When you write your code, the last thing you want to do is go back to it and change it again and again whenever you implement a new functionality. You want the legacy code to work, be tested to work, and allow new functionality to be built on top of it.

In the same way, when we design the system, it should able to extend without changing existing architecture. For example using pub/sub pattern with message brokers is a good example for that. If we want to notify some other microservices, instead of calling their services, we can publish an event and they can consume them from message broker.

By this way, if there will be a new microservice need to consume this event, we don’t change anything for existing architecture, the only thing to subscribe that event from message broker that’s all. So our design open for extensions and close for modifications.

PUBSUB Model is very good example