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

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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 comprises 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 concurretly. 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 process is being accomplished on a given thread, the CPU can to maneuver ahead with the execution move. If the brand new process is related to a special thread, the CPU has to carry out a context change. That is fairly an costly operation, as a result of the state of the previous thread should be saved, the brand new one must be restored earlier than we will carry out our precise process.


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)



In case you run the code above a number of instances, it is doable 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 just isn’t assured. You’ll be able to 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 important thread is a particular one, all of the person interface associated updates ought to occur on this one. In case you are making an attempt to replace a view from a background thread in an iOS software you will may get an warning / error or perhaps a crash. In case you are blocking the principle thread with a protracted operating software your total UI can grow to be unresponsive, that is why it’s good to have a number of threads, so you possibly can transfer your computation-heavy operations into background threads.

It is a quite common method to work with a number of threads, however this may result in undesirable knowledge races, knowledge corruption or crashes on account of reminiscence points. Sadly many of the Swift knowledge sorts are usually not thread protected 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 making an attempt to change the identical dictionary from a number of threads. That is known as a data-race. You’ll be able to detect these sort 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 by utilizing a daily Grand Central Dispatch primarily based method. We’ll create a brand new serial dispatch queue to stop concurrent writes, it will syncronize all of the write operations, however after all 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 method is a fairly in style answer, we may create a generic class that hides the interior non-public storage and the lock queue, so we will have a pleasant public interface that you should utilize safely with out coping with the interior safety mechanism. For the sake of simplicity we’re not going to introduce generics this time, however I’ll 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 gradual because the total 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 fashion customers can learn a lot quicker (concurrently), however writes shall be nonetheless synchronized via 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)


After all we may velocity up the mechanism with dispatch limitations, alternatively we may use an os_unfair_lock, NSLock or a dispatch semaphore to create similiar thread-safe atomic objects.


One essential takeaway is that even when we are attempting to pick one of the best obtainable choice by utilizing sync we’ll at all times block the calling thread too. Which means nothing else can run on the thread that calls synchronized features from this class till the interior 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’ll be able to (by accident) block threads and stop futher code execution
  • You’ll be able to create a impasse if a number of duties are ready for one another
  • Coping with (completion) blocks and reminiscence references are error inclined
  • It is very easy to neglect to name the correct synchronization block


That is various code simply to offer thread-safe atomic entry to a property. Even though we’re utilizing a concurrent queue with limitations (locks have issues too), the CPU wants to change context each time we’re calling these features from a special thread. As a result of synchronous nature we’re blocking threads, so this code just isn’t probably the most environment friendly.

Luckily Swift 5.5 affords a protected, trendy and general significantly better various. ?

Introducing Swift actors


Now let’s refactor this code utilizing the new Actor sort launched in Swift 5.5. Actors can defend inner state via knowledge isolation guaranteeing that solely a single thread can have entry to the underlying knowledge construction at a given time. Lengthy story quick, every part inside an actor shall be thread-safe by default. First I am going to present you the code, then we’ll discuss 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 sorts, similar to lessons. They’ll have strategies, properties, they will implement protocols, however they do not assist inheritance.

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


The very first massive distinction is that we needn’t present a lock mechanism anymore to be able to present learn or write entry to our non-public storage property. Which means we will safely entry actor properties throughout the actor utilizing a synchronous means. Members are remoted by default, so there’s a assure (by the compiler) that we will 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 an alternative of parallelism, we’ve to maneuver away from common queues and use structured concurrency to carry out duties in parallel. Additionally the concurrentPerform operate just isn’t an asynchronous operation, it will block the caller thread till all of the work is finished throughout the block.


Working with async/await signifies that the CPU can work on a special process when awaits for a given operation. Each await name is a potentional 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 mandatory worth. The new Swift concurrency APIs are constructed on prime a cooperative thread pool, the place every CPU core has simply the correct quantity 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 likewise signifies that whenever you work together with async features and await for a operate the CPU can work on different duties as an alternative of blocking the thread on the decision facet.


So again to the instance code, since actors have to guard their inner states, they solely permits us to entry members asynchronously whenever you reference from async features or exterior the actor. That is similar to the case after we had to make use of the lockQueue.sync to guard our learn / write features, however as an alternative of giving the flexibility to the system to perfrom different duties on the thread, we have totally 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 will carry out our duties asynchronously, however since we’re accessing the actor operate (from an async context / exterior the actor) we’ve to make use of the await key phrase earlier than the set name, even when the operate just isn’t 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 get rid of 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. Afterward we’ll be capable of outline customized executors for our actors, however this characteristic just isn’t obtainable but.


At the moment there’s a international executor implementation (related to every actor) that enqueues the duties and runs them one-by-one, if a process just isn’t operating (no rivalry) it will be scheduled for execution (primarily based on the precedence) in any other case (if the duty is already operating / beneath rivalry) 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 exterior 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 will proceed simply the place we left off work. After all you possibly can create async features inside actors, whenever you name these strategies you will at all times have to make use of await, irrespective of in case you are calling them from the actor or exterior.


There may be 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 discuss nonisolated features, actor reentrancy, international actors and lots of extra. I am going to undoubtedly create extra articles about actors in Swift and canopy these matters within the close to future, I promise. Swift 5.5 goes to be a terrific launch. ?


Hopefully this tutorial will aid you to begin working with actors in Swift. I am nonetheless studying rather a lot concerning the new concurrency APIs and nothing is written in stone but, the core staff remains to be altering names and APIs, there are some proposals on the Swift evolution dasbhoard that also must be reviewed, however I believe the Swift staff did an incredible job. Thanks everybody. ?

Honeslty actors seems like magic and I already love them. ?