Backend & systems·Queues, streams and delivery semantics
you publish the event once and three different services each get their own copy without the publisher knowing they exist.
Publish-subscribe
Also calledpub/sub, fan-out
A pattern where publishers emit to a topic and every interested subscriber receives a copy. It removes the publisher's knowledge of consumers, which is what makes adding a fourth consumer a zero-change operation upstream. The trade is that nobody owns the contract, so a field the publisher considers cosmetic can be load-bearing for a subscriber it has never heard of.