Swift actors tutorial – a newbie’s information to string secure concurrency

Thread security & knowledge races

Earlier than we dive in to Swift actors, let’s have a simplified recap of laptop concept first.

An occasion of a pc program is named course of). A course of accommodates smaller directions which are going to be executed sooner or later in time. These instruction duties might be carried out one after one other in a serial order or concurrently. The working system is utilizing a number of threads) to execute duties in parallel, additionally schedules the order of execution with the assistance of a scheduler). 🕣

After a job is being accomplished on a given thread), the CPU can to maneuver ahead with the execution circulate. If the brand new job is related to a special thread, the CPU has to carry out a context swap. That is fairly an costly operation, as a result of the state of the previous thread have to be saved, the brand new one ought to be restored earlier than we are able to carry out our precise job.

Throughout this context switching a bunch of different oprations can occur on completely different threads. Since trendy CPU architectures have a number of cores, they will deal with a number of threads on the similar time. Issues can occur if the identical useful resource is being modified on the similar time on a number of threads. Let me present you a fast instance that produces an unsafe output. 🙉

var unsafeNumber: Int = 0
DispatchQueue.concurrentPerform(iterations: 100) { i in
    print(Thread.present)
    unsafeNumber = i
}

print(unsafeNumber)

If you happen to run the code above a number of instances, it is attainable to have a special output every time. It’s because the concurrentPerform technique runs the block on completely different threads, some threads have greater priorities than others so the execution order will not be assured. You possibly can see this for your self, by printing the present thread in every block. Among the quantity adjustments occur on the principle thread, however others occur on a background thread. 🧵

The principle thread is a particular one, all of the consumer interface associated updates ought to occur on this one. In case you are attempting to replace a view from a background thread in an iOS software you may may get an warning / error or perhaps a crash. In case you are blocking the principle thread with a protracted operating software your whole UI can change into unresponsive, that is why it’s good to have a number of threads, so you may transfer your computation-heavy operations into background threads.

It is a quite common method to work with a number of threads, however this will result in undesirable knowledge races, knowledge corruption or crashes on account of reminiscence points. Sadly a lot of the Swift knowledge varieties will not be thread secure by default, so if you wish to obtain thread-safety you often needed to work with serial queues or locks to ensure the mutual exclusivity of a given variable.

var threads: [Int: String] = [:]
DispatchQueue.concurrentPerform(iterations: 100) { i in
    threads[i] = "(Thread.present)"
}
print(threads)

The snippet above will crash for positive, since we’re attempting to switch the identical dictionary from a number of threads. That is known as a data-race. You possibly can detect these form of points by enabling the Thread Sanitizer beneath the Scheme > Run > Diagnostics tab in Xcode. 🔨

Now that we all know what’s an information race, let’s repair that through the use of a daily Grand Central Dispatch based mostly method. We’ll create a brand new serial dispatch queue to forestall concurrent writes, this may syncronize all of the write operations, however in fact it has a hidden value of switching the context each time we replace the dictionary.

var threads: [Int: String] = [:]
let lockQueue = DispatchQueue(label: "my.serial.lock.queue")
DispatchQueue.concurrentPerform(iterations: 100) { i in
    lockQueue.sync {
        threads[i] = "(Thread.present)"
    }
}
print(threads)

This synchronization approach is a fairly well-liked resolution, we may create a generic class that hides the inner non-public storage and the lock queue, so we are able to have a pleasant public interface that you should utilize safely with out coping with the inner safety mechanism. For the sake of simplicity we’re not going to introduce generics this time, however I will present you a easy AtomicStorage implementation that makes use of a serial queue as a lock system. 🔒

import Basis
import Dispatch

class AtomicStorage {

    non-public let lockQueue = DispatchQueue(label: "my.serial.lock.queue")
    non-public var storage: [Int: String]
    
    init() {
        self.storage = [:]
    }
        
    func get(_ key: Int) -> String? {
        lockQueue.sync {
            storage[key]
        }
    }
    
    func set(_ key: Int, worth: String) {
        lockQueue.sync {
            storage[key] = worth
        }
    }

    var allValues: [Int: String] {
        lockQueue.sync {
            storage
        }
    }
}

let storage = AtomicStorage()
DispatchQueue.concurrentPerform(iterations: 100) { i in
    storage.set(i, worth: "(Thread.present)")
}
print(storage.allValues)

Since each learn and write operations are sync, this code might be fairly sluggish because the whole queue has to attend for each the learn and write operations. Let’s repair this actual fast by altering the serial queue to a concurrent one, and marking the write operate with a barrier flag. This manner customers can learn a lot sooner (concurrently), however writes will likely be nonetheless synchronized by means of these barrier factors.

import Basis
import Dispatch

class AtomicStorage {

    non-public let lockQueue = DispatchQueue(label: "my.concurrent.lock.queue", attributes: .concurrent)
    non-public var storage: [Int: String]
    
    init() {
        self.storage = [:]
    }
        
    func get(_ key: Int) -> String? {
        lockQueue.sync {
            storage[key]
        }
    }
    
    func set(_ key: Int, worth: String) {
        lockQueue.async(flags: .barrier) { [unowned self] in
            storage[key] = worth
        }
    }

    var allValues: [Int: String] {
        lockQueue.sync {
            storage
        }
    }
}

let storage = AtomicStorage()
DispatchQueue.concurrentPerform(iterations: 100) { i in
    storage.set(i, worth: "(Thread.present)")
}
print(storage.allValues)

In fact we may pace up the mechanism with dispatch limitations, alternatively we may use an os_unfair_lock, NSLock or a dispatch semaphore to create related thread-safe atomic objects.

One essential takeaway is that even when we try to pick one of the best out there possibility through the use of sync we’ll at all times block the calling thread too. Because of this nothing else can run on the thread that calls synchronized capabilities from this class till the inner closure completes. Since we’re synchronously ready for the thread to return we will not make the most of the CPU for different work. ⏳

We will say that there are various issues with this method:

• Context switches are costly operations
• Spawning a number of threads can result in thread explosions
• You possibly can (unintentionally) block threads and forestall additional code execution
• You possibly can create a impasse if a number of duties are ready for one another
• Coping with (completion) blocks and reminiscence references are error susceptible
• It is very easy to neglect to name the right synchronization block

That is various code simply to supply thread-safe atomic entry to a property. Although we’re utilizing a concurrent queue with limitations (locks have issues too), the CPU wants to modify context each time we’re calling these capabilities from a special thread. As a result of synchronous nature we’re blocking threads, so this code will not be probably the most environment friendly.

Fortuitously Swift 5.5 presents a secure, trendy and total a lot better various. 🥳

Introducing Swift actors

Now let’s refactor this code utilizing the new Actor kind launched in Swift 5.5. Actors can defend inside state by means of knowledge isolation making certain that solely a single thread can have entry to the underlying knowledge construction at a given time. Lengthy story brief, all the things inside an actor will likely be thread-safe by default. First I am going to present you the code, then we’ll speak about it. 😅

import Basis

actor AtomicStorage {

    non-public var storage: [Int: String]
    
    init() {
        self.storage = [:]
    }
        
    func get(_ key: Int) -> String? {
        storage[key]
    }
    
    func set(_ key: Int, worth: String) {
        storage[key] = worth
    }

    var allValues: [Int: String] {
        storage
    }
}

Process {
    let storage = AtomicStorage()
    await withTaskGroup(of: Void.self) { group in
        for i in 0..<100 {
            group.async {
                await storage.set(i, worth: "(Thread.present)")
            }
        }
    }
    print(await storage.allValues)
}

To start with, actors are reference varieties, identical to lessons. They’ll have strategies, properties, they will implement protocols, however they do not help inheritance.

Since actors are carefully associated to the newly launched async/await concurrency APIs in Swift you need to be conversant in that idea too if you wish to perceive how they work.

The very first large distinction is that we needn’t present a lock mechanism anymore with a purpose to present learn or write entry to our non-public storage property. Because of this we are able to safely entry actor properties inside the actor utilizing a synchronous means. Members are remoted by default, so there’s a assure (by the compiler) that we are able to solely entry them utilizing the identical context.

What is going on on with the brand new Process API and all of the await key phrases? 🤔

Effectively, the Dispatch.concurrentPerform name is a part of a parallelism API and Swift 5.5 launched concurrency as a substitute of parallelism, we’ve to maneuver away from common queues and use structured concurrency to carry out duties in parallel. Additionally the concurrentPerform operate will not be an asynchronous operation, it will block the caller thread till all of the work is completed inside the block.

Working with async/await signifies that the CPU can work on a special job when awaits for a given operation. Each await name is a possible suspension level, the place the operate may give up the thread and the CPU can carry out different duties till the awaited operate resumes & returns with the required worth. The new Swift concurrency APIs are constructed on prime a cooperative thread pool, the place every CPU core has simply the correct amount of threads and the suspension & continuation occurs “just about” with the assistance of the language runtime. That is way more environment friendly than precise context switching, and in addition signifies that once you work together with async capabilities and await for a operate the CPU can work on different duties as a substitute of blocking the thread on the decision aspect.

So again to the instance code, since actors have to guard their inside states, they solely permits us to entry members asynchronously once you reference from async capabilities or outdoors the actor. That is similar to the case once we had to make use of the lockQueue.sync to guard our learn / write capabilities, however as a substitute of giving the power to the system to carry out different duties on the thread, we have completely blocked it with the sync name. Now with await we may give up the thread and permit others to carry out operations utilizing it and when the time comes the operate can resume.

Inside the duty group we are able to carry out our duties asynchronously, however since we’re accessing the actor operate (from an async context / outdoors the actor) we’ve to make use of the await key phrase earlier than the set name, even when the operate will not be marked with the async key phrase.

The system is aware of that we’re referencing the actor’s property utilizing a special context and we’ve to carry out this operation at all times remoted to eradicate knowledge races. By changing the operate to an async name we give the system an opportunity to carry out the operation on the actor’s executor. In a while we’ll be capable to outline customized executors for our actors, however this function will not be out there but.

At the moment there’s a world executor implementation (related to every actor) that enqueues the duties and runs them one-by-one, if a job will not be operating (no competition) it will be scheduled for execution (based mostly on the precedence) in any other case (if the duty is already operating / beneath competition) the system will simply pick-up the message with out blocking.

The humorous factor is that this doesn’t vital signifies that the very same thread… 😅

import Basis

extension Thread {
    var quantity: String {
        "(worth(forKeyPath: "non-public.seqNum")!)"
    }
}

actor AtomicStorage {

    non-public var storage: [Int: String]
    
    init() {
        print("init actor thread: (Thread.present.quantity)")
        self.storage = [:]
    }
        
    func get(_ key: Int) -> String? {
        storage[key]
    }
    
    func set(_ key: Int, worth: String) {
        storage[key] = worth + ", actor thread: (Thread.present.quantity)"
    }

    var allValues: [Int: String] {
        print("allValues actor thread: (Thread.present.quantity)")
        return storage
    }
}


Process {
    let storage = AtomicStorage()
    await withTaskGroup(of: Void.self) { group in
        for i in 0..<100 {
            group.async {
                await storage.set(i, worth: "caller thread: (Thread.present.quantity)")
            }
        }
    }    
    for (ok, v) in await storage.allValues {
        print(ok, v)
    }
}

Multi-threading is tough, anyway similar factor applies to the storage.allValues assertion. Since we’re accessing this member from outdoors the actor, we’ve to await till the “synchronization occurs”, however with the await key phrase we may give up the present thread, wait till the actor returns again the underlying storage object utilizing the related thread, and voilá we are able to proceed simply the place we left off work. In fact you may create async capabilities inside actors, once you name these strategies you may at all times have to make use of await, irrespective of in case you are calling them from the actor or outdoors.

There’s nonetheless rather a lot to cowl, however I do not need to bloat this text with extra superior particulars. I do know I am simply scratching the floor and we may speak about non-isolated capabilities, actor reentrancy, world actors and plenty of extra. I am going to positively create extra articles about actors in Swift and canopy these matters within the close to future, I promise. Swift 5.5 goes to be an excellent launch. 👍

Hopefully this tutorial will assist you to begin working with actors in Swift. I am nonetheless studying rather a lot in regards to the new concurrency APIs and nothing is written in stone but, the core workforce continues to be altering names and APIs, there are some proposals on the Swift evolution dashboard that also must be reviewed, however I feel the Swift workforce did an incredible job. Thanks everybody. 🙏

Actually actors looks like magic and I already love them. 😍