Azure Functions Event Hub Trigger

This document provides detailed information about configuring and using the Event Hub trigger for Azure Functions. The Event Hub trigger allows your Azure Function to be invoked automatically when new events are available in an Azure Event Hub.

Overview

The Event Hub trigger integrates seamlessly with Azure Event Hubs, a highly scalable data streaming platform and event ingestion service. When events are sent to an Event Hub, the trigger monitors specific consumer groups for new messages and executes your function code accordingly.

Key features include:

• Scalability: Handles massive volumes of events.
• Reliability: Ensures at-least-once processing with checkpoints.
• Partition Awareness: Processes events from partitions in parallel.
• Consumer Groups: Supports multiple applications reading from the same Event Hub concurrently without interfering with each other.

Configuration

The Event Hub trigger is configured using attributes in your function code or through the function.json file.

function.json Configuration

For a typical Event Hub trigger, your function.json will look like this:

{
  "scriptFile": "../run.py",
  "bindings": [
    {
      "name": "myEventHubTrigger",
      "type": "eventHubTrigger",
      "direction": "in",
      "eventHubName": "my-event-hub",
      "connection": "EventHubConnection",
      "consumerGroup": "$Default",
      "cardinality": "many"
    }
  ]
}

Attribute-based Configuration (C# Example)

In languages like C#, you can use attributes:

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

public static class EventHubTriggerFunction
{
    [FunctionName("EventHubTriggerCSharp")]
    public static void Run(
        [EventHubTrigger("my-event-hub", Connection = "EventHubConnection", ConsumerGroup = "$Default")] EventData[] events,
        ILogger log)
    {
        log.LogInformation($"C# Event Hub trigger function processed a batch of {events.Length} events.");

        foreach (var eventData in events)
        {
            log.LogInformation($"Event Body: {System.Text.Encoding.UTF8.GetString(eventData.Body.Array, eventData.Body.Offset, eventData.Body.Count)}");
        }
    }
}

public class EventData
{
    public byte[] Body { get; set; }
    // Other EventData properties like Properties, SequenceNumber, etc.
}

Key Configuration Properties:

• name: The name of the event parameter in your function code.
• type: Must be eventHubTrigger.
• direction: Must be in.
• eventHubName: The name of your Event Hub.
• connection: The name of the application setting that contains your Event Hub connection string.
• consumerGroup: The consumer group that your function will use to read from the Event Hub. The default is $Default.
• cardinality: Specifies how events are passed to your function.
  • one: A single event.
  • many: A batch of events (default).
  • all: All events available (can lead to very large batches).
• partitionId: (Optional) Specifies a specific partition to listen to.

Examples

Python Example

This example shows a Python function triggered by an Event Hub.

import logging
import azure.functions as func

def main(eventHubMessages: [func.EventHubEvent]):
    for message in eventHubMessages:
        logging.info('Python Event Hub trigger function received a message: %s',
                     message.get_body().decode('utf-8'))

JavaScript Example

This example demonstrates a JavaScript function triggered by an Event Hub.

module.exports = async function (context, events) {
    context.log(`JavaScript Event Hub trigger function processed a batch of ${events.length} events.`);

    for (const event of events) {
        context.log(`Event Body: ${event.body}`);
    }
};

Troubleshooting

Here are some common issues and how to resolve them:

Function Not Triggering

• Check Connection String: Ensure the application setting specified in the connection property is correctly configured and contains a valid Event Hub connection string.
• Event Hub and Consumer Group Names: Verify that the eventHubName and consumerGroup values in your configuration exactly match your Event Hub setup.
• Permissions: Make sure the identity your Function App is using has the necessary read permissions on the Event Hub.
• Event Hub Activity: Confirm that events are actually being sent to the specified Event Hub. You can check this in the Azure portal metrics for the Event Hub.

Checkpointing Issues

The Event Hub trigger uses checkpoints to track which events have been processed. If checkpoints are not managed correctly, you might see duplicate processing or missed events.

• Storage Account: For durable checkpointing, ensure a AzureWebJobsStorage connection string is configured for your Function App. This is often managed automatically in Azure.
• Consumer Group Conflicts: If multiple applications or functions are using the same consumer group on the same Event Hub, it can lead to unexpected behavior. Consider using dedicated consumer groups.

For more advanced scenarios and detailed configuration options, please refer to the official Azure Functions Event Hub documentation.