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SwiftNIO tutorial – The echo server

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Intoducing SwiftNIO

In the event you used a excessive stage net framework, equivalent to Vapor, previously, you may had some interplay with occasion loops or guarantees. Properly, these elementary constructing blocks are a part of a low stage community framework, known as SwiftNIO, which I’ll discuss 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 thing on this information, so hopefully you may 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 elements, you may learn a number of different articles, however in a nutshell this mannequin permits us to speak with distant computer systems simply. πŸ’¬

As an instance that you’ve got two machines, related by the community. How do they convey with one another? Properly, similar to whenever you ship a daily letter, first you must specify the deal with of the recipient. So as to ship a message to a different pc, you must know its digital deal with too. This digital deal with known as IP deal with and it seems like this:

So you’ve got obtained the deal with, however typically this isn’t sufficient, as a result of a constructing can have a number of flats and you must specify the precise letterbox to be able to attain the precise particular person. This could 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. πŸ’Œ

After you’ve got specified the precise deal with, 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 answer is to easily ship it with out figuring out a lot concerning the supply standing, the digital model of this strategy known as Person Datagram Protocol (UDP).

The opposite (extra dependable) methodology is to get a receipt concerning the supply, this manner you may ensure that the letter really arrived and the recipient obtained 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. Once you talk by way of the community, this methodology known as Transmission Management Protocol (TCP).

Okay, that is greater than sufficient community idea, I do know it is a excessive stage abstraction and never totally correct, however hopefully you may get the essential concept. 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. πŸ“ͺ

The fundamental constructing blocks of SwiftNIO

What do you do should you count on a letter? Other than the thrill, most individuals continuously test their mailboxes to see if it is already there or not. They’re listening for the noises of the postman, similar to pc packages hear on a given port to test if some knowledge arrived or not. πŸ€“

What occurs if a letter arrives? Initially you must go and get it out from the mailbox. So as to get it you must 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 seems like a spam, you may throw it away, but when it is an essential letter you may most 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 applying’s code. The channel’s duty 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, it’s best to think about it because the hallway to the mailbox, actually the messages are going journey to you through a channel. πŸ“¨


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


The ChannelHandler is the motion that you may carry out whenever you open the letter. The channel handler has an enter and an output kind, which you should utilize 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 once they do.

The fashionable 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 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 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.


The EventLoopGroup manages threads and occasion loops. The MultiThreadedEventLoopGroup goes to steadiness out consumer over the obtainable threads (occasion loops) this manner 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 kind, however I suppose this was greater than sufficient idea for now, so I am not going to dive into these sort of objects, however spare them for upcoming articles. πŸ˜‡

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(
    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 subsequent step is to change the primary undertaking file, we are able to simply create the SwiftNIO primarily based TCP server through the use of the ServerBootstrap object. First now we have to instantiate a MultiThreadedEventLoopGroup with plenty 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 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 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 watch for 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, to be able to deal with an occasion occurring on the channel now we have 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)
            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)

In the event 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. Understand 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 sport backend and lots of different cool and loopy performant stuff utilizing SwiftNIO. I hope this tutorial will make it easier to to get began with SwiftNIO, I am additionally studying lots concerning the framework these days, so please forgive me (and even appropriate me) if I missed / tousled one thing. πŸ˜…

So once more: SwiftNIO a (low-level) non-blocking event-driven community software framework for top efficiency protocol servers & shoppers. It is like Netty, however written for Swift.

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