Function Compute:Result callback

How it works

The following figure shows the process of result callback.

Scenario

• Save discarded events for later use

  If an asynchronous request fails to be executed even after the system repeatedly tries to execute it based on the retry policy, Function Compute discards the request. If the destination for failed invocations is configured, Function Compute pushes the context information of the failed request to a message service such as Message Queue for Apache RocketMQ for subsequent processing. You can also set the destination service to another function. Function Compute automatically pushes the context information of the failed request to this function to execute the custom error handling logic.

• Notify downstream services of execution results

  After a request is successfully executed, Function Compute pushes the context information of the request to the downstream destination service.

Supported destination services for asynchronous invocations

Take note of the following items when you configure the destination service for asynchronous invocations:

• Event content sent to the destination for the asynchronous invocation

  The following sample code provides an example of event content when the destination for the asynchronous invocation is MNS, Function Compute, or Message Queue for Apache RocketMQ.

  {
      "timestamp": 1660120276975,
      "requestContext": {
          "requestId": "xxx",
          "functionArn": "acs:fc:::services/{serviceName}/functions/{functionName}",
          "condition": "FunctionResourceExhausted", 
          "approximateInvokeCount": 3
      },
      "requestPayload": "",
      "responseContext": {
          "statusCode": 200,
          "functionError": ""
      },
      "responsePayload": ""
  }

  Table 1. Parameter descriptions

  Parameter	Description
  timestamp	The timestamp of the invocation.
  requestContext	The context of the request.
  requestContext.requestId	The request ID of the asynchronous invocation.
  requestContext.functionArn	The Alibaba Cloud Resource Name (ARN) of the function that is asynchronously invoked.
  requestContext.condition	The error code of the invocation.
  requestContext.approximateInvokeCount	The number of execution times of the asynchronous invocation. A value greater than 1 indicates that Function Compute has retried your function.
  requestPayload	The original payload of the function request.
  responseContext	The context of the response.
  responseContext.statusCode	The status code that is returned by the system for the invoked function. A status code other than 200 indicates that a system error occurred.
  responseContext.functionError	The error message of the invocation.
  responsePayload	The original payload that is returned after the function is executed.

  The following sample code provides an example of event content when the destination for the asynchronous invocation is EventBridge. For more information, see Overview.

  {
      "datacontenttype": "application/json",
      "aliyunaccountid": "143xxxx",
      "data": {
          "requestContext": {
              "condition": "",
              "approximateInvokeCount": 1,
              "requestId": "0fcb7f0c-xxxx",
              "functionArn": "acs:fc:::services/xxxx.LATEST/functions/xxxx"
          },
          "requestPayload": "",
          "responsePayload": "",
          "responseContext": {
              "functionError": "",
              "statusCode": 200
          },
          "timestamp": 1660120276975
      },
      "subject": "acs:fc:::services/xxxx.LATEST/functions/xxxx",
      "source": "acs:fc",
      "type": "fc:AsyncInvoke:succeeded",
      "aliyunpublishtime": "2021-01-03T09:44:31.233Asia/Shanghai",
      "specversion": "1.0",
      "aliyuneventbusname": "xxxxxxx",
      "id": "ecc4865xxxxxx",
      "time": "2021-01-03T01:44:31Z",
      "aliyunregionid": "cn-shanghai-vpc",
      "aliyunpublishaddr": "199.99.xxx.xxx"
  }

• Payload limits

  The payload that is supported by a destination for the asynchronous invocation cannot exceed the following limits:

  • MNS: 64 KB
  • Function Compute: 128 KB
  • EventBridge: 64 KB
  • Message Queue for Apache RocketMQ: 4 MB
• Loop avoidance

  When you configure a destination for the asynchronous invocation, make sure that you do not create a loop. For example, you configure Function B as the destination for successful asynchronous invocations of Function A and Function A as the destination for successful asynchronous invocations of Function B. An asynchronous invocation and execution of Function A may start an infinite loop in which Function A and Function B continuously invoke each other.

Configure a destination for an asynchronous invocation

1. Log on to the Function Compute console.
2. In the left-side navigation pane, click Services & Functions.
3. In the top navigation bar, select a region.
4. On the Services page, click the desired service.
5. On the Functions page, click the function that you want to modify.
6. On the function details page, click the Asynchronous Mode Configurations tab.
7. On the Asynchronous Mode Configurations tab, configure the parameters.
   • Configure the destination for successful invocations
     1. In the Destination for Successful Invocation section, click Modify.
     2. In the Modify Destination for Successful Invocation panel, set the Invoke Other Services upon Success parameter to Enable and then set the Destination Service parameter. The following table describes the parameters.

        Parameter	Description
        Invoke Other Services upon Success	If you set this parameter to Enable, the result of the successful invocation of the function is sent to the destination service.
        Destination Service	Function Compute. If you select Function Compute, you must configure the following parameters: Service Name: the name of the specified service. Version or Alias: the alias or version of the specified service. Function Name: the name of the specified function. Note Only event functions can be configured as destination services. HTTP functions cannot be configured as destination services.
        	MNS. If you select MNS, you must configure the following parameters: Destination Type: the type of the destination that receives the results of successful invocations. Valid values: Queue A queue-based messaging model provides highly reliable and concurrent message consumption services. Each message in a queue can be consumed by only one client. Topic A topic-based messaging model is used to send messages from one publisher client to multiple subscriber clients. MNS topics can be pushed as messages by using multiple methods. Queue: the name of the MNS queue. This parameter is required when the Destination Type parameter is set to Queue. Topic: the name of the MNS topic. This parameter is required when the Destination Type parameter is set to Topic.
        	If you select Message Queue for Apache RocketMQ, you must set the following parameters:Message Queue for Apache RocketMQ Instances: the destination instance. Topic: the destination topic.
        	EventBridge. If you select EventBridge, you must configure the Custom Event Bus parameter.

     3. Click OK.
   • Configure the destination for failed invocations
     1. In the Destination for Failed Invocation section, click Modify.
     2. In the Modify Destination for Failed Invocation panel, set the Invoke Other Services upon Failure parameter to Enable and then set the Destination Service parameter.

        For more information about how to configure the required parameters, see the description of configuring the destination to receive the results of successful invocations.

     3. Click OK.
   After the function is triggered, you can view the following information from the configured destination service after the function execution succeeds or fails:

   {
       "timestamp": 1660120276975,
       "requestContext": {
           "requestId": "xxx",
           "functionArn": "acs:fc:::services/{serviceName}/functions/{functionName}",
           "condition": "FunctionResourceExhausted",
           "approximateInvokeCount": 3
       },
       "requestPayload": "",
       "responseContext": {
           "statusCode": 200,
           "functionError": ""
       },
       "responsePayload": ""
   }

Handling of callback failures

If the service role does not have the permissions on the destination services or the destination service is unavailable, the callback to the destination service may fail. You can handle the errors based on the metrics and logs provided by Function Compute. The following table describes the common errors and corresponding system actions.

Result callback metrics

When a callback fails, Function Compute records the corresponding metrics and displays them in the console. Log on to the Function Compute console. In the left-side navigation pane, choose Advanced Features > Monitoring Dashboard. On the page that appears, click the name of the service whose metrics you want to view in the Service Name column. The following table describes the metrics for the invocation of destination services.

For more information about metrics, see Monitoring metrics.

References