场景中，建议您根据前一个活动的结果排定不同组活动。使用独家选择模式，您可以创建灵活的工作流程，以满足应用程序的复杂要求。

Amazon SWF 没有特定的独家选择行动。要实现排他性选择，您必须编写决策者逻辑，以便根据先前活动的结果做出决策。以下为适用于排他选择的某些应用程序：
+ 如果前一个活动的结果不成功，执行清除活动
+ 根据客户购买的是基本还是高级计划来排定不同的活动
+ 根据客户订单历史执行不同的客户身份验证活动

在电子商务示例中，您可以使用排他选择根据信用卡付费结果给订单发货或取消订单。在下图中，如果信用卡付费成功，决策程序将排定 Ship Order 和 Record Completion 活动任务。否则，它将排定 Cancel Order 和 Email Customer 活动任务。

![客户订单工作流程图](http://docs.aws.amazon.com/zh_cn/amazonswf/latest/developerguide/images/swf-dg-customer-order-workflow.png)


如果信用卡成功付费，决策程序将排定 `ShipOrder` 活动。否则，决策程序将排定 `CancelOrder` 活动。

在这种情况下，给决策程序编程以解析历史记录并决定信用卡是否付费成功。要实现此操作，您要有与下列内容相类似的逻辑

```
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
```

如果信用卡付费成功，决策程序应响应 `RespondDecisionTaskCompleted` 以排定 `ShipOrder` 活动。

```
https://swf.us-east-1.amazonaws.com
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"
          }
      }
  ]
}
```

如果信用卡未付费成功，决策程序应响应 `RespondDecisionTaskCompleted` 以排定 `CancelOrder` 活动。

```
https://swf.us-east-1.amazonaws.com
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"
          }
      }
  ]
}
```

如果 Amazon SWF 能够验证 `RespondDecisionTaskCompleted` 操作中的数据，Amazon SWF 将返回与下面类似的成功 HTTP 响应。

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