Azure Event Hubs Developer's Guide

Consumer Guide

This guide provides comprehensive information for developers looking to consume events from Azure Event Hubs. Understanding how to efficiently and reliably read from Event Hubs is crucial for building scalable and responsive real-time data processing applications.

Understanding Consumer Groups

Consumer groups are a fundamental concept in Event Hubs. Each consumer group allows a separate application or a distinct instance of an application to read from the same Event Hub independently, without affecting other consumers. Each consumer group maintains its own view of the event stream and its own offset.

Independent Consumption: Applications can process the same data stream without interference.
Load Balancing: Within a consumer group, multiple instances of an application can coordinate to distribute the load of processing partitions.
Replayability: Different consumer groups can start reading from different points in the event history.

When you create an Event Hub, a default consumer group ($Default) is automatically created. You can create additional consumer groups as needed for your application's architecture.

Creating Consumer Groups

Consumer groups can be created via the Azure portal, Azure CLI, Azure PowerShell, or programmatically using the Event Hubs SDKs.

Reading Events from Event Hubs

To read events, your application will typically connect to an Event Hub using a connection string and specify the Event Hub name, consumer group name, and the desired partition(s) to read from.

The process generally involves:

Establishing a connection to the Event Hub endpoint.
Creating an Event Hub receiver.
Iterating to receive events.

Receiving Methods

Event Hubs SDKs often provide different methods for receiving events:

Polling: Periodically checking for new events.
Push/Event-driven: Registering a callback function that is invoked when new events are available.

The push model is generally preferred for real-time processing as it reduces latency and avoids unnecessary polling overhead.

Event Processing Strategies

The way you process events depends on your application's requirements. Common strategies include:

At-least-once: Ensure an event is processed at least once. This may lead to duplicate processing if errors occur before acknowledging the event.
At-most-once: An event is processed at most once. This is simpler but risks data loss if failures occur during processing.
Exactly-once (with caveats): Achieving true exactly-once processing is complex and often requires application-level logic or specific frameworks that can deduplicate and manage state reliably across distributed systems.

Event Hubs itself guarantees that events within a partition are delivered in order. The responsibility of managing processing semantics typically lies with the consumer application.

Offset Management

The offset represents the position within a partition from which a consumer group should start reading. Reliable offset management is critical for ensuring data is not lost or reprocessed unnecessarily.

Client-side checkpointing: The consumer application is responsible for storing and retrieving offsets. This is common with libraries like the Event Hubs Processor.
Service-side checkpointing: Event Hubs can manage offsets for you, often integrated with services like Azure Storage or Azure Cosmos DB.

Note: For production applications, using a robust checkpointing mechanism is highly recommended to ensure fault tolerance and state recovery.

Error Handling and Resilience

Robust error handling is vital for any Event Hubs consumer:

Connection Errors: Implement retry logic for transient network issues.
Processing Errors: Decide whether to retry processing, send the event to a dead-letter queue, or log the error and continue.
Partition Ownership: When multiple instances of a consumer application run within the same consumer group, Event Hubs automatically balances partition ownership. Your application should be able to handle changes in partition assignment gracefully.

Warning: Unhandled exceptions during event processing can lead to unexpected behavior, including data loss or infinite retry loops. Implement comprehensive error handling and logging.

Libraries and SDKs

Azure provides official SDKs for various languages, making it easier to integrate with Event Hubs:

These SDKs abstract away much of the complexity of interacting with Event Hubs, providing high-level APIs for sending, receiving, and managing events.

Advanced Topics

Partition Key: Producers can use partition keys to ensure events with the same key are sent to the same partition, which is useful for maintaining order for specific entities.
Event Ordering: Event Hubs guarantees order within a partition. Consumers must handle ordering if cross-partition ordering is required (which is typically not guaranteed).
Large Message Handling: Strategies for handling messages larger than Event Hubs' limits, often involving Event Grid or Azure Blob Storage.
Schema Evolution: Managing changes in the schema of your event data over time.