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Messaging questions cover queues, delivery guarantees, consumer groups, backpressure and the fan-out strategies behind feeds and notification systems.
Message queues decouple producers from consumers, which is what lets parts of a system fail, slow down or be redeployed independently. In interviews they appear whenever work can be done asynchronously, and the interesting questions are all about guarantees: whether a message can be lost, delivered twice, or arrive out of order.
At-least-once delivery is the common default, and it means consumers must be idempotent — saying that unprompted is a strong signal. The other high-value detail is bounding the queue: an unbounded buffer converts overload into unbounded latency and then a crash, so naming what happens at the boundary shows you have operated one of these. For feed-shaped questions, the fan-out-on-write versus fan-out-on-read trade and the celebrity-account problem are the expected discussion.
12 system design · 0/6/6 easy/medium/hard
At-least-once unless you state otherwise, which makes consumer idempotency a requirement rather than a nicety. Exactly-once is usually at-least-once plus deduplication on a stable key, and it is worth saying so.
On write for ordinary accounts, so reads are a single cheap lookup. On read for very high-follower accounts, because one post would otherwise become millions of writes. Production systems run both.