Home IOS Development SwiftNIO tutorial – The echo server

SwiftNIO tutorial – The echo server

SwiftNIO tutorial – The echo server


Intoducing SwiftNIO

When you used a excessive stage internet framework, corresponding to Vapor, up to now, you may had some interplay with occasion loops or guarantees. Nicely, these basic constructing blocks are a part of a low stage community framework, known as SwiftNIO, which I’ll 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 every part on this information, so hopefully you will perceive every part even in case you are 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’ll be able to learn a number of different articles, however in a nutshell this mannequin permits us to speak with distant computer systems simply. 💬

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

So you have received the handle, however generally this isn’t sufficient, as a result of a constructing can have a number of flats and it’s a must to specify the precise letterbox with the intention to attain the precise individual. This may occur with computer systems too, the letterbox known as port quantity and the total handle of the goal will 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. 💌

After you have specified the precise handle, you will 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 resolution is to easily ship it with out realizing a lot concerning the supply standing, the digital model of this strategy known as Consumer Datagram Protocol (UDP).

The opposite (extra dependable) methodology is to get a receipt concerning the supply, this fashion you’ll be able to be sure that the letter truly 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. Whenever you talk by the community, this methodology known as Transmission Management Protocol (TCP).

Okay, that is greater than sufficient community concept, I do know it is a excessive stage abstraction and never solely correct, however hopefully you will get the essential concept. 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 in the event you anticipate a letter? Aside from the thrill, most individuals continually test their mailboxes to see if it is already there or not. They’re listening for the noises of the postman, similar to laptop packages hear on a given port to test if some knowledge 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 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 in some way first, then based mostly on the envelope you’ll be able to carry out an motion. If it appears like a spam, you will throw it away, but when it is an necessary letter you will most probably open it, learn the contents and ship again a solution as quickly as doable. Let’s stick to this analogy, and let me clarify this once more, however this time utilizing SwiftNIO phrases.


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 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 through a channel. 📨


The ChannelPipeline describes a set of actions about the best way to deal with the letters. One doable 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 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 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 necessary phrases, bear with me for a second, I’ll 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, corresponding to “knowledge acquired”) to occur after which fires some type of callback after they do.

The trendy CPUs have a restricted variety of cores, apps will most 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 out there 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 might be executed utilizing the identical context.


The EventLoopGroup manages threads and occasion loops. The MultiThreadedEventLoopGroup goes to steadiness out shopper over the out there threads (occasion loops) this fashion the applying 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 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: [
    dependencies: [
            url: "https://github.com/apple/swift-nio",
            from: "2.0.0"
    targets: [
            name: "Server",
            dependencies: [
                    name: "NIO",
                    package: "swift-nio"

The following step is to change the primary venture file, we will simply create the SwiftNIO based mostly TCP server through the use of the ServerBootstrap object. First now we 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 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 out there ChannelOptions, you’ll be able to check out the NIO supply code, it additionally comprises some excellent docs about these items. The ultimate step is to bind the server bootstrap object to a given host and port, and look forward to incoming connections. 🧐

import NIO

public struct Server {
    public static func predominant() 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 taking place on the channel now we have can create a customized ChannelInboundHandler object. Contained in the channelRead perform 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

remaining class EchoHandler: ChannelInboundHandler {

    typealias InboundIn = ByteBuffer
    typealias OutboundOut = ByteBuffer

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

    func channelReadComplete(
        context: ChannelHandlerContext
    ) {

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

        context.shut(promise: nil)

When you run the app and connect with it utilizing the telnet 8888 command you’ll be able to enter some textual content and the server will echo it again to you. Understand that this can be 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 recreation backend and plenty of different cool and loopy performant stuff utilizing SwiftNIO. I hope this tutorial will aid you to get began with SwiftNIO, I am additionally studying quite a bit concerning the framework recently, 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 top efficiency protocol servers & purchasers. It is like Netty, however written for Swift.



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