Just an aside, but message passing also makes a huge difference if you want to implement a distributed consensus protocol such as Viewstamped Replication or Raf

Just an aside, but message passing also makes a huge difference if you want to implement a distributed consensus protocol such as Viewstamped Replication or Raft. Compared to typical request/response connection-oriented protocols (e.g. TCP or HTTP), message passing maps 1-to-1 to the distributed consensus domain, where the client can send a request message to the leader and then fully expect to get the response message back from another subsequent leader (after a view change or leader election). I ran into this while implementing Viewstamped Replication in Zig for https://github.com/coilhq/tigerbeetle https://github.com/coilhq/tigerbeetle, where we had a typical TCP request/response protocol to begin with, and then realized that a message bus would make things much easier. It's just really nice to be able to say, send this message to process A or process B (if you can) and not worry about much else beyond timeouts and retries, which are needed for distributed consensus anyway, and straight away it opens up the door for alternative underlying transport protocols such as QUIC or UDT, multicast etc. Message passing is just a really good abstraction to have in place above all this in general.