Alan Kay says the following about the meaning of OOP:¹¹1 Stefan Ram, “Dr. Alan Kay on the Meaning of “Object-Oriented Programming”,” July 23, 2003. 

OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things.

This applies to microservices as well:

• messaging: Microservices talk to each other over HTTP (or at least some kind of network protocol).

• local retention: Each microservice has its own state. (I’m treating a microservice as a collection of running instances.)

• protection and hiding of state-process: A microservice’s internal state can only be changed through messages.

• extreme late-binding of all things: Parts of a microservices architecture can (and are) deployed independently, changing their APIs and the way they communicate at runtime.