CalculatorServiceClient creates an RPC client that conforms to the CalculatorService interface, but delegates work to remote instances.

CalculatorServiceServer accepts your "real" CalculatorService instance (the thing that really does the work), and returns a set of endpoint routes which allow this service to be consumed via the gateways/listeners you configure in main().

MockCalculatorService allows you to program behaviors into a mock instance of CalculatorService.