WebSocket
- Kotlin course
- Advanced
- Lesson 23/83
- 5 min
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.
Practice Add a WebSocket route
Define fun Application.module(). Inside it open routing { webSocket("/ws") { } }. In main, print println("WebSocket ready").
Starter code
// WebSocket
// write here
import io.ktor.server.application.*
import io.ktor.server.routing.*
import io.ktor.server.websocket.*
fun main() {
}Sample solution
fun Application.module() {
routing {
webSocket("/ws") {
}
}
}
fun main() {
println("WebSocket ready")
}Expected output
WebSocket ready
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the webSocket route
routing {
____("/ws") {
}
}routing {
webSocket("/ws") {
}
}Explanation: The webSocket block opens a route.
2 Which scenario suits WebSocket?
- Monthly report
- One time file download
- Real time chat Correct answer
- Static page display
Explanation: Chat needs real time data; WebSocket fits.
3 Match WebSocket parts
| Term | Meaning |
|---|---|
webSocket |
Opens route |
incoming |
Incoming data |
outgoing |
Outgoing data |
Frame.Text |
Text frame |