Websockets

In addition to using RPC-style interfaces, KVision support two-way type-safe connections via websockets, based on Kotlin coroutines channels.

Note: Since channels API in Kotlin is still marked as experimental, you should treat websockets support in KVision as experimental feature, too.

Common code

The way the websocket connections are defined is in many ways similar to defining remote methods (see previous chapters). You start by declaring an interface method in the common code, which will be used in the frontend and backend parts. This method needs to have a specific signature:

suspend (ReceiveChannel<M>, SendChannel<N>) -> Unit

When you establish the websocket connection, you will be able to send objects of type M from the client to the server and send objects of type N from the server to the client. Both types M and N need to fulfill the criteria described in this chapter. Of course type M can be the same as N.

interface IWsService {
suspend fun wservice(input: ReceiveChannel<Int>, output: SendChannel<String>) {}
}

Frontend application

To actually create a websocket connection, you call a different method, which is generated by KVision compiler plugin and has reversed types (!), because on the frontend side we will send M and receive N objects. When the method is called, the connection is established and it lasts as long as the method is running. The connection is terminated when the method is finished. It's a suspending method, so you can easily use loops (even while(true) ) and structured concurrency with coroutineScope builders.

val ws = WsService()
GlobalScone.launch {
ws.wsservice { output /*: SendChannel<Int>*/, input /*: ReceiveChannel<String>*/ ->
coroutineScope {
launch {
while(true) {
val i = Random.nextInt()
output.send(i)
delay(1000)
}
}
launch {
for (str in input) {
println(str)
}
}
}
}
}

Backend code

On the backend side you just have to implement the interface method. It will be automatically called when a new client is connected, and it should run as long as the connection is active.

actual class WsService : IWsService {
override suspend fun wservice(input: ReceiveChannel<Int>, output: SendChannel<String>) {
for (i in input) {
output.send("I'v got: $i")
}
}
}

Of course the server can send data to the output channel at any time. So you can even ignore the input at all.

actual class WsService : IWsService {
override suspend fun wservice(input: ReceiveChannel<Int>, output: SendChannel<String>) {
while(true) {
val i = Random.nextInt()
output.send("Have a random: $i")
delay(500)
}
}
}

If you want to have more control over the client connection and more information about it, you can inject WebSocketServerSession when using Ktor module. This object gives you access to, among others, the ApplicationCall object.

actual class WsService : IWsService {
@Inject
lateinit var wsSession: WebSocketServerSession
override suspend fun wservice(input: ReceiveChannel<Int>, output: SendChannel<String>) {
// wsSession.call.request
}
}

When using Spring Boot module, you can autowire WebSocketSession object.

@Service
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE)
actual class WsService : IWsService {
@Autowired
lateinit var webSocketSession: WebSocketSession
override suspend fun wservice(input: ReceiveChannel<Int>, output: SendChannel<String>) {
//
}
}

When using Javalin, you can inject WsContext and WsSession objects.

actual class WsService : IWsService {
@Inject
lateinit var wsCtx: WsContext
@Inject
lateinit var wsSession: Session
override suspend fun wservice(input: ReceiveChannel<Int>, output: SendChannel<String>) {
//
}
}

Disconnection

When the user leaves or closes the browser page the websocket connection is closed. On the backend side you will find both input and output channels closed and then you should return from the backend method.

On the other hand, when the server is stopped the connection is closed as well. But on the frontend side you have the possibility to re-establish the connection when the server is online again. Just run your frontend method again after a few seconds.

val ws = WsService()
GlobalScope.launch {
while (true) {
ws.wsservice { output, input ->
coroutineScope {
// ...
}
}
delay(5000)
}
}