2.1.1 Message Queues
SQS, RabbitMQ, Kafka (log), Redis lists: the producer enqueues; workers dequeue and process.
Why it matters
Slow work (email, video, billing) should not sit on a web thread. Queues absorb spikes.
How it is used
Publish after the DB commit (or use outbox). Set visibility timeouts. Dead-letter poison messages. Scale consumers with queue depth.