WebSocket in Kotlin: Real-Time Applications from A to Z

WebSocket in Kotlin: Building Real-Time Applications from Scratch

Modern web applications increasingly require instant data transfer between client and server. Traditional HTTP, operating on a request-response principle, is not suitable for tasks requiring constant two-way communication — chats, online games, stock quotes, or real-time notifications. This is where the WebSocket protocol comes to the rescue. In this article, we will explore how to build real-time applications in Kotlin using WebSocket, from basic theory to practical examples.

Kotlin, thanks to its conciseness, safety, and full compatibility with the Java ecosystem, is an excellent choice for server-side development. As the main framework, we will use Ktor — a modern asynchronous framework for Kotlin with built-in WebSocket support.

What is WebSocket and Why Do You Need It?

WebSocket is a protocol that provides full-duplex (two-way) communication between a client and a server over a single persistent TCP connection. Unlike HTTP, where the client always initiates the request, WebSocket allows the server to send data to the client independently at any time.

Key advantages of WebSocket:

• Low latency: No overhead for establishing a new connection for each message.
• Efficiency: Minimal overhead traffic (only 2 bytes per message after connection establishment).
• Persistent connection: The server can send data in real-time without polling from the client.

WebSocket is ideal for:

• Online chats and messengers
• Collaborative document editing
• Stock and financial quotes
• Multiplayer games
• Push notifications and monitoring

Setting Up a Ktor Project for WebSocket

Before writing code, you need to set up the project. The easiest way is through IntelliJ IDEA with the Ktor plugin, or manually via Gradle.

Step 1: Creating the Project

Create a new Gradle project with Kotlin (JVM). Add the following dependencies to the build.gradle.kts file:

plugins {    kotlin("jvm") version "1.9.22"    id("io.ktor.plugin") version "2.3.7"}

application {    mainClass.set("ApplicationKt")}

dependencies {    implementation("io.ktor:ktor-server-core:2.3.7")    implementation("io.ktor:ktor-server-netty:2.3.7")    implementation("io.ktor:ktor-server-websockets:2.3.7")    implementation("ch.qos.logback:logback-classic:1.4.14")}

Step 2: Basic Server Structure

Create a file Application.kt and write a minimal WebSocket server:

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("Welcome to the chat!")                        for (frame in incoming) {                if (frame is Frame.Text) {                    val text = frame.readText()                    send("You wrote: $text")                    println("Received: $text")                }            }        }    }.start(wait = true)}

This code starts a server on port 8080 that accepts WebSocket connections at the /chat endpoint. Upon connection, the client receives a welcome message, and then the server echoes back each text message.

Creating an Advanced Chat with Rooms

Now let's move on to a more complex example — a multi