Server WebSockets

This feature adds WebSockets support to Ktor. WebSockets are a mechanism to keep a bi-directional real-time ordered connection between the server and the client. Each message from this channel is called Frame: a frame can be a text or binary message, or a close or ping/pong message. Frames can be marked as incomplete or final.

In order to use the WebSockets functionality you first have to install it:

If required, you can adjust parameters during the installation of the feature:

    pingPeriod = Duration.ofSeconds(60) // Disabled (null) by default
    timeout = Duration.ofSeconds(15)
    maxFrameSize = Long.MAX_VALUE // Disabled (max value). The connection will be closed if surpassed this length.
    masking = false
    extensions {
        // install(...)
    }
}

Once installed, you can define the webSocket routes for the routing feature:

Instead of the short-lived normal route handlers, webSocket handlers are meant to be long-lived. And all the relevant WebSocket methods are suspended so that the function will be suspended in a non-blocking way while receiving or sending messages.

webSocket methods receive a callback with a instance as the receiver. That interface defines an incoming (ReceiveChannel) property and an outgoing (SendChannel) property, as well as a close method. Check the full WebSocketSession for more information.

Usage as an suspend actor

routing {
    webSocket("/") { // websocketSession
        for (frame in incoming) {
            when (frame) {
                is Frame.Text -> {
                    val text = frame.readText()
                    outgoing.send(Frame.Text("YOU SAID: $text"))
                    if (text.equals("bye", ignoreCase = true)) {
                        close(CloseReason(CloseReason.Codes.NORMAL, "Client said BYE"))
                    }
                }
            }
        }
    }
}

Usage as a Channel

Since the incoming property is a ReceiveChannel, you can use it with its stream-like interface:

You receive a WebSocketSession as the receiver (this), giving you direct access to these members inside your webSocket handler.

interface WebSocketSession {
    val incoming: ReceiveChannel<Frame> // Incoming frames channel
    val outgoing: SendChannel<Frame> // Outgoing frames channel
    fun close(reason: CloseReason)
    // Convenience method equivalent to `outgoing.send(frame)`
    suspend fun send(frame: Frame) // Enqueue frame, may suspend if the outgoing queue is full. May throw an exception if the outgoing channel is already closed, so it is impossible to transfer any message.
    // The call and the context
    val application: Application
    // List of WebSocket extensions negotiated for the current session
    val extensions: List<WebSocketExtension<*>>
    // Modifiable properties for this request. Their initial value comes from the feature configuration.
    var pingInterval: Duration?
    var timeout: Duration
    var masking: Boolean // Enable or disable masking output messages by a random xor mask.
    var maxFrameSize: Long // Specifies frame size limit. The connection will be closed if violated
    // Advanced
    val closeReason: Deferred<CloseReason?>
    suspend fun flush() // Flush all outstanding messages and suspend until all earlier sent messages will be written. Could be called at any time even after close. May return immediately if connection is already terminated.
    fun terminate() // Initiate connection termination immediately. Termination may complete asynchronously.
}

A frame is each packet sent and received at the WebSocket protocol level. There are two message types: TEXT and BINARY. And three control packets: CLOSE, PING, and PONG. Each packet has a payload buffer. And for Text or Close messages, you can call the readText or readReason to interpret that buffer.

enum class FrameType { TEXT, BINARY, CLOSE, PING, PONG }

You can test WebSocket conversations by using the handleWebSocketConversation method inside a withTestApplication block.

class MyAppTest {
    @Test
    fun testConversation() {
        withTestApplication {
            application.install(WebSockets)
            val received = arrayListOf<String>()
            application.routing {
                webSocket("/echo") {
                    try {
                        while (true) {
                            val text = (incoming.receive() as Frame.Text).readText()
                            received += text
                        }
                    } catch (e: ClosedReceiveChannelException) {
                        // Do nothing!
                    } catch (e: Throwable) {
                    }
                }
            }
            handleWebSocketConversation("/echo") { incoming, outgoing ->
                val textMessages = listOf("HELLO", "WORLD")
                for (msg in textMessages) {
                    outgoing.send(Frame.Text(msg))
                    assertEquals(msg, (incoming.receive() as Frame.Text).readText())
                }
                assertEquals(textMessages, received)
            }
        }
    }
}

Standard Events: onConnect, onMessage, onClose and onError

• onConnect happens at the start of the block.

• onMessage happens after successfully reading a message (for example with incoming.receive()) or using suspended iteration with for(frame in incoming).

• onClose happens when the incoming channel is closed. That would complete the suspended iteration, or throw a ClosedReceiveChannelException when trying to receive a message`.

• onError is equivalent to other other exceptions.

In both onClose and onError, the closeReason property is set.

To illustrate this:

webSocket("/echo") {
    println("onConnect")
    try {
        for (frame in incoming){
            val text = (frame as Frame.Text).readText()
            println("onMessage")
            received += text
            outgoing.send(Frame.Text(text))
        }
    } catch (e: ClosedReceiveChannelException) {
        println("onClose ${closeReason.await()}")
    } catch (e: Throwable) {
        println("onError ${closeReason.await()}")
        e.printStackTrace()
    }

In this sample, the infinite loop is only exited with an exception is risen: either a ClosedReceiveChannelException or another exception.