SwiftNIO tutorial – The echo server

Intoducing SwiftNIO

When you used a excessive stage net framework, similar to Vapor, prior to now, you may had some interplay with occasion loops or guarantees. Effectively, these basic constructing blocks are a part of a low stage community framework, known 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 the whole lot on this information, so hopefully you may perceive the whole lot even if you’re an entire newbie to this subject. 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 other people 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 may learn a couple 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 convey with one another? Effectively, identical to whenever you ship an everyday letter, first you need to specify the handle of the recipient. As a way to ship a message to a different pc, you need to know its digital handle too. This digital handle known as IP handle and it appears to be like like this: 127.0.0.1.

So you have bought the handle, however generally this isn’t sufficient, as a result of a constructing can have a number of residences and you need to specify the precise letterbox to be able to attain the precise individual. This will occur with computer systems too, the letterbox known as port quantity and the total handle of the goal could 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 have specified the precise handle, you may 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 resolution is to easily ship it with out understanding a lot concerning the supply standing, the digital model of this strategy known as Consumer Datagram Protocol (UDP).

The opposite (extra dependable) technique is to get a receipt concerning the supply, this fashion you may make it possible for the letter truly arrived and the recipient bought 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. If you talk via the community, this technique known as Transmission Management Protocol (TCP).

Okay, that is greater than sufficient community principle, I do know it is a excessive stage abstraction and never fully correct, however hopefully you may get the essential thought. 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 if you happen to anticipate a letter? Other than the thrill, most individuals always examine their mailboxes to see if it is already there or not. They’re listening for the noises of the postman, identical to pc applications pay attention on a given port to examine if some knowledge arrived or not. 🤓

What occurs if a letter arrives? To start with you need to go and get it out from the mailbox. As a way to get it you need to 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 in some way first, then primarily based on the envelope you may carry out an motion. If it appears to be like like a spam, you may throw it away, but when it is an necessary letter you may probably 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, taking place via the socket (or file descriptor). In different phrases, it is the channel that connects the mailbox with you, it’s best to 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 doable model is to decide primarily based on the envelope, you may throw it away if it appears to be like like a spam, or open it if it appears to be like like a proper letter, it is also an motion if you happen to 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 that you would be able to carry out whenever 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 necessary phrases, bear with me for a second, I will present you some actual examples afterwards. 🐻

EventLoop

The EventLoop works identical 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, similar to “knowledge obtained”) to occur after which fires some form of callback once they do.

The trendy CPUs have a restricted variety of cores, apps will probably affiliate one thread (of execution) per core. Switching between thread contexts can 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’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 likely be executed utilizing the identical context.

EventLoopGroup

The EventLoopGroup manages threads and occasion loops. The MultiThreadedEventLoopGroup goes to stability out consumer over the obtainable 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 elements

There are another SwiftNIO elements, we may speak extra about Futures, Guarantees and the ByteBuffer sort, however I suppose this was greater than sufficient principle for now, so I am not going to dive into these form 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 Package deal Supervisor. Subsequent you need to add SwiftNIO as a package deal dependency contained in the Package deal.swift file.

import PackageDescription

let package deal = 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 following step is to change the primary challenge file, we are able to simply create the SwiftNIO primarily based TCP server through the use of the ServerBootstrap object. First we’ve 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 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 obtainable ChannelOptions, you may check out the NIO supply code, it additionally incorporates some excellent docs about this stuff. The ultimate step is to bind the server bootstrap object to a given host and port, and watch for incoming connections. 🧐

import NIO

@principal
public struct Server {
    
    public static func principal() 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, to be able to deal with an occasion taking place on the channel we’ve can create a customized ChannelInboundHandler object. Contained in the channelRead operate it’s doable to unwrap the inbound knowledge 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

last class EchoHandler: ChannelInboundHandler {

    typealias InboundIn = ByteBuffer
    typealias OutboundOut = ByteBuffer

    func channelRead(
        context: ChannelHandlerContext,
        knowledge: NIOAny
    ) {
        let enter = self.unwrapInboundIn(knowledge)
        guard
            let message = enter.getString(at: 0, size: enter.readableBytes)
        else {
            return
        }
        
        var buff = context.channel.allocator.buffer(capability: message.depend)
        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)
    }
}

When you run the app and hook up 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 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 assist you to to get began with SwiftNIO, I am additionally studying lots 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.