Asynchronous Processing and Messaging
How queues, pub/sub, and log-based streaming let systems stay responsive under load — the delivery semantics, ordering rules, backpressure, and outbox patterns that decide whether async saves you or sinks you.
Decoupling with queues and events: message queues vs pub/sub vs log-based streaming, delivery semantics (why exactly-once delivery is a myth — do idempotent processing), ordering, backpressure, the transactional outbox, and dead-letter queues.