Function Compute：Result callback

How it works

The following figure shows the process of result callback.

Scenarios

• Save discarded events for later use

  If an asynchronous request fails to be executed even after the system repeatedly tries to execute the request based on the retry policy, Function Compute discards the request. If a destination is configured for failed invocations, Function Compute pushes the context information of a failed request to a message service such as ApsaraMQ for RocketMQ for subsequent processing. You can also set another function as the destination service. Function Compute automatically pushes the context information of a 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. For example, you have configured Function Compute to automatically decompress ZIP files that are uploaded to Object Storage Service (OSS) and you want to receive notifications after a ZIP file is decompressed, you can configure the result callback feature for the decompression function.

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 Simple Message Queue (formerly MNS), Function Compute, or ApsaraMQ for RocketMQ is configured as the destination for asynchronous invocation.

  {
    "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 description

  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 to invoke 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 EventBridge is configured as the destination for asynchronous invocation. 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:

  • Simple Message Queue (formerly MNS): 64 KB

  • Function Compute: 128 KB

  • EventBridge: 64 KB

  • ApsaraMQ for 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 begin 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. In the left-side navigation pane, click Services & Functions.

2. In the top navigation bar, select a region. On the Services page, click the desired service.

3. On the Functions page, click the name of the function that you want to manage. On the function details page, click the Asynchronous Configurations tab.

4. On the Asynchronous Configurations tab, configure the parameters based on your business requirements.

   • 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 Invoke Other Services upon Success to Enable and configure the Destination Service parameter. The following table describes the Destination Service parameter.

        Parameter	Description
        Destination Service	Function Compute. If you select Function Compute for the Destination Service parameter, you must configure the following parameters: Service Name: the name of a service in Function Compute. Version or Alias: the alias or version of the specified service. Function Name: the name of a function in the service. Note Only event functions can be configured as destination services. HTTP functions cannot be configured as destination services in Function Compute.
        	Simple Message Queue (formerly MNS). If you select Simple Message Queue (formerly MNS) for the Destination Service parameter, 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. SMQ topics can be pushed as messages by using multiple methods. Queue: the name of the Simple Message Queue (formerly MNS) queue. This parameter is required when the Destination Type parameter is set to Queue. Theme: the name of the Simple Message Queue (formerly MNS) topic. This parameter is required when the Destination Type parameter is set to Topic.
        	ApsaraMQ for RocketMQ. If you select ApsaraMQ for RocketMQ for the Destination Service parameter, you must configure the following parameters: Instances: the ApsaraMQ for RocketMQ instance. Topic: the name of the ApsaraMQ for RocketMQ topic.
        	EventBridge. If you select EventBridge for the Destination Service parameter, 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, select Enable for the Invoke Other Services upon Failure parameter and configure the Destination Service parameter.

        The Destination Service configuration for failed invocations is similar to that for successful invocations. For more information, see the Configure the destination for successful invocations section of this topic.

     3. Click OK.

   After the function is triggered, you can view the following information from the configured destination service when 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 service 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 the metrics in the console. Log on to the Function Compute console. In the left-side navigation pane, choose Advanced Features > Monitoring. 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