Swift structured concurrency tutorial – The.Swift.Dev.

Introducing structured concurrency in Swift

In my earlier tutorial we have talked about the brand new async/await characteristic in Swift, after that I’ve created a weblog submit about thread secure concurrency utilizing actors, now it’s time to get began with the opposite main concurrency characteristic in Swift, referred to as structured concurrency. 🔀

What’s structured concurrency? Effectively, lengthy story quick, it is a new task-based mechanism that enables builders to carry out particular person process gadgets in concurrently. Usually while you await for some piece of code you create a possible suspension level. If we take our quantity calculation instance from the async/await article, we may write one thing like this:

let x = await calculateFirstNumber()
let y = await calculateSecondNumber()
let z = await calculateThirdNumber()
print(x + y + z)

I’ve already talked about that every line is being executed after the earlier line finishes its job. We create three potential suspension factors and we await till the CPU has sufficient capability to execute & end every process. This all occurs in a serial order, however typically this isn’t the conduct that you really want.

If a calculation is determined by the results of the earlier one, this instance is ideal, since you should use x to calculate y, or x & y to calculate z. What if we might prefer to run these duties in parallel and we do not care the person outcomes, however we’d like all of them (x,y,z) as quick as we are able to? 🤔

async let x = calculateFirstNumber()
async let y = calculateSecondNumber()
async let z = calculateThirdNumber()

let res = await x + y + z
print(res)

I already confirmed you the way to do that utilizing the async let bindings proposal, which is a type of a excessive stage abstraction layer on prime of the structured concurrency characteristic. It makes ridiculously straightforward to run async duties in parallel. So the large distinction right here is that we are able to run all the calculations without delay and we are able to await for the end result “group” that comprises each x, y and z.

Once more, within the first instance the execution order is the next:

• await for x, when it’s prepared we transfer ahead
• await for y, when it’s prepared we transfer ahead
• await for z, when it’s prepared we transfer ahead
• sum the already calculated x, y, z numbers and print the end result

We may describe the second instance like this

• Create an async process merchandise for calculating x
• Create an async process merchandise for calculating y
• Create an async process merchandise for calculating z
• Group x, y, z process gadgets collectively, and await sum the outcomes when they’re prepared
• print the ultimate end result

As you may see this time we do not have to attend till a earlier process merchandise is prepared, however we are able to execute all of them in parallel, as a substitute of the common sequential order. We nonetheless have 3 potential suspension factors, however the execution order is what actually issues right here. By executing duties parallel the second model of our code may be means quicker, for the reason that CPU can run all of the duties without delay (if it has free employee thread / executor). 🧵

At a really fundamental stage, that is what structured concurrency is all about. In fact the async let bindings are hiding a lot of the underlying implementation particulars on this case, so let’s transfer a bit right down to the rabbit gap and refactor our code utilizing duties and process teams.

await withTaskGroup(of: Int.self) { group in
    group.async {
        await calculateFirstNumber()
    }
    group.async {
        await calculateSecondNumber()
    }
    group.async {
        await calculateThirdNumber()
    }

    var sum: Int = 0
    for await res in group {
        sum += res
    }
    print(sum)
}

In response to the present model of the proposal, we are able to use duties as fundamental items to carry out some type of work. A process may be in one in all three states: suspended, operating or accomplished. Job additionally help cancellation and so they can have an related precedence.

Duties can type a hierarchy by defining baby duties. At present we are able to create process teams and outline baby gadgets by way of the group.async perform for parallel execution, this baby process creation course of may be simplified through async let bindings. Kids routinely inherit their guardian duties’s attributes, reminiscent of precedence, task-local storage, deadlines and they are going to be routinely cancelled if the guardian is cancelled. Deadline help is coming in a later Swift launch, so I will not speak extra about them.

A process execution interval is named a job, every job is operating on an executor. An executor is a service which may settle for jobs and arranges them (by precedence) for execution on out there thread. Executors are at present supplied by the system, however in a while actors will have the ability to outline customized ones.

That is sufficient idea, as you may see it’s attainable to outline a process group utilizing the withTaskGroup or the withThrowingTaskGroup strategies. The one distinction is that the later one is a throwing variant, so you may attempt to await async capabilities to finish. ✅

A process group wants a ChildTaskResult sort as a primary parameter, which needs to be a Sendable sort. In our case an Int sort is an ideal candidate, since we’ll gather the outcomes utilizing the group. You’ll be able to add async process gadgets to the group that returns with the right end result sort.

We will collect particular person outcomes from the group by awaiting for the the subsequent component (await group.subsequent()), however for the reason that group conforms to the AsyncSequence protocol we are able to iterate by way of the outcomes by awaiting for them utilizing a normal for loop. 🔁

That is how structured concurrency works in a nutshell. The most effective factor about this entire mannequin is that by utilizing process hierarchies no baby process shall be ever capable of leak and preserve operating within the background by chance. This a core cause for these APIs that they need to at all times await earlier than the scope ends. (thanks for the strategies @ktosopl). ❤️

Let me present you a number of extra examples…

Ready for dependencies

If in case you have an async dependency in your process gadgets, you may both calculate the end result upfront, earlier than you outline your process group or inside a bunch operation you may name a number of issues too.

import Basis

func calculateFirstNumber() async -> Int {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 2) {
            c.resume(with: .success(42))
        }
    }
}

func calculateSecondNumber() async -> Int {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 1) {
            c.resume(with: .success(6))
        }
    }
}

func calculateThirdNumber(_ enter: Int) async -> Int {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 4) {
            c.resume(with: .success(9 + enter))
        }
    }
}

func calculateFourthNumber(_ enter: Int) async -> Int {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 3) {
            c.resume(with: .success(69 + enter))
        }
    }
}

@primary
struct MyProgram {
    
    static func primary() async {

        let x = await calculateFirstNumber()
        await withTaskGroup(of: Int.self) { group in
            group.async {
                await calculateThirdNumber(x)
            }
            group.async {
                let y = await calculateSecondNumber()
                return await calculateFourthNumber(y)
            }
            

            var end result: Int = 0
            for await res in group {
                end result += res
            }
            print(end result)
        }
    }
}

It’s price to say that if you wish to help a correct cancellation logic you have to be cautious with suspension factors. This time I will not get into the cancellation particulars, however I am going to write a devoted article concerning the matter sooner or later in time (I am nonetheless studying this too… 😅).

Duties with totally different end result varieties

In case your process gadgets have totally different return varieties, you may simply create a brand new enum with related values and use it as a standard sort when defining your process group. You should use the enum and field the underlying values while you return with the async process merchandise capabilities.

import Basis

func calculateNumber() async -> Int {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 4) {
            c.resume(with: .success(42))
        }
    }
}

func calculateString() async -> String {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 2) {
            c.resume(with: .success("The which means of life is: "))
        }
    }
}

@primary
struct MyProgram {
    
    static func primary() async {
        
        enum TaskSteps {
            case first(Int)
            case second(String)
        }

        await withTaskGroup(of: TaskSteps.self) { group in
            group.async {
                .first(await calculateNumber())
            }
            group.async {
                .second(await calculateString())
            }

            var end result: String = ""
            for await res in group {
                change res {
                case .first(let worth):
                    end result = end result + String(worth)
                case .second(let worth):
                    end result = worth + end result
                }
            }
            print(end result)
        }
    }
}

After the duties are accomplished you may change the sequence components and carry out the ultimate operation on the end result primarily based on the wrapped enum worth. This little trick will can help you run all type of duties with totally different return varieties to run parallel utilizing the brand new Duties APIs. 👍

Unstructured and indifferent duties

As you may need observed this earlier than, it isn’t attainable to name an async API from a sync perform. That is the place unstructured duties may help. A very powerful factor to notice right here is that the lifetime of an unstructured process just isn’t sure to the creating process. They will outlive the guardian, and so they inherit priorities, task-local values, deadlines from the guardian. Unstructured duties are being represented by a process deal with that you should use to cancel the duty.

import Basis

func calculateFirstNumber() async -> Int {
    await withCheckedContinuation { c in
        DispatchQueue.primary.asyncAfter(deadline: .now() + 3) {
            c.resume(with: .success(42))
        }
    }
}

@primary
struct MyProgram {
    
    static func primary() {
        Job(precedence: .background) {
            let deal with = Job { () -> Int in
                print(Job.currentPriority == .background)
                return await calculateFirstNumber()
            }
            
            let x = await deal with.get()
            print("The which means of life is:", x)
            exit(EXIT_SUCCESS)
        }
        dispatchMain()
    }
}

You will get the present precedence of the duty utilizing the static currentPriority property and test if it matches the guardian process precedence (in fact it ought to match it). ☺️

So what is the distinction between unstructured duties and indifferent duties? Effectively, the reply is kind of easy: unstructured process will inherit the guardian context, however indifferent duties will not inherit something from their guardian context (priorities, task-locals, deadlines).

@primary
struct MyProgram {
    
    static func primary() {
        Job(precedence: .background) {
            Job.indifferent {
                
                print(Job.currentPriority == .background)
                let x = await calculateFirstNumber()
                print("The which means of life is:", x)
                exit(EXIT_SUCCESS)
            }
        }
        dispatchMain()
    }
}

You’ll be able to create a indifferent process by utilizing the indifferent methodology, as you may see the precedence of the present process contained in the indifferent process is unspecified, which is certainly not equal with the guardian precedence. By the way in which it is usually attainable to get the present process by utilizing the withUnsafeCurrentTask perform. You should use this methodology too to get the precedence or test if the duty is cancelled. 🙅‍♂️

@primary
struct MyProgram {
    
    static func primary() {
        Job(precedence: .background) {
            Job.indifferent {
                withUnsafeCurrentTask { process in
                    print(process?.isCancelled ?? false)
                    print(process?.precedence == .unspecified)
                }
                let x = await calculateFirstNumber()
                print("The which means of life is:", x)
                exit(EXIT_SUCCESS)
            }
        }
        dispatchMain()
    }
}

There’s yet another massive distinction between indifferent and unstructured duties. Should you create an unstructured process from an actor, the duty will execute straight on that actor and NOT in parallel, however a indifferent process shall be instantly parallel. Which means that an unstructured process can alter inner actor state, however a indifferent process cannot modify the internals of an actor. ⚠️

You can too reap the benefits of unstructured duties in process teams to create extra advanced process constructions if the structured hierarchy will not suit your wants.

Job native values

There’s yet another factor I might like to point out you, we have talked about process native values numerous instances, so this is a fast part about them. This characteristic is mainly an improved model of the thread-local storage designed to play good with the structured concurrency characteristic in Swift.

Typically you want to hold on customized contextual information together with your duties and that is the place process native values are available in. For instance you possibly can add debug info to your process objects and use it to search out issues extra simply. Donny Wals has an in-depth article about process native values, if you’re extra about this characteristic, you need to positively learn his submit. 💪

So in follow, you may annotate a static property with the @TaskLocal property wrapper, after which you may learn this metadata inside an one other process. Any longer you may solely mutate this property by utilizing the withValue perform on the wrapper itself.

import Basis

enum TaskStorage {
    @TaskLocal static var title: String?
}

@primary
struct MyProgram {
    
    static func primary() async {
        await TaskStorage.$title.withValue("my-task") {
            let t1 = Job {
                print("unstructured:", TaskStorage.title ?? "n/a")
            }
            
            let t2 = Job.indifferent {
                print("indifferent:", TaskStorage.title ?? "n/a")
            }
            
            _ = await [t1.value, t2.value]
        }
    }
}

Duties will inherit these native values (besides indifferent) and you’ll alter the worth of process native values inside a given process as properly, however these modifications shall be solely seen for the present process & baby duties. To sum this up, process native values are at all times tied to a given process scope.

As you may see structured concurrency in Swift is rather a lot to digest, however when you perceive the fundamentals all the pieces comes properly along with the brand new async/await options and Duties you may simply assemble jobs for serial or parallel execution. Anyway, I hope you loved this text. 🙏