WebSocket in Kotlin: Building Real-Time Applications

Introduction to WebSocket in Kotlin: from theory to real-time practice

Modern web applications increasingly require instant data exchange between client and server. The traditional HTTP protocol, operating on a "request-response" principle, is not suitable for tasks where information must arrive in real time: online chats, stock quotes, multiplayer games, or push notifications. This is where WebSocket comes to the rescue — a protocol that provides a permanent two-way connection.

Kotlin, being a modern language for JVM, JavaScript and Native, provides excellent tools for working with WebSocket. In this article, we will look at how to create real-time applications in Kotlin using the Ktor library — a popular framework for building asynchronous servers and clients. You will learn how to set up a WebSocket connection, send and receive messages, handle errors, and build scalable solutions.

1. WebSocket basics: why Kotlin and Ktor?

WebSocket is a protocol that, after establishing a connection via an HTTP handshake, switches the channel to a permanent two-way mode. Unlike HTTP, the server can initiate data sending without a client request. This is ideal for real-time scenarios.

Kotlin paired with Ktor offers:

• Asynchrony — Kotlin coroutines allow handling thousands of simultaneous connections without blocking threads.
• Cross-platform — the same code can run on JVM, JavaScript and Native.
• Conciseness — minimum boilerplate, maximum expressiveness.
• Security — built-in support for SSL/TLS, authentication and CORS.

Let's move on to practice and create a simple WebSocket server and client in Kotlin.

2. Creating a WebSocket server in Kotlin with Ktor

2.1. Project setup

First, create a new Gradle or Maven project. Add Ktor dependencies to build.gradle.kts:

dependencies {    implementation("io.ktor:ktor-server-core:2.3.0")    implementation("io.ktor:ktor-server-netty:2.3.0")    implementation("io.ktor:ktor-server-websockets:2.3.0")    implementation("ch.qos.logback:logback-classic:1.4.11")}

2.2. Server implementation

Let's create a WebSocket server that accepts connections, reads messages and responds to them. The example below uses the Ktor library with coroutines:

import io.ktor.server.engine.*import io.ktor.server.netty.*import io.ktor.server.websocket.*import io.ktor.websocket.*import kotlinx.coroutines.delay

fun main() {    embeddedServer(Netty, port = 8080) {        install(WebSockets)        webSocket("/chat") {            send(Frame.Text("Welcome to the chat!"))            for (frame in incoming) {                if (frame is Frame.Text) {                    val text = frame.readText()                    println("Received: $text")                    send(Frame.Text("Echo: $text"))                }            }        }    }.start(wait = true)}

Here we:

• Install the WebSockets plugin.
• Define the /chat endpoint.
• Process incoming frames in a coroutine and send an echo response.

2.3. Error handling and connection closing

In real applications, it is important to correctly handle connection breaks. Let's add exception handling and session closing:

webSocket("/chat") {    try {        send(Frame.Text("Connection established"))        for (frame in incoming) {            when (frame) {                is Frame.Text -> {                    val text = frame.readText()                    if (text.equals("exit", ignoreCase = true)) {                        cl