Isn't it more like, message passing is a way of constraining shared memory to the point where it's possible for humans to reason about most of the time?

Sort of like rust and c. Yes, you can write code with 'unsafe' in rust that makes any mistake c can make. But the rules outside unsafe blocks, combined with the rules at module boundaries, greatly reduce the m * n polynomial complexity of a given size of codebase, letting us reason better about larger codebases.

No, they do bypass it. I don't know what "Technical Program Managers at Google" do but they don't seem to be using a lot of Erlang it seems ;-). ETS tables can be modeled as a process which stores data and then replies to message queries. Every update and read is equivalent to sending a message. The terms are still copied (see note * below). You're not going to read half a tuple and then it will mutate underneath as another process updates it. Traversing an ETS table is logically equivalent to asking a process for individual key-values using regular message passing.

What is different is what these are optimized for. ETS tables are great for querying and looking up data. They even have a mini query language for it (https://www.erlang.org/doc/apps/stdlib/qlc.html). Persistent terms are great for configuration values. None of them break the isolated heap and immutable data paradigm, they just optimize for certain access patterns.

Even dictionary fields they mention, when a process reads another process' dictionary it's still a signal being sent to a process and a reply needing to be received.

* Immutable binary blocks >64B can be referenced, but they are referenced when sending data using explicit messages between processes anyway.

I thought it was obviously wrong. Server A calls Server B, and Server B calls server A. Because when I read the code my first thought was that it is circular. Is it really not obvious? Am I losing my mind?

The mention of `persistent_term` is cool.

Nobody argues that any of these approaches is a silver bullet for all concurrency problems. Indeed most of the problems of concurrency have direct equivalents in the world of single-threaded programming that are typically hard and only partially solved: deadlocks and livelocks are just infinite loops that occur across a thread boundary, protocol violations are just type errors that occur across a thread boundary, et cetera. But being able to rule out some of these problems in the happy case, even if you have to deal with them occasionally when writing more fiddly code, is still a big win.

If you have an actor Mem that is shared between two other actors A and B then Mem functions exactly as shared memory does between colocated threads in a multithreaded system: after all, RAM on a computer is implemented by sending messages down a bus! The difference is just that in the hardware case the messages you can pass to/from the actor (i.e. the atomicity boundaries) are fixed by the hardware, e.g. to reads/writes on particular fixed-sized ranges of memory, while with a shared actor Mem is free to present its own set of software-defined operations, with awareness of the program's semantics. Memory fences are a limited way to bring that programmability to hardware memory, but the programmer still has the onerous and error-prone task of mapping domain operations to fences.

Though they do say that race conditions are purely mitigated by discipline at design time, but then mention race conditions found via static analysis:

> Maria Christakis and Konstantinos Sagonas built a static race detector for Erlang and integrated it into Dialyzer, Erlang’s standard static analysis tool. They ran it against OTP’s own libraries, which are heavily tested and widely deployed.

> They found previously unknown race conditions. Not in obscure corners of the codebase. Not in exotic edge cases. In the kind of code that every Erlang application depends on, code that had been running in production for years.

I imagine that the 4th issue of protocol violation could possibly be mitigated by a typesafe abstracted language like Gleam (or Elixir when types are fully implemented)

Default timeout is 5 seconds. You need to set explicit infinity timeout to not have one.

For the rest - pure untyped actors come with a lot of downsides and provoke engineers to make systems unnecessarily distributed (with all the consistency and timeout issues). There aren't that many problems which can be mapped well directly to actors. I personally find async runtimes with typed front-ends (e.g. Cats/ZIO in Scala, async in Rust, etc) much more robust and much less error-prone.

Sure, if you design a shit system that depends on ETS for shares state there are dangers, so maybe don’t do that?

I’d still rather be writing this system in erlang than in another language, where the footguns are bigger.

Hmm....

> Erlang is the strongest form of the isolation argument, and it deserves to be taken seriously, which is why what happens next matters.

OK I think I know who wrote this.

> The problem isn’t that developers write circular calls by accident. It’s that deadlock-freedom doesn’t compose.

Is there a need to regugriate it in this format? "two protocols that are individually deadlock-free can still combine to deadlock in an actor system." This is the actually meaningful part.

> Forget to set a timeout on a gen_server:call?

People have pointed out its factually wrong in the thread. Eh

> This is the discipline tax. It works when the team is experienced, the codebase is well-maintained, and the conventions are followed consistently. It erodes when any of those conditions weaken, and given enough time and enough turnover they do.

I know this is an LLM tell, but can't point out. It makes me uneasy to read this. Maybe the rule of three? Maybe the reguggeiation of a elementary SE concept in between a technical description? Maybe because it's tryhard to sound smart? All three I guess.

I could go on, but sigh, man don't use these clankers to write prose. They're like negative level gzip compression.

  erlang:process_flag(message_queue_data, on_heap),
  erlang:set_process_info_limit(memory, 1024000).