Azure Functions: Event Hub Output Bindings

Introduction
Configuration
Usage
Examples
Best Practices

Introduction to Event Hub Output Bindings

Azure Functions provide a powerful way to integrate with Azure Event Hubs through output bindings. This allows your functions to easily send messages to an Event Hub without needing to write explicit Event Hub client code. This is particularly useful for scenarios where your function processes data and needs to publish events for downstream consumption.

Output bindings simplify the process of sending data to Event Hubs by abstracting away the complexities of connection management, serialization, and batching. You define the output binding in your function's configuration, and the Azure Functions runtime handles the rest.

Configuring the Event Hub Output Binding

The Event Hub output binding is configured in the function.json file for your function. You define an output binding with the type eventHub.

`function.json` Example

                
{
  "scriptFile": "run.csx",
  "bindings": [
    {
      "name": "inputQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue-items",
      "connection": "AzureWebJobsStorage"
    },
    {
      "name": "outputEventHub",
      "type": "eventHub",
      "direction": "out",
      "eventHubName": "myeventhub",
      "connection": "EventHubConnectionString"
    }
  ]
}
                
            

Key Properties

name: The name of the output binding that you will use in your function code.
type: Must be eventHub for Event Hub output bindings.
direction: Must be out.
eventHubName: The name of the Event Hub to which messages will be sent.
connection: The name of the App Setting that contains the connection string for your Event Hub. This could be a direct Event Hub connection string or a connection string to an Azure Event Hubs namespace.

Ensure the connection string specified in the connection property points to a valid Azure Event Hubs namespace or a specific Event Hub. Using the AzureWebJobsStorage for Event Hub connections is not recommended unless it's explicitly configured for Event Hubs.

Using the Event Hub Output Binding in Code

Once configured, you can use the output binding by referencing its name in your function code. The Azure Functions runtime injects an object that allows you to send messages to the configured Event Hub.

C# Example (.NET 6+ In-Process Model)

                
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;

namespace MyFunctionApp
{
    public static class QueueToEventHub
    {
        [Function("QueueToEventHub")]
        [EventHubOutput("myeventhub", Connection = "EventHubConnectionString")]
        public static string Run([QueueTrigger("myqueue-items", Connection = "AzureWebJobsStorage")] string myQueueItem, FunctionContext context)
        {
            var logger = context.GetLogger();
            logger.LogInformation($"C# Queue trigger function processed: {myQueueItem}");

            // Return the message to be sent to Event Hub
            return myQueueItem;
        }
    }
}
                
            

JavaScript Example (Node.js)

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

    // The myQueueItem is automatically passed to the output binding
    // if it's returned from the function.
    context.bindings.outputEventHub = myQueueItem;
};
                
            

In C# with the in-process model, returning a string from the function decorated with [EventHubOutput] automatically sends that string as a message to the Event Hub. For Node.js, you assign the message to the output binding property (context.bindings.outputEventHub in this case).

Sending Complex Data

You can send complex data types like JSON objects. The Functions runtime will typically serialize them to JSON strings before sending.

                
// Example in C#
public class MyEventData
{
    public string Id { get; set; }
    public string Message { get; set; }
    public DateTime Timestamp { get; set; }
}

[Function("ProcessAndSendEvent")]
[EventHubOutput("myeventhub", Connection = "EventHubConnectionString")]
public static MyEventData Run([QueueTrigger("my-data-queue")] string queueMessage, FunctionContext context)
{
    // Deserialize or create your complex data object
    var eventData = new MyEventData
    {
        Id = Guid.NewGuid().ToString(),
        Message = $"Processed: {queueMessage}",
        Timestamp = DateTime.UtcNow
    };
    return eventData;
}
                
            

Practical Examples

1. Processing IoT Data and Sending to Event Hub

Imagine a scenario where you receive sensor data in a queue, process it (e.g., validate, enrich), and then send the processed data to an Event Hub for real-time analytics.

                
// Assuming a C# function processing a string from a queue
// and sending a structured object to Event Hub
public class SensorReading
{
    public string DeviceId { get; set; }
    public double Temperature { get; set; }
    public double Humidity { get; set; }
    public DateTime ReadTime { get; set; }
}

[Function("ProcessSensorData")]
[EventHubOutput("sensor-data-hub", Connection = "EventHubConnection")]
public static SensorReading Run([QueueTrigger("sensor-readings-queue")] string rawData, FunctionContext context)
{
    var logger = context.GetLogger();
    logger.LogInformation($"Processing raw sensor data: {rawData}");

    // In a real-world scenario, you'd parse rawData into a SensorReading object.
    // For simplicity, let's assume it's already somewhat structured or mocked.
    var sensorReading = new SensorReading
    {
        DeviceId = "DEV-123",
        Temperature = 25.5,
        Humidity = 60.2,
        ReadTime = DateTime.UtcNow
    };

    return sensorReading; // This object will be serialized and sent to Event Hub
}
                
            

2. Aggregating Log Messages

A function could collect log messages from various sources and periodically send aggregated batches to an Event Hub for centralized logging.

Best Practices and Considerations

Connection Management: Use Azure App Settings for your connection strings. Avoid hardcoding credentials.
Event Hub Name: Ensure the eventHubName in your function.json exactly matches the target Event Hub.
Data Format: Decide on a consistent data format (e.g., JSON) for messages sent to Event Hub.
Error Handling: Implement robust error handling within your function to manage potential issues during message processing or sending. The Functions runtime might retry, but explicit handling is good practice.
Batching: Azure Functions and Event Hubs handle some level of batching automatically. For high-throughput scenarios, monitor performance and consider optimizations if needed, though direct output bindings abstract much of this.
Serialization: Be mindful of how your data is serialized. For complex objects, ensure the runtime can serialize them correctly.
Permissions: The Managed Identity or Service Principal used by your Azure Function App must have the necessary `Send` permissions on the target Event Hub namespace or Event Hub.