Announce what happened
When a customer places an order, the order system can announce it with an event. Other parts can then update stock, send a confirmation email, or record the sale. The order system does not need to tell each part what to do.
It is like a station announcement: passengers hear the same news and decide what to do.
Flexible, but harder to track
New responses can be added without changing the order system, but they may happen later. Events can arrive twice or out of order, making it harder to trace what happened. Clear event descriptions, reliable delivery, and monitoring help teams spot problems.