Pub/Sub
Problem
An order is created.
Now:
* warehouse cares,
* billing cares,
* analytics cares,
* notifications care.
The order service shouldn't contain direct integrations with every interested system.
Solution — Publish/Subscribe
Publish an event:
OrderCreated
/ | \
Billing Warehouse Analytics
Subscribers independently consume events they care about.
Core idea: pub/sub lets multiple independent consumers react to the same information.