Azure Event Hubs - Consumer Groups

Consumer Groups

Consumer groups are a fundamental concept in Azure Event Hubs that enable multiple applications or different instances of the same application to read from an Event Hub independently and concurrently. Each consumer group maintains its own offset or position within the event stream of an Event Hub partition.

What is a Consumer Group?

When you send events to an Event Hub, they are stored in ordered partitions. A consumer group allows multiple distinct viewers to read these events. Think of it like multiple people watching the same live TV channel, but each person can pause, rewind, or fast-forward independently of the others. Each consumer group tracks its own progress through the event stream for each partition.

Why Use Consumer Groups?

Parallel Processing: Different applications can process the same events in parallel. For example, one consumer group might process events for real-time analytics, while another processes them for archival to a data lake.
Decoupling: It decouples the producers from the consumers. Producers don't need to know who is consuming the events or how many consumers there are.
Resilience and Readability: If a consumer application fails, other consumer groups can continue to read events. When the failed application recovers, it can resume reading from where it left off, rather than reprocessing events already handled by other groups.
Different Perspectives: Different consumer groups can have different filtering logic or processing requirements for the same events.

Default Consumer Group

When you create an Event Hub, a default consumer group named $Default is automatically created. You can use this group for basic scenarios, but it's generally recommended to create custom consumer groups for production workloads to ensure proper isolation and management.

Creating and Managing Consumer Groups

You can create and manage consumer groups through the Azure portal, Azure CLI, Azure PowerShell, or programmatically using the Event Hubs SDKs.

Key Aspects of Consumer Groups:

Name: Each consumer group has a unique name within an Event Hub.
Offset Tracking: Each consumer group tracks its progress by storing an offset for each partition. This offset indicates the position from which the consumer group will start reading the next event.
Ownership: In a consumer group, only one reader at a time owns a specific partition. This ensures that events are processed by only one instance of a consumer application within that group for each partition.

Example Scenario

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

Consumer Group 1 (Analytics): Reads events to update real-time dashboards and perform anomaly detection.
Consumer Group 2 (Archival): Reads events to store them in Azure Data Lake Storage for historical analysis and compliance.
Consumer Group 3 (Alerting): Reads events and triggers alerts when specific threshold values are met.

Each of these groups can read the same incoming events without interfering with each other, as they all maintain their own independent progress within the Event Hub's partitions.

Important: The number of consumer groups you can create per Event Hub is subject to Azure subscription limits. It's also crucial to understand that within a specific consumer group, a partition is processed by only one consumer instance at a time to guarantee ordered processing and avoid duplicate processing of events for that specific group and partition.

Code Snippet (Illustrative - using Azure SDK)

Below is a conceptual example of how a consumer might interact with a consumer group. The actual implementation details will vary based on the SDK and language.

                
// This is illustrative pseudocode using a hypothetical SDK

import { EventHubConsumerClient, latestEventPosition } from "@azure/event-hubs";

async function processEvents(connectionString, eventHubName, consumerGroupName) {
    const consumerClient = new EventHubConsumerClient(consumerGroupName, connectionString, eventHubName);

    const subscription = consumerClient.subscribe({
        async processEvents(events, context) {
            for (const event of events) {
                console.log(`Received event: ${event.body}`);
                // Process the event for this consumer group
                // e.g., send to database, trigger an alert, etc.
            }
            // The SDK typically handles offset checkpointing automatically or provides methods
            // to explicitly save the progress.
        },
        async processError(err, context) {
            console.error(`Error occurred: ${err.message}`);
        }
    }, {
        startPosition: latestEventPosition // or specific sequence number/offset
    });

    console.log(`Subscribed to Event Hub '${eventHubName}' with consumer group '${consumerGroupName}'`);

    // To stop the subscription later:
    // await subscription.close();
}

// Example usage:
// const connStr = "YOUR_EVENTHUB_CONNECTION_STRING";
// const hubName = "YOUR_EVENTHUB_NAME";
// const groupName = "my-custom-consumer-group";
// processEvents(connStr, hubName, groupName);
                
            

By leveraging consumer groups, you can build robust, scalable, and flexible event-driven architectures with Azure Event Hubs.