Set Message Label/Subject using Azure Functions' Service Bus Output Binding

Question

I am sending messages to service bus using python azure functions with the new programming model which uses output bindings like below:

@bp.service_bus_queue_output(
    arg_name="outputQueueMessage",
    queue_name="local-solar-calculation-complete-queue",
    connection="ServiceBusConnectionString"
)   

I send the message using the .set() command:

                        outputQueueMessage.set(json.dumps(response, ensure_ascii=False))

However, this only allows me to set the message body, how do I customise to include other message properties like subject/label etc.? I need these for my downstream service bus message filters to work but haven't come across a way to set anything other than the message body using the new programming model.Im aware this would have been possible using the old programming model where you create a function.json and dont use decorators.

Note - here's an image of the properties in sb explorer which im trying to set using my function app -

Thanks

Answer 1

Hello Aamir Mogra,

Welcome to the Microsoft Q&A and thank you for posting your questions here.

I understand that you would like to set Message Label/Subject using Azure Functions' Service Bus Output Binding using your function app.

To avoid complexity issue while using ServiceBusClient and any issue that can lead to redundancy and potential mismanagement of resources. These are the things you can do:

1. If custom properties are not strictly required for downstream processing, consider encoding metadata within the message body itself. This approach is simple and leverages the existing output binding but with limit customization:

      import json
      import azure.functions as func
      @bp.service_bus_queue_output(
          arg_name="outputQueueMessage",
          queue_name="local-solar-calculation-complete-queue",
          connection="ServiceBusConnectionString"
      )
      def main(req: func.HttpRequest, outputQueueMessage: func.Out[str]) -> func.HttpResponse:
          response = {
              "data": {"key": "value"},
              "metadata": {"subject": "your_subject", "label": "your_label"}
          }
          outputQueueMessage.set(json.dumps(response, ensure_ascii=False))
          return func.HttpResponse("Message sent with metadata encoded in the body", status_code=200)
   

   This approach keeps the decorator-based model while embedding necessary metadata in the message body for downstream services to parse.
2. You can use manual sending for full customization, if custom properties like subject and label are indispensable, using ServiceBusClient for sending messages is the best alternative. This is an improved implementation with proper resource management and error handling:

      from azure.servicebus import ServiceBusMessage, ServiceBusClient
      import json
      import azure.functions as func
      CONNECTION_STR = "your_service_bus_connection_string"
      QUEUE_NAME = "local-solar-calculation-complete-queue"
      def main(req: func.HttpRequest) -> func.HttpResponse:
          response = {"key": "value"}  # Your message content
          message_body = json.dumps(response, ensure_ascii=False)
          try:
              with ServiceBusClient.from_connection_string(CONNECTION_STR) as client:
                  sender = client.get_queue_sender(queue_name=QUEUE_NAME)
                  message = ServiceBusMessage(
                      body=message_body,
                      application_properties={"subject": "your_subject", "label": "your_label"}
                  )
                  sender.send_messages(message)
              return func.HttpResponse("Message sent with custom properties", status_code=200)
          except Exception as e:
              return func.HttpResponse(f"Error sending message: {e}", status_code=500)
   

   This method explicitly sets custom properties on the ServiceBusMessage and ensures the connection is properly closed.
3. Finally, you can contact Azure Support if neither of the above solutions aligns with the your workflow. The feature requests can be submitted via the official Azure SDK GitHub repository - https://github.com/Azure/azure-sdk-for-python

For more reading and details, check out the following links:

• https://learn.microsoft.com/en-us/python/api/overview/azure/servicebus-readme?view=azure-python
• https://learn.microsoft.com/en-us/azure/azure-functions/functions-reference-python

I hope this is helpful! Do not hesitate to let me know if you have any other questions.

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