Conception for Correlation Message

Abstract Concept

sequenceDiagram
    User->>+CamundaEngine: Start Process
    CamundaEngine->>+PythonPod: Subscribe to Activity Message Topic
    PythonPod->>+CamundaEngine: Complete Activity 
    CamundaEngine->>-CamundaEngine: Runs Process
    PythonPod->>+CamundaEngine: Notify async Message Correlation Message
    CamundaEngine->>-CamundaEngine: complete process

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Your engine's workers are currently capable of executing specific tasks. To mitigate potential timeouts caused by unexpectedly long-running tasks, you can leverage the message correlation feature. This mechanism suspends the process until a notification is received, preventing bottlenecks.

The Camunda Engine REST API requires the following dependency for activationin Spring Boot Rest API Depdency

Follwing Example is feasable to write a message to the camunda server: Documentation Message Correlation Rest Api:

Your request JSON for the correclation feature must include at least the following fields:

{
  "messageName": "aMessage",
  "businessKey": "aBusinessKey",
}

the messageName should be the name of your camunda Message Intermediate Catch Event as in our example here

The business key is a unique identifier that you will generate in the UI for your task.

for example:

To listen to the task in Camunda please use following references create external task lib

from camunda.external_task.external_task import ExternalTask, TaskResult
from camunda.external_task.external_task_worker import ExternalTaskWorker

To subscribe to the task, use following snipped:

ExternalTaskWorker(worker_id="1", config=default_config).subscribe("topic", handle_task)

the topic should be the name, you assign in your activity. for our example we use messageSending

The callback function which handle the task can looked like this:

def handle_task(task: ExternalTask) -> TaskResult:
    key = task.get_business_key()
    print("create task event")
    return task.complete({"var1": 1, "var2": "value"})

This handler will listen to the external task api, if the process is running. if a process runs this will intercept and will go further. if you need more logic, please put in in here.

This event will then stuck in the MessageEventListener, until it gets a notification, that it can run further. To archive it, you can use following snipped:

def sendTaskResponse(businessKey):

    print("using businesskey", businessKey)
    time.sleep(2)
    url = "http://localhost:8080/engine-rest/message"
    data = {"businessKey": businessKey, "messageName": "sample_response"}
    print("used url is " ,url)
    print("used json data is ", data)
    response = requests.post(url, headers={'Content-Type': 'application/json'}, json=data)

    if response.status_code == 204:
        print("Request successful")
        print(response)
    else:
        print("Request failed")
        print(response)

Normally, the Camunda REST API endpoint for sending messages is located at /engine-rest/message. The server expects a POST request with appropriate parameters. As mentioned earlier, you'll require both the businessKey and the correct messageName for this operation.

To execute it in our script asynchronous to you can following adition to the handle_task:

def handle_task(task: ExternalTask) -> TaskResult:
    key = task.get_business_key()

    print("business key from task event", key)
    # create a seperated thread for sendTaskResponse in
    e = threading.Event()
    t1 = threading.Thread(target=sendTaskResponse, args=[key])
    t1.start()
    print("create task event")

    return task.complete({"var1": 1, "var2": "value"})