What is Pub/Sub

Pub/sub (publish-subscribe) is a messaging pattern where publishers send messages to a named topic without knowing who will receive them, and subscribers receive all messages from topics they subscribe to.

How it works

A publisher writes a message to a topic. The message broker delivers a copy of that message to every subscriber of that topic. This is fan-out — one message reaches many consumers. The publisher and subscribers are completely decoupled.

This differs from a message queue, where each message goes to exactly one consumer. In pub/sub, every subscriber gets every message.

Where it is used

Kafka topics, Redis pub/sub, Google Cloud Pub/Sub, Amazon SNS, and RabbitMQ fanout exchanges all implement the pub/sub pattern. Common use cases include event notifications, real-time updates pushed via WebSockets, and decoupled microservice communication.

Why it matters

Pub/sub enables loose coupling. Adding a new subscriber is a configuration change — the publisher is unmodified. Removing a subscriber affects nothing. This makes systems extensible without coordination between teams.

For the full explanation including durable vs ephemeral subscriptions, see How Pub/Sub Works.