SwiftNIO tutorial – The echo server

Intoducing SwiftNIO

If you happen to used a excessive degree net framework, similar to Vapor, prior to now, you may had some interplay with occasion loops or guarantees. Properly, these elementary constructing blocks are a part of a low degree community framework, referred to as SwiftNIO, which I will discuss on this tutorial.

Don’t fret if you have not heard about occasion loops or non-blocking IO simply but, I am going to attempt to clarify every little thing on this information, so hopefully you will perceive every little thing even if you’re a whole newbie to this matter. Let’s begin with some fundamentals about networks and computer systems.

Let’s discuss 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 in regards to the underlying elements, you may learn a number of different articles, however in a nutshell this mannequin permits us to speak with distant computer systems simply. 💬

For instance that you’ve two machines, linked by the community. How do they impart with one another? Properly, similar to if you ship a daily letter, first it’s a must to specify the tackle of the recipient. With a purpose to ship a message to a different laptop, it’s a must to know its digital tackle too. This digital tackle is known as IP tackle and it appears like this: 127.0.0.1.

So you have acquired the tackle, however typically 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 with a view to attain the precise individual. This may occur with computer systems too, the letterbox is known as port quantity and the complete tackle of the goal might be created by combining the IP tackle and the port quantity (we name this full tackle as a community socket tackle or just socket, e.g. 127.0.0.1:80). 💌

After you have specified the precise tackle, 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 figuring out a lot in regards to the supply standing, the digital model of this strategy is known as Consumer Datagram Protocol (UDP).

The opposite (extra dependable) methodology is to get a receipt in regards to the supply, this fashion you may ensure that the letter truly arrived and the recipient acquired it. Though, the postman can open your letter and alter your message, nevertheless it’ll be nonetheless delivered and you will get a notification about this. While you talk by means of the community, this methodology is known as Transmission Management Protocol (TCP).

Okay, that is greater than sufficient community concept, I do know it is a excessive degree abstraction and never fully correct, however hopefully you will get the fundamental concept. Now let’s discuss what occurs contained in the machine and the way we are able to place an precise digital letterbox in entrance of the imaginary home. 📪

The fundamental constructing blocks of SwiftNIO

What do you do should you count on a letter? Aside from the thrill, most individuals continually verify their mailboxes to see if it is already there or not. They’re listening for the noises of the postman, similar to laptop applications pay attention on a given port to verify if some information arrived or not. 🤓

What occurs if a letter arrives? Initially it’s a must to go and get it out from the mailbox. With a purpose to get it it’s a must to stroll by means of the hallway or down the steps or you may ask another person to ship the letter for you. Anyway, ought to get the letter one way or the other first, then primarily based on the envelope you may carry out an motion. If it appears like a spam, you will throw it away, but when it is an necessary letter you will almost definitely open it, learn the contents and ship again a solution as quickly as doable. Let’s follow 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 appliance’s code. The channel’s duty is to deal with inbound and outbound occasions, occurring by means of the socket (or file descriptor). In different phrases, it is the channel that connects the mailbox with you, you must 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 tips on how to deal with the letters. One doable model is to decide primarily based on the envelope, you will throw it away if it appears like a spam, or open it if it appears like a proper letter, it is also an motion should you reply to the letter. Actions are referred to as as channel handlers in SwiftNIO. Briefly: a pipeline is a predefined sequence of handlers.

ChannelHandler

The ChannelHandler is the motion you could carry out if you open the letter. The channel handler has an enter and an output kind, which you should utilize to learn the message utilizing the enter and reply to it utilizing the output. Okay, simply two extra necessary phrases, bear with me for a second, I will 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, similar to “information obtained”) to occur after which fires some sort of callback after they do.

The trendy CPUs have a restricted variety of cores, apps will almost definitely 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 principle takeaway right here is that your channel handler is at all times 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 stability out consumer over the accessible threads (occasion loops) this fashion the appliance goes to be environment friendly and each thread will deal with nearly the identical quantity of purchasers.

Different parts

There are another SwiftNIO parts, we may speak extra about Futures, Guarantees and the ByteBuffer kind, however I suppose this was greater than sufficient concept 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 Package deal Supervisor. Subsequent it’s a must to add SwiftNIO as a bundle dependency contained in the Package deal.swift file.

import PackageDescription

let bundle = Package deal(
    identify: "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 principle mission file, we are able to simply create the SwiftNIO primarily based TCP server through the use of the ServerBootstrap object. First we’ve got to instantiate a MultiThreadedEventLoopGroup with various 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 include two handlers, the primary one is the built-in BackPressureHandler, the second goes to be our customized made EchoHandler object.

If you’re within the accessible ChannelOptions, you may 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 anticipate incoming connections. 🧐

import NIO

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

        defer {
            attempt! 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 = attempt serverBootstrap.bind(
            host: defaultHost,
            port: defaultPort
        )
        .wait()

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

As I discussed this, with a view to deal with an occasion occurring on the channel we’ve got can create a customized ChannelInboundHandler object. Contained in the channelRead operate 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

closing 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)
    }
}

If you happen to run the app and connect with it utilizing the telnet 127.0.0.1 8888 command you may enter some textual content and the server will echo it again to you. Needless to say this can be a quite simple TCP server, with out HTTP, however it’s doable to write down express-like HTTP servers, JSON API servers, even a recreation backend and plenty of different cool and loopy performant stuff utilizing SwiftNIO. I hope this tutorial will enable you to to get began with SwiftNIO, I am additionally studying loads in regards to the framework currently, so please forgive me (and even appropriate me) if I missed / tousled one thing. 😅

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