The definition of a microservice is surprisingly simple. Since a service is defined by its public interface, a microservice is a service with a micro-public interface: a micro- front door.

Having a micro-public interface makes it easier to understand both the function of a single service and its integration with other system components. Reducing a service’s functionality also limits its reasons for change and makes the service more autono‐ mous for development, management, and scale.

In addition, it explains the practice of microservices not exposing their databases. Exposing a database, making it a part of the service’s front door, would make its pub‐ lic interface huge. For example, how many different SQL queries can you execute on a relational database? Since SQL is quite a flexible language, the likely estimate would be infinity. Hence, microservices encapsulate their databases. The data can only be accessed through a much more compact, integration-oriented public interface.