"What part of microservices says things must be separated by way of an arbitrary wire protocol?" That's not a requirement, cf. Erlang/OTP. But to qualify as a

"What part of microservices says things must be separated by way of an arbitrary wire protocol?" That's not a requirement, cf. Erlang/OTP. But to qualify as a microservice, you need it decoupled enough that you can independently deploy and/or restart individual services. Pretty sure that's a minimum to call yourself microservice-based, and hard to do by just using classes in C++ or something. Arguably you should also have quite a bit of control over dependencies and their versions. If your service isn't ready for python3, it can keep using python2, or whatever. Taking this to the extreme would generally mean a black box that serializes everything over a network, so that any service can be implemented on any stack. I'm not sure that's fundamental to the idea of microservices, though.