> Like what does your polytree look like if you add a messaging pub/sub type system into it.

A message bus is often considered a clean way to deal with a cycle, and would exist outside the tree. I hear your point about the graph disappearing entirely if you use a message bus for everything, but this would probably either be for an exceptionally rare problem-space, or because of accidental complexity.

Message busses (implemented correctly) work because:

* If the recipient of the message is down the message will still get delivered when it comes back up. If we use REST calls for completion callbacks then the sender might have to do retries and whatnot over protracted periods.

* We can deal with poison messages. If a message is causing a crash or generally exceptional behavior (because of unintentional incompatible changes), we can mark it as poisoned and have a human look at it - instead of the whole system grinding to a halt as one service keeps trashing another.

REST/RPC should be for something that can provide an answer very quickly, or for starting work that will be signaled as complete in another way. Using a message bus for RPC is just as much of a smell as using RPC for eventing.

And, as always, it depends. The line may be somewhere completely different for you. But, and I have seen this multiple times, a directed cycle in a distributed system's architecture turns it into a distributed monolith: eventually you will reach a situation where everything needs to deploy at the same time. Many, many, engineers can talk about their lessons in this - and you are, as always, free to ignore people talking about the consequences of their mistakes.