For what cases was the retry chain mechanism and DLT (Dead Letter Topic) used?

The retry-queue mechanism and Dead Letter Topic (DLT) are used for reliable message processing in distributed systems.

The retry-queue is used when message processing temporarily fails — the message is sequentially resent to queues with increasing delays to give the system time to recover.

DLT is used for messages that could not be processed after several attempts — they are sent to a separate queue for subsequent analysis and manual intervention.

Example: when integrating with an external API, if the service is unavailable, the message moves to the retry-queue with delays of 1 min, 5 min, 15 min. If after all attempts the error persists, the message is sent to DLT for diagnostics.