Well, I'd say you've seen SoA setups that do that, maybe. But those don't sound like microservices :) Perhaps that's not a strong point though. Let me be a bit

Well, I'd say you've seen SoA setups that do that, maybe. But those don't sound like microservices :) Perhaps that's not a strong point though. Let me be a bit clearer on my point because I was wrong to say that you have to treat a service as being totally isolated, what I should have said that they are isolated, whether you treat them that way or not. There is a physical boundary between two computers. You can try to ignore that boundary, you can implement distributed transactions, etc, but the boundary is there - if you do the extra work to try to pretend it isn't, that's a lot of extra work to do the wrong thing. Concretely, you can write: rpc_call(&mut my_state) But under the hood what has to happen, physically, is that your state has to be copied to the other service, the service can return a new state (or an update), and the caller can then mutate the state locally. There is no way for you to actually transfer a mutable reference to your own memory to another computer (and a service should be treated as if it may be on another computer, even if it is colocated) without obscene shenanigans. You can try to abstract around that isolation to give the appearance of shared mutable state but it is just an abstraction, it is effectively impossible to implement that directly. But shared mutable state is trivial without the process boundary. It's just... every function call. Any module can take a mutable pointer and modify it. And that's great for lots of things, of course, you give up isolation sometimes when you need to.