|The following is an excerpt from Reactive Microservices Architecture: Design Principles For Distributed Systems — By Jonas Bonér .|
Microservices are often stateful entities: they encapsulate state and behavior, in similar fashion to an Object or an Actor, and isolation most certainly applies to state and requires that you treat state and behavior as a single unit.
Unfortunately, ignoring the problem by calling the architecture “stateless”—by having “stateless” controller-style services that are pushing their state down into a big shared database, like many web frameworks do—won’t help as much as you would like and only delegate the problem to a third-party, making it harder to control— both in terms of data integrity guarantees as well as scalability and availability guarantees.
What is needed is that each Microservice take sole responsibility for their own state and the persistence thereof. Modelling each service as a Bounded Context can be helpful since each service usually defines its own domain, each with its own Ubiquitous Language. Both these techniques are taken from the Domain-Driven Design (DDD) toolkit of modelling tools. Of all the new concepts introduced here, consider DDD a good place to start learning. Microservices are heavily influenced by DDD and many of the terms you hear in context of Microservices come from DDD.
When communicating with another Microservice, across Bounded Contexts, you can only ask politely for its state—you can’t force it to reveal it. Each service responds to a request at its own will, with immutable data (facts) derived from its current state, and never exposes its mutable state directly.