Can a queue-triggered Azure function return a value?

An additional update:

As suggested by Konstantinos, I modified the function to return an HTTP response (adding a $return binding in function.json) and returning an HTTP response in the code:

return func.HttpResponse('some message', status_code=400)

This does not work. When the function is invoked, I get this error in the invocation log:

Unable to cast object of type 'System.String' to type 'Microsoft.AspNetCore.Http.HttpRequest'.

The same binding and return code do work in my HTTP-triggered function.

So my conclusion is that, for now, a queue-triggered function cannot have any $return binding.

@Amir Katz This is more of a design problem. The solution would depend on what your scenario requires when there is a failure.

Since you can't really "respond" back to the sender directly, you would have to record the status of the message processing in a separate system like a database or output a message into a different queue for further processing.

Your client application would get this status from this separate system on its own.

Hello @Amir Katz

Yes, you can use the $return binding in your function.json file to indicate whether the function has succeeded or failed.

To indicate success, you can set the $return binding to an empty object, like this:

{
  "bindings": [
    {
      "name": "myQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue",
      "connection": "AzureWebJobsStorage"
    },
    {
      "name": "$return",
      "type": "http",
      "direction": "out"
    }
  ]
}

To indicate failure, you can set the $return binding to an object that contains an is_exception property with a value of true, like this:

{
  "bindings": [
    {
      "name": "myQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue",
      "connection": "AzureWebJobsStorage"
    },
    {
      "name": "$return",
      "type": "http",
      "direction": "out"
    }
  ],
  "disabled": false,
  "scriptFile": "__init__.py",
  "entryPoint": "main",
  "bindings": {
    "myQueueItem": {
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue",
      "connection": "AzureWebJobsStorage"
    },
    "$return": {
      "type": "http",
      "direction": "out"
    }
  }
}

In your Python function code, you can set the return value based on whether the function succeeded or failed. For example, you can set the return value to an empty dictionary to indicate success, or to a dictionary containing an is_exception property set to true to indicate failure:

def main(myQueueItem):
    if invalid_input(myQueueItem):
        return {'is_exception': True}
    else:
        return {}

By setting the $return binding in your function.json file and returning the appropriate value in your Python function code, you can indicate whether the function has succeeded or failed and avoid unnecessary retries.

Kindly mark this answer as Accepted in case it helped or post your feedback !

Regards

Hello @Amir Katz

In the V1 programming model of Azure Functions in Python, you can use the $return binding to indicate to the system whether the function has succeeded or failed.

To return a value indicating success, you can use the $return binding as follows:

bash

{
    "bindings": [
        {
            "name": "queueItem",
            "type": "queueTrigger",
            "direction": "in",
            "queueName": "myqueue",
            "connection": "MyStorageAccountAppSetting"
        },
        {
            "name": "$return",
            "type": "queue",
            "direction": "out",
            "queueName": "myqueue",
            "connection": "MyStorageAccountAppSetting"
        }
    ]
}

In your Python code, you can then set the value of $return to a non-empty string to indicate success:

python

import logging

def main(queueItem, $return):
    try:
        # process queueItem
        # if input is invalid, raise an exception
        if is_invalid(queueItem):
            raise Exception("Invalid input")
            
        # process queueItem successfully
        $return = "success"
        
    except Exception as e:
        # handle exception
        logging.error("Error processing queueItem: {}".format(str(e)))
        $return = "failure"

When $return is set to a non-empty string, it indicates success, and the function execution will be marked as completed successfully.

If you set $return to an empty string or do not set it at all, it indicates failure, and the function execution will be retried.

Note that if the function is retried due to failure, the same input message will be retried. So if the input message contains invalid input, the function will continue to fail and be retried until the message is removed from the queue. To prevent this, you may want to remove the message from the queue when you encounter invalid input:

python

import logging
import azure.functions as func

def main(queueItem, $return):
    try:
        # process queueItem
        # if input is invalid, remove the message from the queue
        if is_invalid(queueItem):
            raise Exception("Invalid input")
            return
        
        # process queueItem successfully
        $return = "success"
        
    except Exception as e:
        # handle exception
        logging.error("Error processing queueItem: {}".format(str(e)))
        $return = "failure"
        return

    # remove the message from the queue
    func.Out[str] = queueItem['id']

By removing the message from the queue, you can prevent the function from being retried on the same input message.

Kindly mark this answer as Accepted in case it helped or post your feedback !

Regards