Serverless workflow provides the service integration feature to simplify the interaction between users and cloud services. In this topic, the Message Service (MNS) queues are used with callback to orchestrate tasks that do not involve functions in Function Compute.


Serverless workflow not only allows you to orchestrate functions that are deployed in Function Compute in Function as a Service (FaaS) mode into flows, but also allows you to orchestrate other computing tasks into flows. The topic Perform callbacks on asynchronous tasks under Best Practices describes how to use functions in Function Compute to send messages to MNS queues. In custom environments, after a task executor (worker) receives a message, it notifies Serverless workflow of the task execution result based on the callback. This topic describes how to use MNS queues, a new feature of Serverless workflow. MNS queues further simplify the orchestration of custom task types. Serverless workflow allows you to directly send messages to MNS queues. In this way, you do not need to develop, test, and maintain the function that is deployed in Function Compute for sending the messages, improving the availability and reducing the latency. Compared with sending messages to MNS topics by using functions in Function Compute, using the integrated MNS service to send messages to specified MNS queues has the following benefits:

  • You do not need to develop a function in Function Compute to send messages. This reduces the cost of development, testing, and maintenance.
  • The message delivery delay is reduced, a remote access process is eliminated, and the cold start of Function Compute is avoided.
  • Service dependency is removed and fault tolerance is improved.

Serverless workflow will support more cloud services in the future to make it easier to orchestrate flows that consist of different types of tasks.

Service integration

In the following figure, the three serial tasks are sent by Serverless workflow to the specified MNS queue in sequence. After the messages are sent, Serverless workflow waits for the callback in this step. You can call the ReceiveMessage operation of MNS to pull messages in the worker in a custom environment, such as an Elastic Compute Service (ECS) instance, a container, or a server in an on-premises data center. After the worker receives the messages, it executes the corresponding task based on the message content. After the task ends, the worker calls the ReportTaskSucceeded/Failed operation of Serverless workflow. Serverless workflow continues the step after receiving the task result. After the worker reports the success result, the message is deleted from the MNS queue.



Perform the following step to use this feature:

  1. Prepare for using this feature
  2. Define a flow
  3. Define a worker
  4. Execute the flow and view the result

Step 1: Prepare for using this feature

  1. In the MNS console, create an MNS queue. For more information, see Create a queue.
  2. Serverless workflow assumes the Create execution roles (the role of the RAM user) that you specify in the flow to send messages to the MNS queue in your Alibaba Cloud account. Therefore, you must add MNS SendMessage policies for the role of the RAM user. The following example shows a fine-grained policy. If you do not need the fine-grained policy, you can log on to the Serverless workflow console, and add AliyunMNSFullAccess in System Policy to Flow RAM Role.
    "Statement": [
            "Effect": "Allow",
            "Action": [
            "Resource": [
    "Version": "1"

Step 2: Define a flow

The following code in Flow Definition Language (FDL) defines a task step that can send messages to the MNS queue named fnf-demo and wait for the callback.

version: v1
type: flow
 - type: task
   name: Task_1
   resourceArn: acs:mns:::/queues/fnf-demo/messages  # This task step sends messages to the MNS queue fnf-demo that is under the same account in the same region.
   pattern: waitForCallback  # The task step suspends after the message is sent to the MNS queue and waits until it receives the callback.
      - target: task_token
        source: $context.task.token  # Serverless Workflow queries the task token from the context object.
      - target: key
        source: value
   serviceParams:  # The service integration parameters.
      MessageBody: $  # The mapped input is used as the body of the message you want to send.
      Priority: 1  # The priority of the MNS queue.         

Step 3: Define a worker

The following Python 2.7 code simulates a worker that executes a task. It can run in any environment that can access Serverless workflow and MNS. The worker calls the MNS ReceiveMessage operation for long polling. When it enters a task step with an MNS configuration, Serverless workflow sends a message to the fnf-demo queue. After the worker executes the task, it calls back the ReportTaskSucceeded/Failed operation of Serverless workflow. After Serverless workflow receives the task execution result, it continues the current task step. The worker deletes the message from the queue.

  1. In a virtual environment, install Serverless Workflow, MNS, and Python SDK.
    cd /tmp; mkdir -p fnf-demo-callback; cd fnf-demo-callback
    virtualenv env; source env/bin/activate
    pip install -t . aliyun-python-sdk-core -t . aliyun-python-sdk-fnf -t . aliyun-mns                              
  2. Compile the code for the local task executor
    import json
    import os
    from aliyunsdkcore.client import AcsClient
    from aliyunsdkcore.acs_exception.exceptions import ServerException
    from aliyunsdkcore.client import AcsClient
    from aliyunsdkfnf.request.v20190315 import ReportTaskSucceededRequest
    from mns.account import Account # pip install aliyun-mns
    from mns.queue import *
    def main():
      region = os.environ['REGION']
      account_id = os.environ['ACCOUNT_ID']
      ak_id = os.environ['AK_ID']
      ak_secret = os.environ['AK_SECRET']
      queue_name = "fnf-demo"
      fnf_client = AcsClient(
      mns_endpoint = "" % (account_id, region)
      my_account = Account(mns_endpoint, ak_id, ak_secret)
      my_queue = my_account.get_queue("fnf-demo")
      wait_seconds = 10
        while True:
            print "Receiving messages"
            recv_msg = my_queue.receive_message(wait_seconds)
            print "Received message %s, body %s" % (recv_msg.message_id, recv_msg.message_body)
            body = json.loads(recv_msg.message_body)
            task_token = body["task_token"]
            output = "{\"key\": \"value\"}"
            request = ReportTaskSucceededRequest.ReportTaskSucceededRequest()
            resp = fnf_client.do_action_with_exception(request)
            print "Report task succeeded finished"
            print "Deleted message " + recv_msg.message_id
          except MNSExceptionBase as e:
          except ServerException as e:
            if e.error_code == 'TaskAlreadyCompleted':
              print "Task already completed, deleted message " + recv_msg.message_id
      except ServerException as e:
    if __name__ == '__main__':
  3. Run the worker to long poll the fnf-demo queue. After the worker receives the message, it performs callback to report the result to Serverless workflow.
    # Run the worker process.
    export REGION={your-region}
    export ACCOUNT_ID={your-account-id}
    export AK_ID={your-ak-id}
    export AK_SECRET={your-ak-secret}

Step 4: Execute the flow and view the result

In the Serverless workflow console, execute the flow and run the worker. The result shows that the flow is successful.

Screen Shot 2019-09-17 at 7.51.12 PM