• Basic concepts

Basic concepts

Unlike HTTP, WebSocket provides full-duplex communication. Additionally, WebSocket enables streams of messages on top of TCP. TCP alone deals with streams of bytes with no inherent concept of a message. Before WebSocket, port 80 full-duplex communication was attainable using Comet channels; however, Comet implementation is non-trivial, and due to the TCP handshake and HTTP header overhead, it is inefficient for small messages. WebSocket protocol aims to solve these problems without compromising security assumptions of the web.

The WebSocket example:

fun main() {
    `$`.httpServer().websocket("/websocket/hello")
        .onServerMessageAsync { frame, _ -> onMessage(frame) }
        .onAcceptAsync { connection -> sendMessage("Server", connection) }
        .listen("localhost", 8090)

    val url = "ws://localhost:8090" // (1)
    `$`.httpClient().websocket("$url/websocket/hello")
        .extensions(listOf("permessage-deflate")) // (2)
        .onClientMessageAsync { frame, _ -> onMessage(frame) }
        .connectAsync { connection -> sendMessage("Client", connection) }
}

private suspend fun sendMessage(data: String, connection: WebSocketConnection) = connection.useAwait {
    (1..10).forEach { // (3)
        connection.sendText("WebSocket ${data}. count: $it, time: ${Date()}")
        delay(1000)
    }
}

private fun onMessage(frame: Frame) {
    if (frame is TextFrame) {
        println(frame.payloadAsUTF8)
    }
}

Run it. And the console shows:

Server. message: 1, time: Mon Feb 15 15:35:24 CST 2021
Client. message: 1, time: Mon Feb 15 15:35:24 CST 2021
Server. message: 2, time: Mon Feb 15 15:35:25 CST 2021
Client. message: 2, time: Mon Feb 15 15:35:25 CST 2021
Server. message: 3, time: Mon Feb 15 15:35:26 CST 2021
Client. message: 3, time: Mon Feb 15 15:35:26 CST 2021

......

1. The WebSocket protocol specification defines ws and wss as two new uniform resource identifier (URI) schemes that are used for unencrypted and encrypted connections, respectively. Apart from the scheme name and fragment (# is not supported), the rest of the URI components are defined to use URI generic syntax.
2. The Firefly WebSocket server and client supports some extension protocol, such as permessage-deflate, deflate-frame, x-webkit-deflate-frame, fragment, and identity.
3. The function connection.useAwait closes connection automatically when the readLoop exits.