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SwiftNIO tutorial – The echo server

Intoducing SwiftNIO

Should you used a excessive degree internet framework, similar to Vapor, up to now, you may had some interplay with occasion loops or guarantees. Effectively, these elementary constructing blocks are a part of a low degree community framework, known as SwiftNIO, which I will speak about on this tutorial.

Don’t fret if you have not heard about occasion loops or non-blocking IO simply but, I will attempt to clarify the whole lot on this information, so hopefully you may perceive the whole lot even if you’re a whole newbie to this subject. 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 units. 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 got two machines, linked by the community. How do they convey with one another? Effectively, similar to whenever you ship a daily letter, first it’s a must to specify the tackle of the recipient. As a way to ship a message to a different laptop, it’s a must to know its digital tackle too. This digital tackle known as IP tackle and it appears like this:

So you have acquired the tackle, 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 with the intention to attain the precise individual. This could occur with computer systems too, the letterbox known as port quantity and the complete tackle of the goal may 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. 💌

After you have specified the precise tackle, you may want somebody to truly 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 understanding a lot concerning the supply standing, the digital model of this method known as Consumer Datagram Protocol (UDP).

The opposite (extra dependable) technique is to get a receipt concerning the supply, this fashion you may ensure that the letter really 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. Whenever you talk by way of the community, this technique 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 solely correct, however hopefully you may 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 essential constructing blocks of SwiftNIO

What do you do in the event 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 start 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 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 like a spam, you may throw it away, but when it is an essential letter you may probably 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.


A Channel connects the underlying community socket with the appliance’s code. The channel’s accountability is to deal with inbound and outbound occasions, occurring 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. 📨


The ChannelPipeline describes a set of actions about how one can deal with the letters. One doable model is to decide primarily based on the envelope, you may throw it away if it appears like a spam, or open it if it appears like a proper letter, it is also an motion in the event you reply to the letter. Actions are known as as channel handlers in SwiftNIO. In brief: a pipeline is a predefined sequence of handlers.


The ChannelHandler is the motion that you could carry out whenever you open the letter. The channel handler has an enter and an output sort, which you should 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 will present you some actual examples afterwards. 🐻


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 acquired”) to occur after which fires some type of callback once they do.

The fashionable CPUs have a restricted variety of cores, apps will probably 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 change 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 at all times going to be related to precisely one occasion loop, this implies actions shall be executed utilizing the identical context.


The EventLoopGroup manages threads and occasion loops. The MultiThreadedEventLoopGroup goes to steadiness 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 parts

There are another SwiftNIO parts, we may speak 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 it’s a must to add SwiftNIO as a package deal dependency contained in the Bundle.swift file.

import PackageDescription

let package deal = Bundle(
    identify: "echo-server",
    platforms: [
    dependencies: [
            url: "",
            from: "2.0.0"
    targets: [
            name: "Server",
            dependencies: [
                    name: "NIO",
                    package: "swift-nio"

The following step is to change the primary undertaking file, we will simply create the SwiftNIO primarily based TCP server by utilizing the ServerBootstrap object. First we now have 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 attention-grabbing 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.

If you’re within the obtainable ChannelOptions, you may check out the NIO supply code, it additionally comprises some superb 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

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

        defer {
            strive! eventLoopGroup.syncShutdownGracefully()

        let serverBootstrap = ServerBootstrap(
            group: eventLoopGroup
            worth: 256
            worth: 1
        .childChannelInitializer { channel in
            worth: 1
            worth: 16
            worth: AdaptiveRecvByteBufferAllocator()

        let defaultHost = "" 
        let defaultPort = 8888

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

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

As I discussed this, with the intention to deal with an occasion occurring on the channel we now have 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)
            let message = enter.getString(at: 0, size: enter.readableBytes)
        else {
        var buff = message.depend)
        context.write(wrapOutboundOut(buff), promise: nil)

    func channelReadComplete(
        context: ChannelHandlerContext
    ) {

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

        context.shut(promise: nil)

Should you run the app and hook up with it utilizing the telnet 8888 command you may enter some textual content and the server will echo it again to you. Take into account that 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 assist you to to get began with SwiftNIO, I am additionally studying loads concerning the framework recently, 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 top efficiency protocol servers & purchasers. It is like Netty, however written for Swift.



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