What is the behavior of SQS when a message is not deleted within its visibility timeout?

When a message is not deleted within its visibility timeout in Amazon Simple Queue Service (SQS), the message becomes available for processing again. The visibility timeout is a feature that temporarily prevents other consumers from receiving the same message after it has been fetched for processing. If the consumer does not delete the message within the set time, the visibility timeout expires, and the message is returned to the queue, allowing other consumers to process it.

This behavior is crucial for ensuring that messages are not lost and can be effectively retried by other consumers in cases where the initial processing fails or takes longer than expected. It helps maintain the reliability of message delivery in distributed systems, allowing for fault tolerance and scalability.

Moreover, this mechanism does not permanently remove the message from the queue, nor does it ignore it. Messages are not routed to a dead-letter queue unless specified by the configured redrive policy, typically after reaching a maximum number of processing attempts. Hence, the message becoming available for processing again accurately describes the situation when the visibility timeout is exceeded without a delete action from the consumer.