One of the fundamental laws of microservices is that each service is responsible for storing its own state. Nothing else is allowed access to its backing store.

Given that, once you have more than one service in your architecture, you cannot coordinate transactions across the distinct storage mechanisms - they are distinct databases and are therefore subject to the CAP theorem and other complexities of distributed computing.

Of course, nothing's stopping you from putting all your features into one service to avoid this thorny problem. It's a wise approach for most of us.

When you do that you have a monolithic architecture, not a microservice one.