What is Pub/Sub Messaging?

The publisher doesn't call the subscriber directly and often doesn't even know how many subscribers exist; a message broker (or event bus) sits in between, accepting published messages and fanning them out to every current subscriber of that topic. This decoupling is the whole point: a new subscriber can start listening to a topic without the publisher's code changing at all, and a publisher can be added without every existing subscriber needing to know about it. This contrasts with point-to-point integration, where each connection is a dedicated wire between two specific systems; pub/sub instead lets N systems all publish and subscribe through one shared topic layer, which scales far better as the number of interconnected systems grows. It also naturally supports multiple independent consumers processing the same event for different purposes — one subscriber logs the event for audit, another triggers a notification, a third updates a cache — all from the exact same publish. The tradeoff is added infrastructure and a need to reason about delivery guarantees (at-least-once vs exactly-once), message ordering, and backpressure when a subscriber falls behind the rate messages are published — none of which a simple direct call needs to worry about.