Publish/Subscribe (Pub/Sub)

Publish/subscribe (pub/sub) is a messaging pattern in which senders, called publishers, send messages to a named topic or channel rather than to specific receivers. A broker delivers a copy of each message to every subscriber of that topic.

Why it matters

Publishers and subscribers do not need to know about each other, so you can add a new consumer, such as an analytics service, without changing the sender. This suits event-driven systems, notifications, live updates and fan-out, where one event must trigger several actions. It differs from a work queue, where each message goes to only one worker.

Systems that implement it include Redis Pub/Sub, Google Cloud Pub/Sub, Amazon SNS, Apache Kafka and RabbitMQ exchanges. Guarantees vary: some retain messages, others deliver only to subscribers connected at that moment.

Example

publish  topic=order.created  {"id": 981}
 -> subscriber: email-service    sends receipt
 -> subscriber: stock-service    reserves items
 -> subscriber: analytics        records sale

Common mistakes

Assuming a subscriber that was offline will receive earlier messages; many simple brokers do not store them.
Changing a message format without versioning, breaking unknown subscribers.
Mixing up fan-out topics with load-balanced queues.

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