Review by Chris Lattner: https://forums.swift.org/t/se-0306-actors/45734/4

(Of course I'm heavily biased against inheritance, but I think it's a very good review regardless.)

The language should provide all the necessary abstractions to create an isolated context. Actors can then "just" be a default abstraction provided by the standard library.

Side note: has the Swift cross platform support improved? I was very much looking forward to using Swift as a general purpose language, but Apple didn't seem very interested in making it a true first class cross platform language. I believe IBM also dropped their "Swift for the server" efforts a while ago due to similar concerns.

it seems like these fundamentals are in tension, and tradeoffs need to be made, which just means that there won't be a one size fits all paradigm. maybe someone will create something that is truly magic and does everything, but more likely they're trying to sell you something.

I also don't see why a namespace, like that of a function, won't be enough isolation from peeking between actors.

Somehow Scala got Akka, the actor framework, without having to tailor the language for that.

That's precisely what actors aim to solve. Actors pass messages, not memory.

> Akka

If you look at an actor framework built in a feature-rich language (Orleans), that problem isn't apparent at all. Even Rust, with all the ownership complexity, manages to do elegantly handle actors, without an explicit plan to do so. Your critique is more relevant to Java than actor libraries.

I haven't tried it on Windows yet, but I know that it is officially supported now.

A stated goal is static, compile time, prevention of concurrency error. You can't do that with a concurrency library unless you restrict yourself to purely deterministic code.

If you ditch Foundation and rely on C libs you are able to use it pretty much anywhere.

In my own experimentation, I’ve seen some success with Vapor 4 and PlotHTML

In theory any C based UI libs that support those platforms should work as long as they can be called from Swift.

The most difficult thing I’ve done with Swift/C was packaging a Python interpreter inside a Mac app with a few lxml dependencies called with PythonKit. Might not begin to scratch the surface of what you need though...

Nice job embedding Python in Swift. I haven't done Python, but Wren and Gravity were pretty challenging to get working right. I was going to use a scripting layer for the UI, but eventually came up with a solution that allows for Swift to be used. I sort of mimic the system Apple uses for Playgrounds/Live Preview in Xcode.

I'm not a Swift user, but the approach I've seen is people using it to run a localhost HTTP server for a HTML+JS frontend.

i have been carefully avoiding any objective-c code so far, but i must admit i haven't been to careful with foundation dependencies.

Do you have a blog post explaining what you did and what limitations you encountered ?

You can find some info by searching, there has been a few attempts to get it running on Android. Also just searching around github will net some other attempts.

The foundation dependency is the biggest problem. You can check out https://github.com/apple/swift-corelibs-foundation which gets rid of the objc runtime dependency for other platforms, though you will find yourself writing implementations for each eventually.

I would say if you are going to stay with just Android/iOS to look into maybe moving to Kotlin or C for your common layer and then call it from Swift. Kotlin multi-platform also has been improving recently.

EDIT: Also look at the Swift github, there are some docs about different platforms. https://github.com/apple/swift/blob/main/docs/Android.md

"Actor" in this case is more like a new access modifier than another type.

I don't see how you could really do this in a standard library as cleanly.

The reality is that the ability for this to happen _always_ depends on the motivation of parties to make it happen. With a proprietary language/compiler, you are at the mercy of the owner while with open source you can contribute the changes - but in either case you can't will other parties to see the business value and do the work for you.

There are still first-party projects to help with Swift on the Server (which is necessarily cross-platform), swift has official containers on docker hub, there is and Apple does contribute to these efforts. X86 Ubuntu, Centos and Amazon Linux are part of Apple's official CI, while the community contributes CI for various other chip architectures, Android, and Windows.

However, the bulk of Apple's investment in Swift is obviously in their own interests - just like any other open source contributor. IBM and Google both stopped contributing officially, but from what I've seen it appears that has more slowed community work than stopped it.

That’s hard to answer without a “since when”, but the answer probably is “yes”. See https://swift.org/platform-support. There are Windows binaries now, for example.

Many of Apple’s libraries aren’t available on non-Apple platforms, though, so many would say it hasn’t improved enough to be competitive with other languages.

The only issue for me is that I don't use Swift Package Manager, hence, the build system (Bazel) integration is simply not there (the BSP support from sourcekit-lsp is dead).

Wasn’t it baked into Erlang (well, OTP, but like there’s a distinction), where the model originated?

[1] https://en.wikipedia.org/wiki/History_of_the_Actor_model

It's a nice language but useless if you want to use it as a generic purpose tool.

I think the argument is vastly overstated that Swift is not useful without Apple frameworks. It's similar to C# - you have the language, and then you have Windows APIs. Nobody would argue that C# is "useless" outside of Windows.

Compare Swift on Windows versus .NET (VB, C#, F#) on macOS.

My only point is that that the argument I seem to see often that Swift is not useful outside of macOS/iOS because you don't have SwiftUI/CoreData/whatever is not a good argument.

If you can find a language that you actually like to work in then why not use it on other platforms?

The dependencies really come down to the Apple frameworks. Ditch those and it's no different than any other general purpose language.

Myriad reasons. If—for whatever reason—someone likes to program in Swift, having high-quality implementations available on various platforms is a win.

> Other platforms tend to allow you much better choices.

This assumes an objective measure of programming language goodness. Whether a language is better than Swift is an opinion. Opinions vary.

    // Safe: this operation is the only one that has access to the actor's isolated
    // state right now, and there have not been any suspension points between
    // the place where we checked for sufficient funds and here.
    balance = balance - amount
    
    // Safe: the deposit operation is placed in the `other` actor's mailbox; when
    // that actor retrieves the operation from its mailbox to execute it, the
    // other account's balance will get updated.
    await other.deposit(amount: amount)

If I have a closed system of actors constantly transferring money among themselves, and I poll to see the total amount of money in the system, will it fluctuate?

[1] https://swift.org/blog/swift-atomics/

Actor model in general doesn't guarantee the ordering of the messages.

The is possibility that all the withdraw messages arrive first and all the deposits later.

Balance poll result makes sense only with assumption "all messages before this point of time have been processed"

    Once upon a time, there was a company called Taligent, which was created by IBM and Apple to develop a set of    
    tools and libraries like Cocoa. About the time Taligent reached the peak of its mindshare, I met one of its 
    engineers at a trade show. I asked him to create a simple application for me: A window would appear with a  
    button, and when the button was clicked, the words "Hello, World!" would appear in a text field. The engineer 
    created a project and started subclassing madly: subclassing the window and the button and the event handler. 
    Then he started generating code: dozens of lines to get the button and the text field onto the window. After 45 
    minutes, I had to leave. The app still did not work. That day, I knew that the company was doomed. A couple of years   
    later,Taligent quietly closed its doors forever.

  import SwiftUI

  @main
  struct FooApp: App {
 @State private var isClicked: Bool = false
 var body: some Scene {
  WindowGroup {
   VStack {
    Button("Click me!") { isClicked.toggle() }
    if isClicked { Text("Hello, World!") }
   }.frame(width: 640, height: 480)
  }
 }
  }

    Taligent: The whole point of adding all those abstractions is to reduce complexity.

And when people do everything "wrong", we get amazing things like Smalltalk and Cocoa.

Hmm...

We don't get this type public proposals for Swift UI, I wish Swift UI would be open source, because the documentation hasn't been great in my experience and at least this type of proposals get publicly documented and discussed before being implemented.

The whole XAML/WPF thing just seem to me a bit of a dig's dinner.

Is it just me, or is Swift getting really complex? As I read through Lattners evaluation, I found myself wondering "hadn't heard of that @Thing yet. I wonder how many there are that I don't know about."

Wile I was impressed about some of the nuance that goes into the combinatorial complexity of the many design patterns Swift wants to capture under its broad umbrella to make it a language for everyone, I kind of have to accept that I will probably never have the bandwidth to acquire the High Priest level of knowledge to really get the thing. Which means the odds of ending up with unexpected and surprising behavior actually rise. I feel more and more this way about Kotlin, Rust, and even Python these days.

Reading/writing code in these increasingly multi-paradigm ecosystems is starting to feel like reading epic poetry (e.g. Dante or Beowulf), but where different sections are written in different languages, with different prose and rhyming styles, some pieces a mix of two left for the reader to figure out.

I used to stress about getting these systems enough to code idiomatically in them, to use them "right." But more and more, I'm finding myself developing personal dietary menus in each, where I pick a nutritious subset that I can wrangle into some sort of expressive consistency, and just write code in that subset, idiomatic-fling-of-the-season be damned.

That said, I think Swift does sometimes feel like it's missing a metaprogramming system. Like property wrappers and function builders could probably be generated by a sufficiently powerful macro system, but instead they exist as one-off features with significant complexity.

Probably a fair few, although I wouldn't blame you for not knowing about something like @Sendable - since that is closely associated with the actor proposal.

When I start looking into the subject further, I start falling into a rabbit hole of how Actors are sometimes treated as a database, that is hard to query, and provides no transaction-ability across actors. There is some argument that Actors could be reimplemented using Postgres' "SELECT ... FOR UPDATE", since you can lock the row for the duration of the transaction.

Anyone else have experience managing large amounts of data inside of Actors have a say about this?

The particular system my team inherited gave us nothing but issues: cluster coordination issues, so quorum wasn't met and things wouldn't start cleanly, network partitioning issues so nodes would randomly be considered dead, actors "becoming" (the term used IIRC) other types of actors based on messages received... the whole thing was just.. too ephemeral and organic for our tastes. I used to get excited thinking about systems like that, but I've since grown more conservative in my technology / architectural preferences (i.e. choose boring technology). We actually ended up replacing it with exactly what I suggested: a plain-old set of services talking via a message-broker. It's stupid simple and we've all slept better.

Edit: I will say that the use of actors constrained to a single service to handle concurrency is probably way more supportable than in a clustered mode.

>a plain-old set of services talking via a message-broker

That said, I think you're absolutely right with distributed Akka. I'm very hesitant to fully embrace it, and we use simple service-level APIs to communicate between nodes. I understand the developers have made a lot of progress on the functionality of remote Akka over the years, but it's just not as tried and true as I would like. Using Aeron for message transport for example, is something that may be the best tool available, but is really hard to sell to my org when simple services are more approachable and maintainable.

>actors "becoming" (the term used IIRC) other types of actors based on messages received

Yeah... I didn't understand why my org was using the Classic Akka instead of the new and improved Typed Akka for a long time. But cool-looking things like this just aren't worth it sometimes. Especially when Classic Akka "just works".

Actors can be a near-sliver bullet for some things, but you have to think about solutions differently. I always though a bank was a good example. One actor per-account, how do you think about money in flight and such? Probably in a fundamentally different way than you would in postgres.

> hard to query

That one is easy though, CQRS

Systems with explicit communication, i.e. based on queues, encourage the programmer to limit the number of possible ways in which worker threads can interact with the thread that manages mutable state. They also require something like a big switch statement where all possible interactions are listed in one place. It seems like in actor-based code, all the possible interactions with the mutable state could be spread across the code base.

On the other hand, these hand-written message loops and switch statements are essentially the same code that the compiler would generate for an actor's mailbox processing loop. One could argue that writing them by hand is a waste of effort. Programmers simply need to use discipline. (Similar to virtual dispatch in C++, for example.)

I am curious about the opinions of programmers more familiar with concurrent architectures. Does the actor model make it easy to write concurrent spaghetti code?

I think you could write concurrent spaghetti code in any environment just as easily, to be honest. I also think it really depends on which actor-model implementation you're using. Each offers their own unique experience, as I've found that there's a noticeable difference in the Scala+Akka approach vs Elixir+OTP, for example.

The actor model is just one way of reasoning about concurrency. Your mailbox serializes/linearizes your interactions so you don't necessarily need locks. Because you get that for free, you can write "single-threaded" code to handle each message. Messages can be sent across a network, so now you can concurrently interact with remote actors too. The simplicity of the model stops there.

Concurrent code gets complicated very quickly by nature. If you architect your application to have messages causing ripple effects in your system, your application behavior is going to be very difficult to reason about. But that's nothing new, either. Instead of sending messages, concurrent function calls could produce the same issue.

Is reasoning about message flows and behaviors hard? Yes, but that's just a byproduct of concurrency. Like you said, "programmers simply need to use discipline", but everyone will disagree on what discipline looks like. Each actor model implementation will have their "best practices" to mitigate the complexity of concurrent interactions, as will each organization using said implementation for their project. At my work, the way we use Scala+Akka was very structured and not at all the way I expected, having been used to Elixir+OTP.

As I think the issues of the actor model are actually issues with reasoning about concurrency in general, I would pick actors over coroutines just for the up-front structure and simplicity it provides.

Maybe it's just me, but message flows and behavior is a lot easier to think about than concurrent access to shared state.

In an Erlang world, it's fairly easy to look at a process's code and see what it does for each message it might get, and think about if it's correct or not. Of course, it's sometimes hard to see which messages it might get from other code; also the system behavior can be a challenge to understand at times, although generally going back to making sure each actor does the right thing, and making sure the right number are running gets you most of the way there.

With the re-entrant Swift actors proposed in the link, I'm not so sure it will be easy. It looks like if your actor A's method needs to call another actor and read the response, that opens actor A to running other methods between the call and response; that makes it hard to think about again.

In general, I agree with you. I think the actor model makes concurrent code readable, understandable, and potentially easier to maintain due to that readability. People inexperienced with concurrency might find it a lot more approachable than the alternatives; an actor feels so similar to an object at times and are easier to use than threads or green-threads. I've personally found that actors also give me performance right out of the box, when I could have really screwed something up with locking.

But other than that, doesn't it all boil down to the same thing?

Actors serialize access/modification of state through ordered messages. Locks serialize access/modification of state through acquiring the lock. And when you have complex access or modification patterns in your code, the actor model doesn't help all that much. To me it's like each message acquires the lock to the actor state.

Sure, you can see what messages cause what behavior or state change for an actor A, but the rest of the system can still be complex. Which actors send a given message to actor A, and why or when do they do this? It's not obvious, and the same complexity can be found in event-driven systems, lock-y threaded systems, etc.

>With the re-entrant Swift actors proposed in the link, I'm not so sure it will be easy.

I'm personally not convinced about re-entrant actors. I don't see why you'd give up the guarantee of blocking until a message is fully handled.

They state:

"Moreover, it helps ensure that actors can make timely progress within a concurrent system, and that a particular actor does not end up unnecessarily blocked on a long-running asynchronous operation (say, downloading a file)."

This example is particularly weak in my opinion. An actor could block on the long-running async operation, of course. But isn't that the brute-force/naive approach? I can't see any one doing this in practice.

You could create a long-running actor whose only job is to sequentially handle requests to download files and send the file descriptors back as a message response. You could create a worker pool to do that as well. You could decompose your one actor into a staged pipeline of actors, so any requests that block can go to one pipeline, and other swiftly handled messages can go to another pipeline. Of course, none of these are simple solutions, but neither is designing an actor with re-entrancy in mind.

>that makes it hard to think about again

I agree. However, they do seem to be pushing the envelope and not just reinventing the wheel. I'm curious to see how this plays out.

> Actors serialize access/modification of state through ordered messages. Locks serialize access/modification of state through acquiring the lock. And when you have complex access or modification patterns in your code, the actor model doesn't help all that much. To me it's like each message acquires the lock to the actor state.

I think yes and no. The big thing for me is it's easy to mess up locks. I've locked one thing and modified another, or read outside the lock, then locked to write, and the value changed. I also just did something similar except there's a mix of locks and atomics (kernel side of pthread umutexes needs to use atomics to mark the mutex as contested).

Actors serialization of work via mailbox (and its implicit locking) makes it hard to make basic mistakes. Of course, complex flows are still complex.

We're definitely in agreement on re-entrant actors. On the plus side, if it doesn't work well, I expect they'll change course.

The "actor properties can only be accessed via self." constraint seems a very elegant and simple static expression that is understandable in a program. Love it.

On top of that, the rest of the proposal appears to stick to "message passing is method invocation" way of thinking about objects and therefore syntactically both get mixed up. I have a hunch that the language would be better off being explicit about message passing and keep it separate from "invoke an async method". (The proposal requires automatic conversion to message passing when it sees an async method invocation on an actor.)

The "access immutable properties directly" facility should reduce a lot of boilerplate that would otherwise crop up, but actor usage itself should stay away from styles that require such cross-actor access (opinion).

What would be the benefit of this? (I haven't read the proposal in detail, but) If you can invoke an async method of an actor, it still has to be run in serial with the execution of messages to avoid races.

Besides, in the implementation I doubt that "messages" will be anything more than delayed async methods (and the implementation notes seem to support this). Actual actor systems rarely treat messages as first-class serializable entities - unless they hope to execute across multiple machines.

Having programmed in Erlang, I feel that the notation (i.e. syntax) influences moment to moment thinking. So if there is overlapping notation between method invocation and message passing, I expect increased opportunity to code and design incorrectly, resulting in subtle bugs.

Making "async message passing" syntactically explicit and obivous clarifies a lot and prevents expectations of calling other methods, facing compiler errors, or worse having to deal with specified behaviours that may not be what I had in mind.

Edit: one example of this confusion in the spec is the whole part about inheritance in actors ... which looks like a terrible idea to me.

I can't think of many instances where I would use STM over CAS (e.g. an atom in Clojure); and the times I have needed something akin to STM (e.g. building an incremental dataflow engine with transactions) I wanted more control over the runtime behavior than Clojure's STM provided.

Plenty of business systems need transactional guarantees and either durability or distribution (or both), which makes an external store like Postgres or Redis a ton more attractive than STM within a single process.

Features like postgres's serialized transaction isolation -- which as a non expert I think of as basically database implemented STM -- are really flexible -- but most ecosystems programming models don't really expose this capability in a full proof and natural way. You have to work to too hard to take full advantage of the capability ...

The other problem is that Intel tried to add STM to their hardware, but it has so many bugs that it's been disabled in multiple CPU generations. I only know one program that uses it and it's a PS3 emulator, not a database.

I totally agree. It's actually a big part of the reason I pulled back from Swift development for personal projects, since it appears that the core team has no problem shoe-horning in half-baked features if it's required to make the necessary impact at the keynote of WWDC.

However I think that example (along with property wrappers) shouldn't be used to impugn the language design decisions as a whole. Protocol-oriented programming is extremely powerful in Swift, and since I have switched to mostly using Rust over the past year for personal projects, there are a lot of things about Swift I really miss.

To be fair, that 'shoe-horning' took nearly a year and a half. Apple was willing to ship the feature but took their time and got lots of community feedback before a version of it became officially part of the language. I believe SwiftUI has already migrated over to the standardized version of it.

SwiftUI is in some ways amazing - because you (rightfully) get this feeling that they had several ridiculously-overqualified compiler experts working in tandem with UX experts and low-level framework engineers on it.

But the flip side is that you don't have a lot of parallels to draw experience from when using it - you must gain experience by working with SwiftUI itself.

So let me tell you, as someone who was a very active member of the community when these features were announced, this is not what that felt like. I, like the rest of the community, found out about function builders by noticing code presented on a slide at WWDC which would not compile under the then-current version of Swift. There was then a large debate within the community, where members of the core team presented post-hoc rationalizations about how "Swift was always intended as a language which would support DSL's and declarative programming". But the message was clear - this feature was going into the language, with zero prior community review or input, because it was required for the business goals of Apple.

The thing that made this particularly jarring for many in the community is that it was so far out of character for the general language evolution process. If anything Swift had been known for being slow at adopting new features. Every addition or change had to meet a very high standard of 1. feeling cohesive with the language, 2. not limiting the future design space, and 3. not presenting risks with respect to the ABI.

Highly necessary features like variadic generics with obvious utility languished for years, cross-platform support and tooling remained in this grey zone of working in some select cases but not others, and here were radical changes to the language being made which were not known to the community at all. For me and a lot of others it clarified things about the direction and governance philosophy of the language.

> But the flip side is that you don't have a lot of parallels to draw experience from when using it - you must gain experience by working with SwiftUI itself.

What would be the big differences you would see with other hot-reloading FRP-style frameworks like React and Flutter?

Yes, and this is one of the reasons that having well defined walls (and having names for those) is really important for commercially-backed open source projects.

It is perhaps clearer to say there is Swift, the language and open source implementation which has an open process for contributing changes. Then there is Apple iSwift, a vendor fork of that language, still open source, that Apple uses to leverage Swift for their platforms. This is similar to the relationship Apple has with clang and used to have with gcc (writing their own precompiled headers and blocks features back in the day, plus objective C itself at one time).

Function Builders were added to iSwift, and then Apple spent over a year getting it into the Swift language proper. Changes which affected the syntax of a mainline Swift feature were not off the table, although it would have affected the timeline of Apple being able to migrate developers to it.

Ugh. I agree. I haven't actually used SwiftUI in anger yet, so I try to reserve my judgement, but the whole thing seems like a lot of magic and complexity to me (not to mention reports of some performance issues/gotchas).

Didn't they also add some weird dynamic JavaScripty property getters or something?

How so? Rust has an equivalent feature (impl Trait) for example. It solves a legitimate problem.

My impression is that when working with Swift your first tool of choice should be a value type, so a strict or an enum. Actors would only be desirable when you need class-like behaviors and need concurrency. Maybe I’m missing something, but I suspect many of us will never use actors directly at all. I certainly don’t see any parts of my code which demand this kind of structure.

But concurrency is a really interesting case. Concurrency solutions are highly specific. I think we'd all agree there is no "answer" for concurrency, which puts it square in the library category. And there are consequences to getting your language's concurrency solution wrong. For example, I'd point to Clojure's built-in STM as a huge swing-and-a-miss. Otoh Erlang is the poster child for why doing concurrency right really does require language-level support.

What's the way out? Imo either build your language around concurrency from day 1, ie Erlang, Go, Pony, or accept that concurrency will always be a best effort good-enough solution in your language.

Precisely. "Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp." https://en.wikipedia.org/wiki/Greenspun%27s_tenth_rule

As much as I actually do enjoy working with Swift (real type classes!!), there are some parts that are a big WTF.

This is exactly what the new async/await feature solves though. Actors are built on top of that.

They weren’t under any pressure at all to have any other ‘async’ story early on and so intentionally chose to focus on other fundamentals and take their time to build something that would be well designed and additive.

The early versions of Swift didn't even have a standard Result type, IIRC.

So even today, you will see some Swift functions that have `throws` in the signature and some that return `Result<T>`.

It was always awkward.

I agree that they are not idiomatic, which is the point I’m making.

There was a perfectly working solution that had no barriers to its use with swift, so it made sense not to rush the replacement.

Maybe it was the right call in the end (I'm of the opinion that it literally doesn't matter. Apple could ask us to develop in COBOL or even something as crazy as Objective-C and we'd all still do it), but I still think it's fair to say it was weird/surprising to have awkward/difficult async tools for an official mobile OS language.

I guess in relation to your original statement, I am refuting it.

They had a strong solution which was pretty easy to use, and as good as the language they were replacing, and well integrated with the platform.

The idea that they ‘didn’t have a strong solution’ doesn’t really hold from my point of view.

It was strong, and working which gave them the luxury of time to develop an idiomatic solution as the language matured.

If there really hadn’t been a strong solution in place, I would have agreed with you.

It is frustrating, but it seems like they are finally getting there. It’s frustrating since a language like Go had a good concurrency model almost from the start. But Go had different constraints and arguably has less surface area to cover.

Go is twice as old (since initial public release) and still doesn't have decent support for generalized algorithms, for example. They have struggled just as long to come up with a more usable error model.

By contrast, a language like Rust is far more parsimonious with introducing keywords, but I think I encountered 10 new concepts in the first month I was working with the language, all of which I had to understand at some level in order to progress with my project.

Why?

As far as I can tell, both Erlang and Akka (the two cited examples that implement actors) do both concurrent and distributed systems.

There is already a distributed programming library on the system called XPC, so people already have experience with it, but you certainly can't program as if every method call might become remote. Mainly the problem is every call can fail, and sometimes retrying is correct and sometimes it isn't, but also the costs of passing a large function parameter become different cross-process and then more different cross-machine.

Note ObjC already had some language features for an older library (DO) like 'inout' parameters, but they were actually removed because XPC instead only uses callbacks.

If that works it's going to be quite interesting. But i feel that it goes against what i've learned from go language design decisions : concurrency is such a deep concern that you have to build the whole language around it, as well as from erlang, where they basically designed the virtual machine around the actor requirements.

It's going to be some interesting times..

One of the biggest benefits of using actors is that whether an actor exists on the same machine or across the network can be abstracted away from you. 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. Not considering this use case would make it a very limiting language feature.

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.

>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.

It's not about the message passing operation looking the same. The developers erroneously assumed that message delivery was guaranteed. The core issue here is not unique to actor systems.

Erlang/Elixir and Akka do not guarantee message delivery (even for a local case) from what I understand; guaranteed message delivery may mean different things in different contexts (like message queued in mailbox vs message is received from mailbox). In my opinion, developers should always program defensively when writing networked applications using something like the circuit-break pattern or making operations idempotent as you mentioned. When using actor systems, developers should not assume guaranteed message delivery unless the tool allows them to.

I'm not familiar with other actor systems so I can't speak on their guarantees, but Erlang has outlined the reasoning why messages shouldn't be considered to be guaranteed here [1].

[1] If I send a message, is it guaranteed to reach the receiver?: https://erlang.org/faq/academic.html#idp32844816

Erlang also bakes OTP, that is a library for messaging semantics and process behaviours (that processes have to implement to be OTP) and introduces the concept of a "call", where a unique reference is created for the message being sent and only when the receiver processes the message and "replies" (with the answer and the reference) is the "call" considered complete and allows to be sure the message was processed to the point of sending that reply. This is the solution the "ack" mentioned in the linked doc refers to. It's not inherent in send because send is async and the only way to have it know that, is to wait for an ack.

(you can implement the call semantics with plain processes, but it's such a normal thing that in OTP it's baked at a lower level for the process behaviours included in OTP, mostly all the gen_* behaviours)

All of this breaks down in distributed settings because it's physically impossible to guarantee. Your message may be received but the answer back may not because the network glitched or the hardware blew before the response was sent. These are problems of distributed systems though. You can be sure that if you get a reply from a call that the message was received. You still need to take (or not) care of some of the failure modes accordingly to your requirements (be it having idempotency, retry logic, nodes behaving as queue processors, etc). Some failure modes encode the reason as well, for instance a failed call to a non-existent pid in a functioning node that is reachable is different than a failed call to a non-reachable node, but a failure in a node that went down or a node that is alive but not reachable is impossible to discern without additional things.

Distributed programming is not a little add-on you can slap on top of a non-distributed system if history has taught us anything.

In the original manifesto: https://gist.github.com/lattner/31ed37682ef1576b16bca1432ea9...

https://github.com/apple/swift-evolution/blob/main/proposals...

...but it seems like its still an open question in the RFC?

I would expect actors to operate on their own state and on immutable data, and things such as transactional data coordination to be separate.

That said, I don't have nearly the experience with actors as many on HN - are you thinking of a particular actor-based system w.r.t touched state?

However, after having read a lot of the swift proposal relative to concurrency, i am still unsure if the end result of production code is going to be lots of actors running in parallel, or just lots of async calls multiplexed on a few OS threads.

The issue with dispatch is that it wasn't communicated enough that you want to have a few base queues and then a tree of other serial queues on top of that, so people end up running too many CPU threads at once, on a device that often has less than one core free at a time.