Actor-model programming for Swift

by iOS Example

flynn

Flynn grafts Actor-Model programming onto Swift, providing a new level of safety and performance for your highly concurrent Swift code. Flynn is heavily inspired by the Pony programming language. Here's what you need to know:

Actors are concurrency safe Swift classes

Using Actors to separate concurrent logic provides safety, performance, and efficiency.

class ConcurrentDatastore: Actor {
  // Everything inside this actor is safe and cannot
  // be accessed concurrently by any other thread
  private var storage: [String: String] = [:]
  
  ...
}

Behaviors are asynchronous method calls

Actors provide behaviors (which look like normal method calls at the call site) that execute asynchronously from the caller's perspective.

let datastore = ConcurrentDatastore()
datastore.beStore("SomeKey", "SomeValue")

From the Actor's perspective, behaviors execute synchronously (in the same order they are sent for the calling code).

class ConcurrentDatastore: Actor {
  ...
  // Behaviors are called asynchronously but execute synchronously on the Actor
  private func _beStore(_ key: String, _ value: String) {
    storage[key] = value
  }
}

Actors run on schedulers

Unlike other attempts to bring Actor-Model programming to Swift, Flynn does not use DispatchQueues. Instead, Flynn includes a modified version of the Pony language runtime. This makes actors in Flynn much more light-weight than DispatchQueues; you can have millions of actors all sending messages to each other incredibly efficiently.

Use FlynnLint

Flynn provides the scaffolding for safer concurrency but it relies on you, the developer, to follow the best practices for safe concurrency. FlynnLint will help you by enforcing those best practices for you at compile time. This keeps you out of numerous concurrency pitfalls by not allowing unsafe code to compile:

In this example, we have a public variable on our Counter Actor. Public variables are not allowed as they can be potentially accessed from other threads, breaking the concurrency safety the Actor-Model paradigm provides us.

Remote actors are actors which run elsewhere

RemoteActors are an advanced topic and are likely useful only to a subset of developers.

As of v0.2 Flynn has a new kind of actor, the RemoteActor. RemoteActors behave similarly to Actors; they have internal state which is concurrency safe and you interact with them by calling behaviors. RemoteActors are intended to execute outside of your normal Flynn environment, usually that's in another process running on an different machine. Since RemoteActors run elsewhere, they are more restrictive then normal Actors (for instance, you cannot choose to unsafely expose access RemoteActors). Please see the RemoteActor documentation for more details.

Products

Have you released something using Flynn? Let us know!

Installation

Flynn is a fully compatible with the Swift Package Manager.

Swift Package Manager

If you use swiftpm, you can add Flynn as a dependency directly to your Package.swift file.

dependencies: [
    .package(url: "https://github.com/KittyMac/Flynn.git", .upToNextMajor(from: "0.1.1")),
],

Xcode

To integrate with Xcode, simply add it as a package dependency by going to

File -> Swift Packages -> Add Package Dependency

and pasting the url to this repository. Follow the instructions to complete the dependency addition. Check the releases page for release versions or choose master branch for the bleeding edge.

Flynn is most effective when used with FlynnLint. FlynnLint helps protect you from accidentally introducing data races in your highly concurrent code by enforcing Flynn's best programming practices.

It is HIGHLY RECOMMENDED that you use FlynnLint!

FlynnLint is included in the Flynn repository in the meta folder. Just add a new "Run Script Phase" with:

FLYNNLINTSWIFTPM=${SRCROOT}/.build/checkouts/flynn/meta/FlynnLint
FLYNNLINTXCODE=${BUILD_ROOT}/../../SourcePackages/checkouts/flynn/meta/FlynnLint

if [ -f "${FLYNNLINTSWIFTPM}" ]; then
    ${FLYNNLINTSWIFTPM} ${SRCROOT}
elif [ -f "${FLYNNLINTXCODE}" ]; then
    ${FLYNNLINTXCODE} ${SRCROOT}
else
    echo "warning: Unable to find FlynnLint, aborting..."
fi

Example:

  • Place the FlynnLint build phase before the "Compile Sources" phase
  • Place the FlynnLint build phase before any other code linters

FlynnLint processes any and all directories provided as arguments. If you want to restrict it to a subset of directories, simply list each directory after the call to FlynnLint. For example, if you use swiftpm and your source files are in /Sources and /Tests, then the following would lint just those directories:

${FLYNNLINTSWIFTPM} ${SRCROOT}/Sources ${SRCROOT}/Tests

GitHub