Pub/Sub (Publish–Subscribe)

Pub/Sub works like a radio station. The station (the publisher) broadcasts on a channel (a 'topic'). It doesn't know or care who's listening. Anyone tuned in (a subscriber) hears the broadcast. The publisher never talks directly to the listeners. A middleman (the 'broker') delivers the message to everyone who subscribed to that channel.

1. 1Subscribers register interest in a topic by name — for example, 'orders'.
2. 2A publisher sends a message to a topic. It doesn't know or care who — or how many services — are subscribed.
3. 3The broker looks up every subscriber for that topic. It delivers a copy of the message to each one.

Use pub/sub when one event needs to trigger several unrelated things, without those things knowing about each other. For example, a new order should notify billing, email, and shipping — but the order-placing code shouldn't need to know the details of all three.

• ⚠Having the publisher call each subscriber directly, instead of going through a topic. That tightly links services together — exactly what pub/sub is meant to avoid.
• ⚠Assuming a topic with no subscribers is an error. Often it's completely fine for nobody to be listening yet.