Azure Documentation

Event Processors

Event processors are a crucial component for consuming events from Azure Event Hubs. They provide a scalable and fault-tolerant way to read and process events from one or more partitions within an Event Hub.

What are Event Processors?

An event processor is an abstraction that simplifies reading events from Event Hubs. It handles the complexities of connecting to Event Hubs, managing partition ownership, tracking checkpoints (the progress of event consumption), and receiving events. You implement the logic for how to process each received event.

Key Concepts of Event Processors

• Partition Ownership: Event processors work cooperatively to divide the workload of processing partitions. A partition is typically owned by only one event processor instance at a time to avoid duplicate processing. This ownership is dynamic and can be rebalanced if processors join or leave the processing group.
• Checkpoints: To ensure reliable processing, event processors record "checkpoints." A checkpoint marks the position of the last successfully processed event for a given partition within a consumer group. If a processor fails or restarts, it can resume processing from the last recorded checkpoint, preventing data loss or reprocessing.
• Consumer Groups: A consumer group is a named view of an Event Hub. Each consumer group can have its own independent set of event processors reading from the same Event Hub. This allows multiple applications or components to consume events from the same data stream without interfering with each other.
• Load Balancing and Rebalancing: When multiple instances of an event processor are running for the same consumer group, they automatically coordinate to distribute the partitions among themselves. If an instance is added or removed, the partitions are rebalanced among the remaining instances to ensure all partitions are being processed.

Event Processor Host

In the context of the older Event Processor Host library (for .NET), this component managed the lifecycle of event processors, handling partition ownership, checkpointing, and event receiving. While still functional, modern development often leverages the Azure SDK's event processor capabilities directly or through libraries like the BlobCheckpointStore for Java.

Implementing Event Processing Logic

When you create an event processor, you typically provide handlers for:

• Processing Events: A method that receives a batch of events and contains your custom logic to process them (e.g., store in a database, trigger another service, perform transformations).
• Handling Errors: A method to gracefully handle exceptions that occur during event processing or connection issues.
• Starting and Stopping: Methods to execute code when the processor starts or stops for a given partition.

Example (Conceptual - .NET SDK)

using Azure.Messaging.EventHubs;
using Azure.Messaging.EventHubs.Consumer;
using Azure.Messaging.EventHubs.Processor;
using Azure.Storage.Blobs;
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;

// ...

string eventHubsConnectionString = "...";
string eventHubName = "my-event-hub";
string blobStorageConnectionString = "...";
string blobContainerName = "eventhub-checkpoints";

BlobServiceClient blobServiceClient = new BlobServiceClient(blobStorageConnectionString);
BlobContainerClient containerClient = blobServiceClient.GetBlobContainerClient(blobContainerName);

EventProcessorClient processor = new EventProcessorClient(containerClient, EventHubConsumerClient.DefaultConsumerGroupName, eventHubsConnectionString, eventHubName);

processor.ProcessEventAsync += ProcessEventHandler;
processor.ProcessErrorAsync += ProcessErrorHandler;

await processor.StartProcessingAsync();

// ... later ...

await processor.StopProcessingAsync();

async Task ProcessEventHandler(ProcessEventArgs eventArgs)
{
    Console.WriteLine($"Received event: {Encoding.UTF8.GetString(eventArgs.Data.EventBody.ToArray())}");
    // Your custom event processing logic here
    await eventArgs.UpdateCheckpointAsync(eventArgs.CancellationToken); // Mark as processed
}

Task ProcessErrorHandler(ProcessErrorEventArgs eventArgs)
{
    Console.WriteLine($"Error in processor: {eventArgs.Exception.Message}");
    return Task.CompletedTask;
}
            

Event processors are the backbone of reliable event-driven architectures using Azure Event Hubs, enabling you to build scalable and resilient applications that react to incoming data streams.