Consumers in Azure Event Hubs

Consumers are client applications that read events from an Azure Event Hub. They work in conjunction with partitions and consumer groups to enable scalable and reliable event processing.

The Role of Consumers

Consumers are responsible for:

• Connecting to an Event Hub.
• Reading a stream of events from one or more partitions.
• Processing these events, which could involve storing them, transforming them, triggering actions, or forwarding them to other services.
• Maintaining their progress (checkpointing) to ensure that events are not missed if a consumer restarts or fails.

Consumer Groups

A consumer group is a named view of a consumer, or a group of consumers, that reads from an Event Hub. Each consumer group maintains its own offset or state within a partition. This allows multiple applications to read from the same Event Hub independently, without interfering with each other.

For example, you might have one consumer group for archiving data to long-term storage, and another consumer group for real-time analytics. Both groups can read all events from the Event Hub, but they process them differently and track their progress independently.

Every Event Hub has a default consumer group named $Default. You can create additional consumer groups as needed for your specific use cases.

Partition Ownership and Load Balancing

Consumers within a consumer group work together to read events from the partitions of an Event Hub. The Event Hubs service, with the help of consumer libraries (like the Azure SDKs), automatically distributes the partitions among the active consumers in a consumer group. This process is called partition ownership and load balancing.

• When a new consumer joins a group, it negotiates ownership of partitions with existing consumers.
• Each partition is owned by at most one consumer at any given time within a consumer group.
• If a consumer fails or disconnects, its partitions are reassigned to other active consumers in the group.

This dynamic distribution ensures that event processing can scale horizontally by adding more consumer instances and that the system remains resilient to individual consumer failures.

Checkpointing

Checkpointing is a crucial mechanism for ensuring reliable event consumption. Consumers periodically record the offset of the last successfully processed event for each partition they are reading from. This record, the checkpoint, is stored by the Event Hubs service or an external store (like Azure Blob Storage).

When a consumer restarts, it can retrieve the last checkpoint for each partition and resume reading from the event immediately following the checkpointed offset. This prevents data loss and duplicate processing within the same consumer group.

Consumer SDKs

Azure provides several SDKs to help you build consumer applications. The recommended approach is to use the Azure Event Hubs client libraries, which abstract away much of the complexity of connecting, reading events, managing checkpoints, and handling partition ownership.

Commonly used SDKs include:

• .NET: Azure.Messaging.EventHubs
• Java: com.azure:azure-messaging-eventhubs
• Python: azure-eventhubs
• JavaScript/TypeScript: @azure/event-hubs

These libraries often integrate with other Azure services like Azure Blob Storage for checkpoint management, simplifying the development of robust event processing solutions.

For detailed examples and usage patterns, refer to the respective language SDK documentation.