> You're just sending messages, so you can send a message over the wire and it would be as simple to the developer as sending it locally
If you are expecting a reply, or some side-effect in another system, as a result of the message you sent (and usually you would expect that, otherwise you wouldn't send the message in the first place) then it's not that simple.
If the actor is in the same OS process on the same machine, then message delivery is reliable, and you know you'll either get a reply OR a signal that the other actor died. If, on the other hand, the actor is on another machine across the network then the semantics are different. You cannot always differentiate between the remote actor dying and an intermittent network connection error. So you need to take that into account in your protocol design - for example by making operations idempotent.
I've many times seen Erlang code where the developers didn't make this distinction - because the message passing operation looks the same, remote or not - and as a result the system is not resilient to network failures.
If you are expecting a reply, or some side-effect in another system, as a result of the message you sent (and usually you would expect that, otherwise you wouldn't send the message in the first place) then it's not that simple. If the actor is in the same OS process on the same machine, then message delivery is reliable, and you know you'll either get a reply OR a signal that the other actor died. If, on the other hand, the actor is on another machine across the network then the semantics are different. You cannot always differentiate between the remote actor dying and an intermittent network connection error. So you need to take that into account in your protocol design - for example by making operations idempotent.
I've many times seen Erlang code where the developers didn't make this distinction - because the message passing operation looks the same, remote or not - and as a result the system is not resilient to network failures.