Every node becomes an "edge" in it's own right, and doesn't necessarily have global coherence with the rest of the system.
There already are static typed actor systems ( e.g. Orleans) which work well, but my point is that I believe OTP is more flexible for better or worse. Whether that flexibility is worth it to you for what you get is another matter.
Also I'm not sure how to think about binary compatibility between upgrades in such a system
Yep, I develop one myself. And have gone to the extent of not allowing senders to even post a message if it's of the wrong type (processes in nodes publish the types they accept to a central store). I initially went along with the 'accept anything' approach (which Akka really majors on too), but found that for the large systems I was developing that it became a real headache to deal with.
> but my point is that I believe OTP is more flexible for better or worse. Whether that flexibility is worth it to you for what you get is another matter.
Yep, fair enough, if it works for you, who am I to complain? It's not worth it for me, because I feel quite strongly that the code I write should understand the types it's working with. It feels like this super-late binding can give false positives, appear to work, when in fact it's not. That scares the shit out of me when systems get large.
All I am trying to do here is enumerate the difficulties in trying to Type "OTP" in Erlang (its main selling point) , and am not commenting on all possible actor systems.
Currently, that function declares it will match on the pattern of those 4 attributes rather than a static type. Now, you update the Node and modify the type on the sending Node to have 5 attributes.
With pattern matching on the 4, everything still works. With static types on the struct the contract is now out of sync.
On a suitably complex/large system this is a recipe for disaster. Things start to slowly rot. It is far better to maintain the old function, accepting the old struct, map it to the new struct and forward it on to the new function that accepts the new struct. Let the old one consume anything that's already queued, or being sent from other nodes that haven't yet been upgraded whilst the new one takes the new format.
To achieve that, we follow the design of Protocol Buffers:
1) Each field in each struct has both a name and a numeric id. Only ids are used for serialization, so field names can be changed at any time.
2) All fields are marked as optional or repeated, never required. Most code is written to handle missing fields gracefully.
3) Changing the type or id of an existing field is forbidden. (Note that changing the contents of a nested struct doesn't count as changing its type.)
4) Adding a new field is okay, as long as you use an id that was never used before. (Each struct definition has a comment indicating the next available id to use.)
5) Removing a field is okay if you've checked that no one is using it anymore.
6) As a small but intentional bonus, you can change an optional field to repeated while preserving binary compatibility.
In the end it works out. You can think of breakages that could theoretically happen, but they don't.
If you send a bad message, the receiver will crash or discard it and it is how it is intended to be.
Erlang embrace laws of maths and physics.
