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SwiftNIO tutorial – The echo server

Intoducing SwiftNIO

For those who used a excessive stage internet framework, equivalent to Vapor, previously, you may had some interplay with occasion loops or guarantees. Nicely, these basic constructing blocks are a part of a low stage community framework, referred to 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 the whole lot on this information, so hopefully you will perceive the whole lot 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 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’re curious in regards to 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. 💬

To illustrate that you’ve got two machines, related by the community. How do they impart with one another? Nicely, similar to while you ship an everyday letter, first it’s important to specify the deal with of the recipient. In an effort to ship a message to a different laptop, it’s important to know its digital deal with too. This digital deal with known as IP deal with and it seems to be like this:

So you’ve got obtained the deal with, however generally this isn’t sufficient, as a result of a constructing can have a number of flats and it’s important to specify the precise letterbox in an effort to attain the precise individual. This will occur with computer systems too, the letterbox known as port quantity and the complete deal with of the goal could be created by combining the IP deal with and the port quantity (we name this full deal with as a community socket deal with or just socket, e.g. 💌

After you’ve got specified the precise deal with, 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 in regards to the supply standing, the digital model of this method 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 be sure that the letter really arrived and the recipient obtained 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. Once 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 completely correct, however hopefully you will get the essential thought. Now let’s speak about 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 for those who count on a letter? Aside from the thrill, most individuals continuously examine 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 examine if some knowledge arrived or not. 🤓

What occurs if a letter arrives? To begin with it’s important to go and get it out from the mailbox. In an effort to get it it’s important to stroll by 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 based mostly on the envelope you may carry out an motion. If it seems to be like a spam, you will throw it away, but when it is an essential letter you will probably open it, learn the contents and ship again a solution as quickly as potential. Let’s keep on 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 applying’s code. The channel’s accountability is to deal with inbound and outbound occasions, occurring by 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. 📨


The ChannelPipeline describes a set of actions about find out how to deal with the letters. One potential model is to decide based mostly on the envelope, you will throw it away if it seems to be like a spam, or open it if it seems to be 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. Briefly: a pipeline is a predefined sequence of handlers.


The ChannelHandler is the motion you could carry out while 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 essential 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 (normally I/O associated occasions, equivalent to «knowledge obtained») to occur after which fires some sort of callback after 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 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 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 accessible threads (occasion loops) this fashion the applying goes to be environment friendly and each thread will deal with nearly the identical quantity of shoppers.

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 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 important to add SwiftNIO as a bundle dependency contained in the Package deal.swift file.

import PackageDescription

let bundle = Package deal(
    identify: "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 mission file, we are able to simply create the SwiftNIO based mostly TCP server through the use of 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 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 accessible 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 look forward to incoming connections. 🧐

import NIO

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

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

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

As I discussed this, in an effort 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 potential 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)
            let message = enter.getString(at: 0, size: enter.readableBytes)
        else {
        var buff = context.channel.allocator.buffer(capability: message.rely)
        context.write(wrapOutboundOut(buff), promise: nil)

    func channelReadComplete(
        context: ChannelHandlerContext
    ) {

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

        context.shut(promise: nil)

For those who 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 potential 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 aid you to get began with SwiftNIO, I am additionally studying lots in regards to the framework these days, 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 prime efficiency protocol servers & shoppers. It is like Netty, however written for Swift.



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