SwiftNIO tutorial – The echo server

Intoducing SwiftNIO

In case you used a excessive stage net framework, akin to Vapor, prior to now, you may had some interplay with occasion loops or guarantees. Nicely, these elementary constructing blocks are a part of a low stage community framework, known as SwiftNIO, which I’ll speak about on this tutorial.

Don’t be concerned if you have not heard about occasion loops or non-blocking IO simply but, I will attempt to clarify every part on this information, so hopefully you will perceive every part even in case you are an entire newbie to this matter. Let’s begin with some fundamentals about networks and computer systems.

Let’s speak about TCP/IP

It began on January 1st, 1983. The web was born (as some say) and folks began to formally use the web protocol suite (TCP/IP) to speak between gadgets. If you do not know a lot about TCP/IP and you might be curious concerning the underlying components, you’ll be able to learn a couple of different articles, however in a nutshell this mannequin permits us to speak with distant computer systems simply. ?

As an example that you’ve got two machines, related by the community. How do they convey with one another? Nicely, similar to while you ship a daily letter, first it’s a must to specify the deal with of the recipient. As a way to ship a message to a different laptop, it’s a must to know its digital deal with too. This digital deal with known as IP deal with and it seems like this: 127.0.0.1.

So you have bought the deal with, however generally this isn’t sufficient, as a result of a constructing can have a number of residences and it’s a must to specify the precise letterbox in an effort to attain the precise individual. This could occur with computer systems too, the letterbox known as port quantity and the total deal with of the goal may be created by combining the IP deal with and the port quantity (we name this full deal with as a community socket deal with or just socket, e.g. 127.0.0.1:80). ?

After you have specified the precise deal with, you will want somebody to really ship the letter containing your message. The postal supply service can switch your letter, there are two methods to ship it over to the recipient. The primary answer is to easily ship it with out realizing a lot concerning the supply standing, the digital model of this method known as Consumer Datagram Protocol (UDP).

The opposite (extra dependable) methodology is to get a receipt concerning the supply, this manner you’ll be able to make it possible for the letter truly arrived and the recipient bought it. Though, the postman can open your letter and alter your message, but it surely’ll be nonetheless delivered and you will get a notification about this. Once you talk by way of the community, this methodology known as Transmission Management Protocol (TCP).

Okay, that is greater than sufficient community idea, I do know it is a excessive stage abstraction and never solely correct, however hopefully you will get the fundamental thought. Now let’s speak about what occurs contained in the machine and the way we will place an precise digital letterbox in entrance of the imaginary home. ?

The fundamental constructing blocks of SwiftNIO

What do you do when you anticipate a letter? Other than the joy, most individuals always verify their mailboxes to see if it is already there or not. They’re listening for the noises of the postman, similar to laptop packages pay attention on a given port to verify if some information arrived or not. ?

What occurs if a letter arrives? To begin with it’s a must to go and get it out from the mailbox. As a way to get it it’s a must to stroll by way of the hallway or down the steps or you’ll be able to ask another person to ship the letter for you. Anyway, ought to get the letter one way or the other first, then based mostly on the envelope you’ll be able to carry out an motion. If it seems like a spam, you will throw it away, but when it is an essential letter you will almost certainly open it, learn the contents and ship again a solution as quickly as doable. Let’s persist with this analogy, and let me clarify this once more, however this time utilizing SwiftNIO phrases.

Channel

A Channel connects the underlying community socket with the applying’s code. The channel’s duty is to deal with inbound and outbound occasions, taking place by way of the socket (or file descriptor). In different phrases, it is the channel that connects the mailbox with you, you need to think about it because the hallway to the mailbox, actually the messages are going journey to you through a channel. ?

ChannelPipeline

The ChannelPipeline describes a set of actions about easy methods to deal with the letters. One doable model is to decide based mostly on the envelope, you will throw it away if it seems like a spam, or open it if it seems like a proper letter, it is also an motion when you reply to the letter. Actions are known as as channel handlers in SwiftNIO. Briefly: a pipeline is a predefined sequence of handlers.

ChannelHandler

The ChannelHandler is the motion which you can carry out while you open the letter. The channel handler has an enter and an output kind, which you need to use to learn the message utilizing the enter and reply to it utilizing the output. Okay, simply two extra essential phrases, bear with me for a second, I’ll present you some actual examples afterwards. ?

EventLoop

The EventLoop works similar to a run loop or a dispatch queue. What does this imply?

The occasion loop is an object that waits for occasions (often I/O associated occasions, akin to “information acquired”) to occur after which fires some sort of callback once they do.

The fashionable CPUs have a restricted variety of cores, apps will almost certainly affiliate one thread (of execution) per core. Switching between thread contexts can also be inefficient. What occurs when an occasion has to attend for one thing and a thread turns into accessible for different duties? In SwiftNIO the occasion loop will obtain the incoming message, course of it, and if it has to attend for one thing (like a file or database learn) it’s going to execute another duties within the meantime. When the IO operation finishes it’s going to swap again to the duty and it will name again to your code when it is time. Or one thing like this, however the primary takeaway right here is that your channel handler is all the time going to be related to precisely one occasion loop, this implies actions will probably be executed utilizing the identical context.

EventLoopGroup

The EventLoopGroup manages threads and occasion loops. The MultiThreadedEventLoopGroup goes to steadiness out consumer over the accessible threads (occasion loops) this manner the applying goes to be environment friendly and each thread will deal with nearly the identical quantity of purchasers.

Different elements

There are another SwiftNIO elements, we might speak extra about Futures, Guarantees and the ByteBuffer kind, however I suppose this was greater than sufficient idea for now, so I am not going to dive into these sort of objects, however spare them for upcoming articles. ?

Constructing an echo server utilizing SwiftNIO

You can begin by creating a brand new executable Swift bundle, utilizing the Swift Bundle Supervisor. Subsequent it’s a must to add SwiftNIO as a bundle dependency contained in the Bundle.swift file.

import PackageDescription

let bundle = Bundle(
    title: "echo-server",
    platforms: [
       .macOS(.v10_15),
    ],
    dependencies: [
        .package(
            url: "https://github.com/apple/swift-nio",
            from: "2.0.0"
        ),
    ],
    targets: [
        .executableTarget(
            name: "Server",
            dependencies: [
                .product(
                    name: "NIO",
                    package: "swift-nio"
                )
            ]
        ),
    ]
)

The subsequent step is to change the primary mission file, we will simply create the SwiftNIO based mostly TCP server by utilizing the ServerBootstrap object. First we now have to instantiate a MultiThreadedEventLoopGroup with quite a lot of threads, utilizing the CPU cores within the system.

Then we configure the server by including some channel choices. You do not have to know a lot about these simply but, the fascinating half is contained in the childChannelInitializer block. We create the precise channel pipeline there. Our pipeline will encompass two handlers, the primary one is the built-in BackPressureHandler, the second goes to be our customized made EchoHandler object.

In case you are within the accessible ChannelOptions, you’ll be able to check out the NIO supply code, it additionally accommodates some excellent docs about this stuff. The ultimate step is to bind the server bootstrap object to a given host and port, and look ahead to incoming connections. ?

import NIO

@most important
public struct Server {
    
    public static func most important() throws {
        let eventLoopGroup = MultiThreadedEventLoopGroup(
            numberOfThreads: System.coreCount
        )

        defer {
            strive! eventLoopGroup.syncShutdownGracefully()
        }

        let serverBootstrap = ServerBootstrap(
            group: eventLoopGroup
        )
        .serverChannelOption(
            ChannelOptions.backlog,
            worth: 256
        )
        .serverChannelOption(
            ChannelOptions.socketOption(.so_reuseaddr),
            worth: 1
        )
        .childChannelInitializer { channel in
            channel.pipeline.addHandlers([
                BackPressureHandler(),
                EchoHandler(),
            ])
        }
        .childChannelOption(
            ChannelOptions.socketOption(.so_reuseaddr),
            worth: 1
        )
        .childChannelOption(
            ChannelOptions.maxMessagesPerRead,
            worth: 16
        )
        .childChannelOption(
            ChannelOptions.recvAllocator,
            worth: AdaptiveRecvByteBufferAllocator()
        )

        let defaultHost = "127.0.0.1" 
        let defaultPort = 8888

        let channel = strive serverBootstrap.bind(
            host: defaultHost,
            port: defaultPort
        )
        .wait()

        print("Server began and listening on (channel.localAddress!)")
        strive channel.closeFuture.wait()
        print("Server closed")
    }
}

As I discussed this, in an effort to deal with an occasion taking place on the channel we now have can create a customized ChannelInboundHandler object. Contained in the channelRead perform it’s doable to unwrap the inbound information right into a ByteBuffer object and write the enter message onto the output as a wrapped NIOAny object.

Problem: write a server that may print colourful messages. Trace: constructing a textual content modifying server.

import NIO

remaining class EchoHandler: ChannelInboundHandler {

    typealias InboundIn = ByteBuffer
    typealias OutboundOut = ByteBuffer

    func channelRead(
        context: ChannelHandlerContext,
        information: NIOAny
    ) {
        let enter = self.unwrapInboundIn(information)
        guard
            let message = enter.getString(at: 0, size: enter.readableBytes)
        else {
            return
        }
        
        var buff = context.channel.allocator.buffer(capability: message.rely)
        buff.writeString(message)
        context.write(wrapOutboundOut(buff), promise: nil)
    }


    func channelReadComplete(
        context: ChannelHandlerContext
    ) {
        context.flush()
    }

    func errorCaught(
        context: ChannelHandlerContext,
        error: Error
    ) {
        print(error)

        context.shut(promise: nil)
    }
}

In case you run the app and connect with it utilizing the telnet 127.0.0.1 8888 command you’ll be able to enter some textual content and the server will echo it again to you. Understand that it is a quite simple TCP server, with out HTTP, however it’s doable to put in writing express-like HTTP servers, JSON API servers, even a sport backend and plenty of different cool and loopy performant stuff utilizing SwiftNIO. I hope this tutorial will make it easier to to get began with SwiftNIO, I am additionally studying so much concerning the framework these days, so please forgive me (and even right me) if I missed / tousled one thing. ?

So once more: SwiftNIO a (low-level) non-blocking event-driven community software framework for prime efficiency protocol servers & purchasers. It is like Netty, however written for Swift.