Publish-Subscribe (Pub/Sub)
Also known as:
Pub/Sub
Publish Subscribe
Fan-out Messaging
Definition
Publish-subscribe is a messaging pattern where publishers send messages to a topic instead of to specific receivers, and any number of subscribers to that topic get a copy. Publishers and subscribers never know about each other, which decouples them and lets you add or remove consumers without touching the sender.
Key Takeaways
- Publishers write to a topic, not to a consumer. This decoupling is the whole point: senders and receivers scale and change independently.
- Unlike a point-to-point message queue where one consumer gets each message, pub/sub fans one message out to many subscribers.
- It is the backbone of event-driven systems and pairs naturally with CQRS and the transactional outbox.
- You still have to decide delivery guarantees. At-least-once delivery means consumers must be idempotent to handle duplicates.
How It Works
- A publisher sends a message to a named topic on a broker.
- The broker keeps a list of subscribers for that topic.
- Each subscriber receives its own copy, either pushed to it or pulled from its own cursor in the log.
- Subscribers process independently, so a slow one does not block the others.
Where It Is Used
- Apache Kafka, Google Pub/Sub, AWS SNS, and Redis Pub/Sub are common implementations.
- Notification systems fan one event out to email, SMS, and push subscribers at once.
- Slack and other real-time apps use pub/sub to route a message to every server holding a relevant WebSocket.