CloudFlow:Asynchronous function invocation

The following operations are complete:

• Optional: Asynchronous invocation is enabled. For more information, see Asynchronous invocation.

  After you enable stateful asynchronous invocation for your functions, you can stop the function instance when the task is being executed. This helps you view the flow from a finer-grained perspective.

• Function Compute is granted the permissions to access Serverless Workflow. This ensures that Serverless Workflow can be called back to execute subsequent steps after the asynchronous function execution is complete. The following snippet shows how to configure the permission policy.

  {
      "Statement": [
          {
              "Effect": "Allow",
              "Action": [
                  "fnf:ReportTaskSucceeded",
                  "fnf:ReportTaskFailed"
              ],
              "Resource": [
                  "*"
              ]
          }
      ],
      "Version": "1"
  }

  For more information about how to grant permission to Function Compute, see Grant Function Compute permissions to access other Alibaba Cloud services.

By default, Serverless Workflow uses the synchronous pattern when Serverless Workflow orchestrates Function Compute tasks to implement task flows. In the synchronous pattern, Serverless Workflow does not proceed to the next step until the flow receives the execution results of the previous function. The following snippet shows a flow that uses the synchronous pattern.

version: v1
type: flow
steps:
  - type: task
    name: mytask
    resourceArn: acs:fc:{region}:{account}:services/{serviceName}.{qualifier}/functions/{functionName}

The following issues may occur when you use the synchronous pattern for function invocation:

• The resource limits to invocation of functions of Function Compute may cause throttling errors.

  By default, a maximum of 300 pay-as-you-go instances are allowed in a single region for an Alibaba Cloud account of Function Compute. When you use Serverless Workflow to invoke a function, the invocation shares the same quota with the function invocations that are initiated by other services. This may cause throttling errors. In this case, you need to define a complex retry policy in your Serverless Workflow flow. However, this does not ensure that the flow is executed successfully.

• If the task requires a long time to execute, a persistent connection must be established between Serverless Workflow and Function Compute. Unstable network conditions may cause unexpected errors.

Serverless Workflow allows you to combine the asynchronous function invocation feature with various integration patterns to solve the preceding issues. At the same time, the combination can meet the following requirements:

• In some scenarios, the system needs to perform subsequent steps without waiting for the execution completion of the current step.

• If unexpected errors occur when the flow is being executed, the system needs to skip the current step and proceed to the next step.

In different scenarios, Serverless Workflow uses the request-response pattern (requestResponse), synchronous pattern(sync), or wait-for-callback pattern (waitForCallback) to asynchronously invoke functions of Function Compute.

- type: task
    ...
    pattern: requestResponse 
    serviceParams:
        InvocationType: Async

- type: task
    ...
    pattern: sync 
    serviceParams:
        InvocationType: Async

- type: task
    ...
    pattern: waitForCallback
    serviceParams:
        InvocationType: Async

In the following examples, Serverless Workflow uses Function Compute as the task node, uses resourceArn to specify Function Compute as the destination service, and uses serviceParams to specify the parameters that are used to invoke functions of Function Compute. serviceParams supports the following parameters:

• InvocationType: the invocation type of functions. The valid values are Sync and Async. Sync indicates synchronous invocation. Async indicates asynchronous invocation.

• Optional:StatefulAsyncInvocationID: the ID of a stateful asynchronous invocation. You can use this parameter to search for the name of the destination task in the Function Compute console.

  Note

  If you set InvocationType to Async and specify a value for StatefulAsyncInvocationID, the function invocation pattern of Serverless Workflow is stateful and asynchronous invocation. The value of the StatefulAsyncInvocationID parameter must be unique for all the functions.

When a flow arrives at the specified step, Severless Workflow asynchronously invokes a function and executes the function. Then, the flow proceeds to the next step without waiting for a callback or the execution completion of the preceding step.

version: v1
type: flow
steps:
  - type: task
    name: mytask
    resourceArn: acs:fc:{region}:{account}:services/{serviceName}.{qualifier}/functions/{functionName}
    pattern: requestResponse  # Async invocation with sync pattern
    serviceParams:
        InvocationType: Async

Serverless Workflow combines the asynchronous function invocation feature and the stateful asynchronous invocation feature to provide supports for job-type scenarios. When you enable the stateful asynchronous invocation feature and use the asynchronous invocation pattern, you can view the function execution status and stop the function execution when and after the function is invoked. This makes the flow observable and easy to operate. The following snippet shows the flow definition language (FDL) of Serverless Workflow.

version: v1
type: flow
steps:
  - type: task
    name: mytask
    resourceArn: acs:fc:::services/{serviceName}.{qualifier}/functions/{functionName}
    pattern: sync  # Async invocation with sync pattern
    inputMappings:
      - target: id
        source: $context.execution.name
    serviceParams:
        InvocationType: Async
        StatefulAsyncInvocationID: $.id