by toast0 | 2025-10-20 06:00:43+00:00
> Whenever you message another process and need a reply there is a risk of deadlock. I didn't find any primtives in OTP for handling this, you have to structure your actor interaction to avoid it. You can't just have a little bit of shared memory.

I see a lot of people that dislike Erlang that run into this or are afraid of running into it.

Receive with timeout is certainly a primative to handle this, and there's things like process aliases so that late responses can be more easily ignored. But, there's nothing structural preventing it. It's just that when you do it, it hurts, so hopefully you stop doing it.

But when you're in a situation whete you've setup circular messaging, it's very easy to identify. You see a process (A) that has is making no progress and/or has a large mailbox. You see the stack trace shows it is messaging process B, you inspect process B to see why it didn't respond and see it is messaging process A and there you go. Or you look at process A's mailbox and see that it has requests related to the request it's already trying to fill.

> The actor model doesn't really offer any benefit over other models while bringing significant downsides.

I find the actor model makes many types of request/response for persistent state to become very simple. A process owns the state, and application level read/write messages are sent to that process. The mailbox serializes access to provide an ordering and to make changes atomic. You certainly have choose your messages; if you have a counter and do a get and a set message, you will lose updates; you need to do increment/decrement/add kind of messages; same thing for larger/more complex datastructures. Linked datastructures are still tricky.

It's also super nice for request/response connection handling. Just write straight line code that reads from the connection when it needs a request and writes to it when the response has been computed. When you need to be asynchronous, it's more complex, but not so terrible. The process becomes a loop waiting for a fully formed request from the client (which then gets sent off to a request processor) or a reply from a processor (which then gets sent to the client) ... Java's Loom green threads will likely socialize this further than Erlang has.

This kind of feeds into your other question, but IMHO, Actors let you focus on a process owning state and the messages it receives and how they change the state. You don't need to model for the computer where those messages come from, that's emergent system behavior. BEAM doesn't need to know, and doesn't ask, in part because as a changable distributed system, there's not really a way to know. What sends what to who is essential complexity of the system, but the system doesn't need to know about it. If you had a full accounting of how messages are sent, then you potentially find message loops, but ... it's easier to just find and fix any loops at runtime. (hotloading code can make it very low cost to deploy changes, which makes it more reasonable to spend less effort finding issues before release)

> Plus, there are major downsides to using an unpopular platform like Erlang/BEAM.

Sure, you have to do more work yourself. On the otherhand, relying on the work of others isn't always less work than doing it yourself. There are at least a few Erlang consulting shops you can go to if you really need something and are unwilling or unable to do it. You can't find a lot of answers on stackexchange, but you also don't find a lot of wrong or outdated answers either.