WebSocket

Learn realtime communication with Ktor WebSocket.

What is WebSocket?

WebSocket is a bidirectional, persistent connection between client and server. Unlike HTTP, the connection stays open; both sides send data instantly.

In Ktor a WebSocket route is declared with routing { webSocket("/ws") { ... } }. Inside, incoming and outgoing channels exchange data. Example: a chat app, live alerts, game data stream. WebSocket is more efficient than polling HTTP repeatedly; it fits real time applications.

incoming and outgoing

Inside a webSocket block you handle incoming frames from the client with incoming, and send frames to the client with outgoing.

Example: for (frame in incoming) { outgoing.send(Frame.Text("Echo")) }. This loop sends a response as each message arrives. Frame.Text is a text frame; Frame.Binary is for binary data. When the connection closes, incoming ends and the loop exits. This structure merges concurrent data flow with Kotlin coroutines.

Exercise Add a WebSocket route

Define fun Application.module(). Inside it open routing { webSocket("/ws") { } }. In main, print println("WebSocket ready").

Show example solution

fun Application.module() {
    routing {
        webSocket("/ws") {
        }
    }
}

fun main() {
    println("WebSocket ready")
}

Test yourself

Complete the webSocket route

routing {
    ____("/ws") {
        
    }
}

Show answer

routing {
    webSocket("/ws") {
        
    }
}

Which scenario suits WebSocket?
- Monthly report
- One time file download
- Real time chat
- Static page display
Show answer

Real time chat — Chat needs real time data; WebSocket fits.
Match WebSocket parts

webSocket

Opens route
incoming

Incoming data
outgoing

Outgoing data
Frame.Text

Text frame

Next lesson: Summary - Ktor server

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