mikalv / swift-distributed-actors

Peer-to-peer cluster implementation for Swift Distributed Actors

Home Page:https://swift.org/blog/distributed-actors/

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Swift Distributed Actors

Peer-to-peer server-side focused clustering transport implementation for Swift Distributed Actors.

NOTE: This is a work in progress, early preview project. All APIs may (and will) change. Please read more about our plans the introduction below.


What are Distributed Actors?

Distributed actors are an early and experimental language feature with which we aim to simplify and push the state-of-the-art of distributed systems programming in Swift, in the same way we did with concurrent programming with local actors and Swift's structured concurrency approach embedded in the language.

Currently we are iterating on the design of distributed actors, and are looking to gather your feedback, use-cases, and general ideas in the proposal’s pitch thread, as well as the Distributed Actors category on the Swift forums. The library and language features described in the proposal, and this blog post are available in nightly toolchains, so please feel free to download them and get a feel for the feature. We are going to be posting updated proposals and other discussion threads on the forums, so if you are interested, please follow the respective category and threads on the Swift forums.

We are most interested in general feedback, thoughts about use-cases, and potential transport implementations you would be interested in taking on. As we mature and design the language feature, the library (introduced below), will be following along serve as the reference implementation of one such advanced and powerful actor transport. If you are interested in distributed systems, contributions to the library itself are also very welcome, and there is much to be done there as well!

In the near future, we will also provide a more complete “reference guide”, examples and and article-style guides written using in the recently open sourced DocC documentation compiler, which in addition to the API documentation available today will teach about the specific patterns and use-cases this library enables.

These proposed language features–as all language features–will go through a proper Swift Evolution process before lifting their experimental status. We invite the community to participate and help us shape the language and APIs through review, contributions and sharing experiences. Thank you very much in advance!

This project is released as "early preview" and all of its APIs are subject to change, or even removal without any prior warning.

The library depends on un-released, work-in-progress, and Swift Evolution review pending language features, and as such we cannot recommend using it in production just yet — the library may depend on specific nightly builds of toolchains etc.

The primary purpose of open sourcing this library early is proving the ability to implement a feature complete, compelling clustering solution using the distributed actor language feature, and co-evolving the two in tandem.

Introduction: Distributed Actors

Distributed actors are the next step in the evolution of Swift's concurrency model.

With actors built-into the language, Swift offers developers a safe and intuitive concurrency model that is a great fit for many kinds of applications. Thanks to advanced semantic checks, the compiler is able to guide and help developers write programs which are free from low-level data races. This isn't where the usefulness of the actor model ends though: unlike other concurrency models, the actor model is also tremendously useful in modeling distributed systems, where thanks to the notion of location transparent distributed actors, we can program distributed systems using the familiar notion of actors, and then easily move it to a distributed, e.g. clustered, environment.

With distributed actors we aim to simplify and push the state of the art of distributed systems programming, the same way we did with concurrent programming with local actors and Swift's structured concurrency models embedded in the language.

This abstraction is not intended to completely hide away the fact that distributed calls are crossing the network though. In a way, we are doing the opposite, and programming with the assumption that calls may be remote. This small, yet crucial, observation allows us to build systems primarily intended for distribution, but that are also testable in local test clusters which may even efficiently simulate various error scenarios.

Distributed actors are similar to (local) actors in the sense that they encapsulate their state, and may only be communicated with through asynchronous calls. The distributed aspect adds to that equation some additional isolation, type system and runtime considerations, however the surface of the feature feels very similar to local actors. Here is a small example of a distributed actor declaration:

// 1) Actors may be declared with the new 'distributed' modifier
distributed actor Worker {

  // 2) An actor's isolated state is only stored on the node where the actor lives.
  //    Actor Isolation rules ensure that programs only access isolated state in
  //    correct ways, i.e. in a thread-safe manner, and only when the state is
  //    known to exist.
  var data: SomeData

  // 3) Only functions (and computed properties) declared as 'distributed' may be accessed cross actor.
  //    Distributed function parameters and return types must be Codable,
  //    because they will be crossing network boundaries during remote calls.
  distributed func work(item: String) -> WorkItem.Result {
    // ...

Distributed actors take away a lot of the boilerplate that we'd normally have to build and re-invent every time we build some distributed RPC system. After all, nowhere in this snippet did we have to care about exact serialization and networking details, we just declare what we need to get done - send work requests across the network! This is quite powerful, and we hope you'll enjoy using actors in this capacity, in addition to their concurrency aspect.

To actually have a distributed actor participate in some distributed system, we must provide it with an ActorTransport, which is a user-implementable library component, responsible for performing all the networking necessary to make remote function calls. Developers provide their transport of choice during the instantiation of a distributed actor, like this:


// 4) Distributed actors must have a transport associated with them at initialization
let someTransport: ActorTransport = ...
let worker = Worker(transport: someTransport)

// 5) Distributed function invocations are asynchronous and throwing, when performed cross-actor,
//    because of the potential network interactions of such call.
//    These effects are applied to such functions implicitly, only in contexts where necessary,
//    for example: when it is known that the target actor is local, the implicit-throwing effect
//    is not applied to such call.
_ = try await worker.work(item: "work-item-32")

// 6) Remote systems may obtain references to the actor by using the 'resolve' function.
//    It returns a special "proxy" object, that transforms all distributed function calls into messages.
let result = try await Worker.resolve(worker.id, using: otherTransport)

This summarizes the distributed actor feature at a very high level. We encourage those interested to read the full proposal available in Swift Evolution, and provide feedback or ask questions in the Distributed Actors category on the Swift Forums.

You can follow along and provide input on the distributed actor language proposal on the Swift forums and Swift Evolution. The full current draft of the language proposal is also available for review, though we expect to make significant changes to it in the near future.

We would love to hear your feedback and see you participate in the Swift Evolution reviews of this exciting new feature!


Developing with nightly toolchains

This library depends on in-progress work in the Swift language itself. As such, it is necessary to download and use nightly built toolchains to develop and use this library until the distributed actor language feature becomes a stable released part of the language.

Obtaining a nightly toolchain

Distributed actors require "latest" nightly toolchains to build correctly.

At this point in time, the 2021-10-26 nightly toolchain is sufficient to build the project. You can download it from https://swift.org/download/.

# Export the toolchain (nightly snapshot or pull-request generated toolchain), e.g.:

export TOOLCHAIN=/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2021-10-26-a.xctoolchain

# Just build the project
$TOOLCHAIN/usr/bin/swift build --build-tests

Running tests & configuring DYLD_LIBRARY_PATH

It is a known limitation of the toolchains that one has to export the DYLD_LIBRARY_PATH environment variable with the path to where the TOOLCHAIN stores the _Distributed library.

It is possible to swift build the project without passing additional environment variables, however in order to run anything, we must provide the location of the _Distributed library in the custom toolchain to the dynamic linker as we start the binary.

To do this on macOS, you have to use the DYLD_LIBRARY_PATH. For security reasons, unless you have System Integrity Protection turned off, this environment variable is automatically stripped out when a process creates a child process. This is why we need to invoke the specific xctest binary, rather than leave it to SwiftPM to handle.

Plain swift test invocations are expected to fail like this:

-> % swift test --filter Receptionist
error: signalled(6): /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2021-10-26-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper /Users/ktoso/code/swift-distributed-actors/.build/x86_64-apple-macosx/debug/swift-distributed-actorsPackageTests.xctest /var/folders/w1/hmg_v8p532d800g08jtqtddc0000gn/T/TemporaryFile.cfl1uX output:
    dyld[72407]: Library not loaded: @rpath/XCTest.framework/Versions/A/XCTest
# expected to fail
-> % xctest .build/x86_64-apple-macosx/debug/swift-distributed-actorsPackageTests.xctest
2021-10-28 15:34:20.890 xctest[69943:24184188] The bundle “swift-distributed-actorsPackageTests.xctest” couldn’t be loaded. Try reinstalling the bundle.
2021-10-28 15:34:20.890 xctest[69943:24184188] (dlopen(/Users/ktoso/code/swift-distributed-actors/.build/x86_64-apple-macosx/debug/swift-distributed-actorsPackageTests.xctest/Contents/MacOS/swift-distributed-actorsPackageTests, 0x0109): Library not loaded: /usr/lib/swift/libswift_Distributed.dylib
  Referenced from: /Users/ktoso/code/swift-distributed-actors/.build/x86_64-apple-macosx/debug/swift-distributed-actorsPackageTests.xctest/Contents/MacOS/swift-distributed-actorsPackageTests
  Reason: tried: '/usr/lib/swift/libswift_Distributed.dylib' (no such file), '/usr/local/lib/libswift_Distributed.dylib' (no such file), '/usr/lib/libswift_Distributed.dylib' (no such file))

Instead, you must find the xctest binary in Xcode:

-> % find /Applications/Xcode-Latest.app  | grep xctest

-> % export XCTEST_BINARY=/Applications/Xcode-Latest.app/Contents/Developer/usr/bin/xctest

Then, you can use:


$TOOLCHAIN/usr/bin/swift build --build-tests && \

or the following, in order to run only the test class DistributedReceptionistTests:

$TOOLCHAIN/usr/bin/swift build --build-tests && \
  -XCTest "DistributedActorsTests.DistributedReceptionistTests" \

If you see such issue, you may need to provide the dynamic library path pointing at the specific toolchain you are using to build the project, like this:

DYLD_LIBRARY_PATH="$TOOLCHAIN/usr/lib/swift/macosx/" <command>

For running only a specific test file, please refer to Running filtered tests, which is hitting a similar limitation.

Swift Syntax dependency versions

⚠️ Please note that the build needs the exact matching Swift Syntax version that is compatible with the toolchain. This manifests as the following error:

The loaded '_InternalSwiftSyntaxParser' library is from a toolchain that is not compatible with this version of SwiftSyntax

In case you encounter this compatibility issue, please check your toolchain used to build, as well as the swift syntax dependency in Package.swift. SwiftSyntax uses the toolchain's internal types to perform its parsing, and those are sadly not API stable. As we are developing this feature on nightly builds of the toolchain, and depend on SwiftSyntax as a library, we need to make sure the two match in a compatible way.

The SwiftSyntax version declared in this project's Package.swift will always be in-sync with the recommended toolchain version we recommend above. If you see this issue after pulling the latest changes from this repository, please check if you don't have to also update the toolchain you use to build it.

This is a current limitation that will be lifted as we remove our dependency on source-generation.


Editing in Xcode is currently unsupported. This project depends on work in progress language features; once they mature enough, editing in Xcode will become available.

Other IDEs It should be possible to open and edit this project in other IDEs. Please note though that since we are using an unreleased Swift 5.6-dev version, some syntax in the Package.swift may not be recognized by those IDE's yet. For example, you may need to comment out all the .plugin sections declarations, in order to import the project into CLion. Once the project is imported though, you can continue using it as usual.


Contributed by Moritz Lang on the forums

I found the following to work for me, but only using CLion:

  • Set the Swift toolchain in CLion to the latest nightly one (for me that was 10-28)
  • Remove the .plugin from Package.swift in some other editor (NOT CLion)
  • Open the project in CLion, resolve the Swift package dependencies
  • Close CLion
  • Re-add the .plugin to Package.swift
  • Re-open the project in CLion, but DO NOT resolve Swift packages within CLion
  • Add DYLD_LIBRARY_PATH to the run configuration
  • Build/Run tests 🎉

I suspect the reason this works is that CLion stores that it already resolved the dependencies inside the .idea folder. Additionally, CLion invokes the build command with the --skip-update flag.


The project currently is emitting many warnings about Sendable, this is expected and we are slowly working towards removing them.

Much of the project's internals use advanced synchronization patterns not recognized by sendable checks, so many of the warnings are incorrect but the compiler has no way of knowing this. We will be removing much of these internals as we move them to use the Swift actor runtime instead.

Source generation (to be removed)

The current approach uses source generation, using a SwiftPM plugin, in order to implement the bridging between function calls and messages. We are actively working on removing this part of the library and replace it with language features powerful enough to express these semantics.

You can view our proposal to replace the source generator with a language proposal in this Swift Evolution post.

Running samples

To run samples, it currently is necessary to provide the DYLD_LIBRARY_PATH environment variable so Swift is able to locate the new _Distributed module. This is a temporary solution, and eventually will not be necessary.

For example, the following will run the SampleDiningPhilosophers example app in distributed mode:

Much of the project's internals use advanced synchronization patterns not recognized by sendable checks, so many of the warnings are incorrect but the compiler has no way of knowing this. We will be removing much of these internals as we move them to use the Swift actor runtime instead.

Running samples


DYLD_LIBRARY_PATH="$TOOLCHAIN/usr/lib/swift/macosx/" \
  $TOOLCHAIN/usr/bin/swift run \
  --package-path Samples \
  SampleDiningPhilosophers dist 


Only an initial version of API documentation is available right now. This is mostly because the API surface of this library is expected to change intensely, along with the evolving language proposal on which this library depends.

You can build API documentation by running the Swift DocC compiler. The recently released DocC compiler is an official part of the Swift project, however it is so new, that it does not ship with official toolchains yet. Therefore, we provide scripts that automate the building and previewing documentation for you. Those will not be necessary once DocC begins to ship with

Do build documentation run:


Upon first invocation, this will download and build the docc tool. Subsequent invocations will be fast, as only the initial call has to download and build the tool itself.

And to preview and browse the documentation as a web-page, run:


Which will result in an output similar to this:

Starting Local Preview Server

You can then keep this preview server running, and re-run the generate_docc.sh script to keep updating the browsed documentation.

Supported Versions


  • Nightly snapshots of Swift 5.6+
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Peer-to-peer cluster implementation for Swift Distributed Actors


License:Apache License 2.0


Language:Swift 97.8%Language:Shell 1.5%Language:TLA 0.3%Language:C 0.3%Language:Dockerfile 0.0%