Azure Event Hubs Documentation

Event Delivery Guarantees

Azure Event Hubs provides robust mechanisms for ensuring that events are delivered reliably to consumers. Understanding these guarantees is crucial for building resilient event-driven applications.

At-Least-Once Delivery

Event Hubs aims to deliver each event at least once. This means that an event might be delivered to a consumer more than once under certain failure conditions. Consumers should be designed to be idempotent to handle duplicate events gracefully.

How Idempotency is Achieved

Idempotency means that performing an operation multiple times has the same effect as performing it once. For event processing, this typically involves:

• Checking for unique event identifiers before processing.
• Using transactional operations for state changes.
• Designing operations so that reprocessing them does not cause side effects.

Message Durability and Fault Tolerance

Event Hubs stores events durably for a configurable retention period. This durability ensures that even if consumers are temporarily unavailable or fail, events are not lost. When a consumer becomes available again, it can resume processing from where it left off.

Consumer Offset Management

Each consumer group maintains its own offset for each partition. The offset is a pointer to the position of the last successfully processed event in a partition. Event Hubs clients (e.g., Event Hubs SDKs) typically manage this offset automatically. When an event is successfully processed, the consumer client updates the offset.

If a consumer crashes, it can restart and use the last committed offset to resume processing without missing any events or reprocessing events that have already been acknowledged.

Checkpointing

Checkpointing is the mechanism by which consumers record their progress. In the context of Event Hubs, this often involves storing the offset (and sometimes a sequence number or partition identifier) to a durable store. This store can be:

• Azure Blob Storage
• Azure Table Storage
• Other custom durable storage solutions

The Event Hubs SDKs, particularly the EventProcessorClient (in .NET) or equivalent abstractions in other languages, abstract much of this complexity, allowing developers to focus on the business logic of event processing.

Example of Checkpointing Logic (Conceptual)

// Pseudocode representing checkpointing
function processEvent(event) {
    try {
        // Process the event's data
        applyBusinessLogic(event.data);

        // Update the offset after successful processing
        // The EventProcessorClient typically handles this automatically
        // by calling checkpoint() after a batch of events is processed.
        commitOffset(event.partitionId, event.offset);

    } catch (error) {
        // Handle processing errors, do NOT commit offset
        logError(error);
        // Depending on the error, you might want to retry or move to dead-letter queue
    }
}

async function commitOffset(partitionId, offset) {
    // This is an abstract representation. Actual SDKs manage this.
    // For example, EventProcessorClient.checkpoint(lastSuccessfullyProcessedEvent).
    await updateCheckpointInDurableStorage(partitionId, offset);
    console.log(`Checkpoint saved for partition ${partitionId} at offset ${offset}`);
}
                

Delivery Guarantees Summary

• At-Least-Once: Events are delivered a minimum of one time. Consumers must handle duplicates.
• Durable Storage: Events are persisted for a configurable retention period.
• Consumer Offset: Each consumer group tracks its progress independently.
• Checkpointing: Consumers record their processing progress to durable storage.

By understanding and implementing strategies for idempotency and reliable offset management, you can build robust applications on top of Azure Event Hubs.