Azure Functions Queue Output Bindings

Overview

Queue output bindings allow your Azure Function to write messages to an Azure Queue Storage queue. This is a common pattern for decoupling services, enabling asynchronous processing, and building event-driven architectures.

Supported Queue Types

Azure Functions supports output bindings for:

• Azure Queue Storage: The standard queue service for Azure.
• Azure Service Bus Queues: For more advanced messaging features like sessions, transactions, and duplicate detection.

Configuration

Output bindings are configured in your function's function.json file. The binding type is typically queue for Azure Queue Storage or serviceBus for Service Bus.

Azure Queue Storage Example (function.json)

{
  "scriptFile": "run.py",
  "bindings": [
    {
      "name": "msg",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue-items",
      "connection": "AzureWebJobsStorage"
    },
    {
      "name": "outputQueueItem",
      "type": "queue",
      "direction": "out",
      "queueName": "myoutputqueue",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
            

Azure Service Bus Queue Example (function.json)

{
  "scriptFile": "run.csx",
  "bindings": [
    {
      "name": "inputQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "input-queue",
      "connection": "ServiceBusConnectionString"
    },
    {
      "name": "outputServiceBusQueue",
      "type": "serviceBus",
      "direction": "out",
      "queueName": "processed-items-queue",
      "connection": "ServiceBusConnectionString"
    }
  ]
}
            

Using the Output Binding in Code

The way you interact with the output binding depends on your chosen language.

C# Example

In C#, the output binding is typically represented by an out parameter, often of type IAsyncCollector<T> or a specific SDK type like string.

using Microsoft.Azure.WebJobs;
using Microsoft.Extensions.Logging;

public static class QueueOutputBindingExample
{
    [FunctionName("QueueOutputTrigger")]
    public static void Run(
        [QueueTrigger("myqueue-items")] string myQueueItem,
        [Queue("myoutputqueue")] out string outputQueueItem,
        ILogger log)
    {
        log.LogInformation($"C# Queue trigger function processed: {myQueueItem}");
        outputQueueItem = $"Processed: {myQueueItem}";
        log.LogInformation($"Message sent to output queue: {outputQueueItem}");
    }
}
            

Python Example

In Python, the output binding is passed as an argument to your function and you use its .set() method.

import logging
import azure.functions as func

def main(msg: func.QueueMessage, outputQueueItem: func.Out[str]) -> None:
    logging.info('Python queue trigger function processed a message: %s',
                 msg.get_body().decode('utf-8'))

    message_body = msg.get_body().decode('utf-8')
    processed_message = f"Processed: {message_body}"
    outputQueueItem.set(processed_message)
    logging.info('Message sent to output queue: %s', processed_message)
            

JavaScript Example

In JavaScript, the output binding is available as a parameter and you assign to it.

module.exports = async function (context, myQueueItem) {
    context.log('JavaScript queue trigger function processed work item', myQueueItem);

    const processedMessage = `Processed: ${myQueueItem}`;
    context.bindings.outputQueueItem = processedMessage;
    context.log('Message sent to output queue:', processedMessage);
};
            

Common Scenarios

• Event Processing: A function is triggered by an event (e.g., blob creation), processes it, and then sends a message to a queue to notify other services.
• Data Transformation: Process incoming data, transform it, and place the transformed data onto an output queue for further processing.
• Workflow Orchestration: Use queues to pass tasks or states between different functions in a multi-step workflow.

Best Practices

• Use descriptive queue names.
• Ensure your connection strings are securely managed (e.g., using App Settings).
• Handle potential errors during message sending to the output queue.
• Consider using poison queue handling for input triggers if your output binding operations are critical.