Event-driven architectures decouple producers from consumers, allowing systems to scale and fail independently. But reliability requires careful design.
Choosing the right queue
AWS offers several queue services. SQS is the most common choice for background processing. It provides at-least-once delivery, configurable visibility timeouts, and dead-letter queues. SNS is useful when you need fan-out to multiple subscribers. EventBridge is the right choice for event buses with complex routing rules.
Idempotent consumers
Since SQS delivers messages at least once, your consumer must handle duplicate messages. An idempotent consumer checks a deduplication store before processing. A simple pattern is to use a database table with a unique constraint on the message ID:
- Check if the message ID exists in the processed table.
- If yes, acknowledge and skip.
- If no, process and insert the message ID.
Retry and dead-letter handling
Messages that fail processing should be retried. Configure the queue visibility timeout to allow enough time for processing. After a maximum number of receives, move the message to a dead-letter queue.
From the dead-letter queue, you can inspect failed messages, fix the issue, and redrive them. Never lose a message because of transient failures.
Monitoring
Set up CloudWatch alarms on queue depth, age of oldest message, and dead-letter queue size. These metrics tell you when consumers are falling behind or when messages are failing consistently.