Events arrive out of order. User updates overwrite each other. Here’s how partition keys, sequence numbers, and causal ordering keep things straight.

Your consumer retried a bad message 10,000 times. It will never succeed. Dead letter queues catch the messages that can’t be processed so the rest of your system keeps moving.

Exactly-once delivery is impossible across boundaries. Here’s the pattern that actually works: at-least-once delivery with idempotent consumers.

Kafka says exactly-once. Your consumer processed the message twice anyway. Here’s why exactly-once is impossible across system boundaries.