Azure Event Hubs Developer's Guide

Understanding Consumer Groups in Azure Event Hubs

Consumer groups are a fundamental concept in Azure Event Hubs, enabling multiple applications or services to read from an Event Hub independently and at their own pace. Each consumer group maintains its own reading offset, allowing for parallel consumption and different processing logic for the same event stream.

What are Consumer Groups?

An Event Hub partitions events based on a partition key or round-robin distribution. A consumer group allows a set of readers to read from all partitions of an Event Hub. Without consumer groups, only one reader could process events from a partition at a time.

Key Concept: Every Event Hub is created with a default consumer group named $Default. You can create additional consumer groups to support various consumption patterns.

Why Use Consumer Groups?

  • Independent Consumption: Different applications can process the same events without interfering with each other. For example, one application might ingest data into a data lake, while another performs real-time analytics.
  • Scalability: Multiple instances of the same application can join the same consumer group to scale processing horizontally. Event Hubs distributes partitions among the consumers within a group.
  • Resilience: If one consumer fails, others in the group can continue processing events.
  • Replaying Events: By resetting the offset of a consumer group, you can reprocess historical events for debugging or reprocessing scenarios.

Creating and Managing Consumer Groups

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

Using the Azure Portal:

  1. Navigate to your Event Hubs namespace in the Azure portal.
  2. Select your Event Hub.
  3. In the left-hand menu, under "Features," click "Consumer groups."
  4. Click "+ Consumer group" to create a new one, providing a unique name and optionally a starting offset.

Using the Azure CLI:

To create a consumer group:


az eventhubs consumer-group create --namespace-name  --event-hub-name  --name
Tip: When creating a consumer group, you can specify a name. If you don't, a unique identifier will be generated. It's best practice to use descriptive names.

Reading Events with Consumer Groups

When you connect to an Event Hub to read events, you specify both the Event Hub name and the consumer group name. This allows Event Hubs to track the reading progress (offset) for that specific consumer group.

Example (Conceptual using a hypothetical SDK):


// Using Azure SDK for .NET (conceptual)
var consumerClient = new EventHubConsumerClient(
    "$Default", // Or your custom consumer group name
    new EventHubClient(eventHubConnectionString)
);

await foreach (PartitionEvent receivedEvent in consumerClient.ReadEventsAsync())
{
    // Process the received event
    Console.WriteLine($"Received event: {Encoding.UTF8.GetString(receivedEvent.Data.EventBody.ToArray())}");
    // The consumer client automatically manages the offset for the $Default consumer group.
}

Consumer Group Offsets and Checkpoints

Each consumer group tracks its progress by storing the offset and sequence number for the last successfully processed message for each partition. This is often referred to as checkpointing. When a consumer restarts, it can resume reading from the last checkpointed offset, preventing duplicate processing and missed events.

The responsibility for checkpointing can lie with the consumer application itself or be managed by a service like Azure Functions or Azure Stream Analytics.

Important: Without proper checkpointing, a consumer might re-read events or miss events if it restarts. Ensure your consumer implementation correctly handles checkpointing.

Default Consumer Group ($Default)

The $Default consumer group is automatically created for every Event Hub. It's useful for simple scenarios or when you want a single default reader. However, for more complex or decoupled architectures, creating named consumer groups is highly recommended.

Best Practices for Consumer Groups

  • Descriptive Naming: Use clear and descriptive names for your consumer groups that reflect their purpose (e.g., data-lake-ingest, real-time-analytics).
  • One Consumer Group per Application/Purpose: Avoid sharing consumer groups between different applications or processing workloads.
  • Manage Offsets: Implement a robust checkpointing strategy to ensure reliable event processing.
  • Monitor Consumer Lag: Keep an eye on the consumer lag for your consumer groups to ensure events are being processed in a timely manner.

By effectively utilizing consumer groups, you can build robust, scalable, and decoupled event-driven architectures with Azure Event Hubs.