Can SQS messages be delivered multiple times?

SQS (Simple Queue Service) is designed to enable communication between distributed software components and can indeed deliver messages more than once. This characteristic is primarily due to the nature of the distributed systems involved. When a consumer processes a message and fails to acknowledge that it has been successfully processed, SQS may deliver the same message again since it believes the consumer has not yet handled it. This can happen due to network failures, processing errors, or timeouts, all of which fall under the at-least-once delivery model that SQS employs.

The design decisions for SQS prioritize reliability and fault tolerance, which is why the possibility of duplicates exists. To manage duplicate deliveries, developers often implement idempotency in their applications. This strategy ensures that processing the same message multiple times does not result in unintended side effects.

While FIFO (First-In-First-Out) queues also exist within SQS and guarantee that messages are delivered in order without duplication, standard SQS queues do permit multiple deliveries. It’s essential for developers working with SQS to design their systems with this behavior in mind to ensure robust application behavior.