SwiftNIO tutorial – The echo server

Intoducing SwiftNIO

If you happen to used a excessive degree net framework, reminiscent of 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’ll discuss 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 thing on this information, so hopefully you will perceive every 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 units. If you do not know a lot about TCP/IP and you’re curious concerning the underlying components, you may learn just a few different articles, however in a nutshell this mannequin permits us to speak with distant computer systems simply. 💬

For example that you’ve got two machines, linked by the community. How do they convey with one another? Properly, similar to once you ship an everyday letter, first you must specify the handle of the recipient. As a way to ship a message to a different laptop, you must know its digital handle too. This digital handle is known as IP handle and it appears like this: 127.0.0.1.

So you’ve got received the handle, however generally this isn’t sufficient, as a result of a constructing can have a number of residences and you must specify the precise letterbox with a view to attain the precise individual. This will occur with computer systems too, the letterbox is known as port quantity and the complete handle of the goal may be created by combining the IP handle and the port quantity (we name this full handle as a community socket handle or just socket, e.g. 127.0.0.1:80). 💌

After you’ve got specified the precise handle, 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 concerning the supply standing, the digital model of this strategy is known as Consumer Datagram Protocol (UDP).

The opposite (extra dependable) technique is to get a receipt concerning the supply, this manner you may ensure that the letter really arrived and the recipient received 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. While you talk via the community, this technique 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 will place an precise digital letterbox in entrance of the imaginary home. 📪

The essential constructing blocks of SwiftNIO

What do you do for those who anticipate a letter? Aside from the thrill, 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? Initially you must go and get it out from the mailbox. As a way to get it you must stroll via 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 based mostly 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 vital letter you will more than likely open it, learn the contents and ship again a solution as quickly as attainable. Let’s stick to 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 accountability is to deal with inbound and outbound occasions, taking place via 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 by way of a channel. 📨

ChannelPipeline

The ChannelPipeline describes a set of actions about easy methods to deal with the letters. One attainable model is to decide based mostly 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 for those who reply to the letter. Actions are referred to as as channel handlers in SwiftNIO. In brief: a pipeline is a predefined sequence of handlers.

ChannelHandler

The ChannelHandler is the motion which you could carry out once you open the letter. The channel handler has an enter and an output sort, which you need to use to learn the message utilizing the enter and reply to it utilizing the output. Okay, simply two extra vital 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 (normally I/O associated occasions, reminiscent of “information acquired”) to occur after which fires some type of callback once they do.

The fashionable CPUs have a restricted variety of cores, apps will more than likely 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 obtainable 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 will execute another duties within the meantime. When the IO operation finishes it will 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 all the time going to be related to precisely one occasion loop, this implies actions might be executed utilizing the identical context.

EventLoopGroup

The EventLoopGroup manages threads and occasion loops. The MultiThreadedEventLoopGroup goes to steadiness out shopper over the obtainable threads (occasion loops) this manner 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 discuss extra about Futures, Guarantees and the ByteBuffer sort, however I suppose this was greater than sufficient concept for now, so I am not going to dive into these type of objects, however spare them for upcoming articles. 😇

Constructing an echo server utilizing SwiftNIO

You can begin by creating a brand new executable Swift package deal, utilizing the Swift Bundle Supervisor. Subsequent you must add SwiftNIO as a package deal dependency contained in the Bundle.swift file.

import PackageDescription

let package deal = Bundle(
    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 undertaking file, we will simply create the SwiftNIO based mostly TCP server by utilizing the ServerBootstrap object. First we’ve got to instantiate a MultiThreadedEventLoopGroup with 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 attention-grabbing 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.

In case you are within the obtainable ChannelOptions, you may check out the NIO supply code, it additionally comprises some excellent docs about this stuff. The ultimate step is to bind the server bootstrap object to a given host and port, and await incoming connections. 🧐

import NIO

@important
public struct Server {
    
    public static func 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 taking place on the channel we’ve got can create a customized ChannelInboundHandler object. Contained in the channelRead perform it’s attainable 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

ultimate 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. Remember the fact that it is a quite simple TCP server, with out HTTP, however it’s attainable to write down express-like HTTP servers, JSON API servers, even a sport backend and lots of different cool and loopy performant stuff utilizing SwiftNIO. I hope this tutorial will enable you to get began with SwiftNIO, I am additionally studying rather a lot concerning 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 software framework for prime efficiency protocol servers & purchasers. It is like Netty, however written for Swift.