It is essential that microservices have a clear boundary. Typically, every microservice has a single identified business capability that it delivers well. Teams have found success adapting the Bounded Context concept proposed in the book Domain-Driven Design by Eric J Evans.
Essentially, for large systems, it is very difficult to create one domain model. Evans talks about splitting the system into different bounded contexts. Identifying the right bounded contexts is the key to success with microservice architecture.
