I work on such a system (multiple "microservices" written in a variety of languages) and we use JSON on top of zeromq. It's super easy to debug without any fanc

I work on such a system (multiple "microservices" written in a variety of languages) and we use JSON on top of zeromq. It's super easy to debug without any fancy tool. And as long as I'm consistent with my API design it's trivial to extend and keep everybody up to date. If performance ever becomes an issue (unlikely in my case) I could probably switch to some binary serialization protocol eventually.