Event Hubs Consumer Groups

In Azure Event Hubs, a consumer group is a named logical view of the event stream. Each consumer group allows a separate application, or a distinct component within an application, to independently consume from the event stream. This is crucial for scenarios where multiple applications need to process the same data without interfering with each other.

Why Use Consumer Groups?

When you create an Event Hub, it comes with a default consumer group named $Default. However, you'll typically need to create additional consumer groups to:

• Isolate Consumer Logic: Different applications can read the same events without affecting each other's progress. For example, one consumer group might be for real-time analytics, another for archiving, and a third for feeding data into a machine learning model.
• Enable Parallel Processing: Within a single application, multiple instances can read from different partitions. Consumer groups facilitate this by providing a unique starting point for each consumer instance.
• Fault Tolerance and Scalability: By distributing the reading load across multiple consumers within a consumer group, you can enhance the resilience and scalability of your event processing pipeline.

Key Concepts of Consumer Groups

1. Naming Conventions

Consumer group names can be up to 64 characters long and must be unique within an Event Hub. They can contain alphanumeric characters and hyphens. The special name $Default is reserved for the default consumer group.

2. Offsets and Event Sequencing

Each consumer group maintains its own position (offset) in the event stream for each partition. This offset indicates the next event to be read by a consumer in that group. When a consumer reads an event, its offset advances. If a consumer restarts, it will resume reading from its last recorded offset within its specific consumer group.

3. Consumer Group Creation

You can create consumer groups using:

• Azure Portal: Navigate to your Event Hub namespace, select the Event Hub, and then go to "Consumer groups".
• Azure CLI: Use commands like az eventhubs consumer-group create.
• Azure SDKs: Available in various programming languages.

4. Default Consumer Group ($Default)

The $Default consumer group is automatically created when an Event Hub is created. It's often used for simple scenarios or as a fallback. However, for production workloads, it's best practice to create dedicated consumer groups for your applications.

5. Multiple Consumer Groups

An Event Hub can have a large number of consumer groups (up to 20,000 by default, but this limit can be increased). This allows for extensive customization of data consumption patterns.

Example Scenario

Imagine an IoT solution sending sensor data to an Event Hub.

• Consumer Group 1 (Analytics): Reads events for real-time dashboard updates.
• Consumer Group 2 (Archiving): Reads events and saves them to Azure Blob Storage for long-term analysis.
• Consumer Group 3 (Alerting): Monitors events for specific conditions that trigger alerts.

Each of these consumer groups will independently track its position in the event stream, ensuring that data is processed correctly by each component.

Best Practices

• Create a dedicated consumer group for each distinct application or service that consumes events from an Event Hub.
• Avoid using the $Default consumer group for production applications if possible.
• Monitor consumer group offsets to ensure data is being processed efficiently and to identify potential bottlenecks.

Related Topics

• Understanding Event Hubs Partitions
• Event Hubs Architecture Overview
• Using the Event Processor Host for Scalable Consumption