Message queue

How this question breaks down

1. 1SemanticsCore22 minDecide what "delivered" means for each message (lost at worst, or duplicated at worst), then make consumers idempotent so at-least-once delivery produces each effect exactly once.
2. 2OrderingCore20 minGive up on one global order, split the stream into partitions by key so each key's messages stay in order, and scale consumers by partition without letting retries, rebalances or resizing scramble a key.
3. 3Dead lettersCore24 minRetry transient failures with growing delays, cap how many times one message may fail, and move the ones that never succeed to a dead-letter queue that someone watches, can inspect and can replay.

Work through the parts in order and you’ve covered the full design. Each part is one topic.