Sending async message from micronaut ServerWebSocket - websocket

Question

I am trying to implement simple WebSocket server using Micronaut (and Groovy). I am new to Micronaut, so learning it as I go, and with it some RxJava seems required. So learning that as well.
I can make the basic example work:
#CompileStatic
#ServerWebSocket("/ws")
class TimeseriesWebSocket {
#OnOpen
public Publisher<String> onOpen(WebSocketSession session) {
println("opening connection")
return session.send("Hello")
}
#OnMessage
public Publisher<String> onMessage(String message, WebSocketSession session) {
println("message received")
return session.send("Thanks for the message")
}
#OnClose
public Publisher<String> onClose(WebSocketSession session) {
println("Closing")
//this seems not to be delivered, I guess due to session being closed already
return session.send("OK")
}
}
So this prints out all the messages I put there and works fine with a client connecting. The client also sees the "Hello" and "Thanks for the message" messages that are return with session.send(..).
Now my problem is, I am trying to send a message outside these methods. Like this:
#CompileStatic
#ServerWebSocket("/ws")
class MyWebSocket {
#OnOpen
public Publisher<String> onOpen(WebSocketSession session) {
println("opening connection")
startPing()
return session.send("Hello")
}
//...(same as above)
public void startPing(WebSocketSession session) {
PingPing ping = new PingPing(session)
ScheduledExecutorService exec = Executors.newSingleThreadScheduledExecutor();
exec.scheduleAtFixedRate(ping, 0, 1, TimeUnit.SECONDS);
}
}
class PingPing {
WebSocketSession session
public PingPing(WebSocketSession session) {
this.session = session
}
#Override
void run() {
println("pinging..")
try {
session.send("Ping...")
} catch (Exception e) {
e.printStackTrace()
}
}
}
This executes but nothing shows up on the client end. Now if I change session.send() to session.sendSync() it works fine. The ping is delivered.
The send() signature is actually
default <T> Publisher<T> send(T message)
I figured at first I should provide the Publisher to some other source to have it sent. But I guess this is not the case. I realized it is sort of a Future object, so if I subscribe to it myself like this:
def publisher = session.send("Ping...")
publisher.subscribe(new Subscriber<GString>() {
#Override
void onSubscribe(Subscription s) {
println("subscription")
}
#Override
void onNext(GString gString) {
println("next")
}
#Override
void onError(Throwable t) {
println("error")
t.printStackTrace()
}
#Override
void onComplete() {
println("complete")
}
})
println("publisher: ${publisher}")
Running the above piece of code (with subscribe), I guess it triggers the session.send() on the current thread and returns a result. But where should I actually call this? On what thread? I looked at the RxJava schedulers but could not quite understand where to call it from.
Further, the result of running the above actually delivers the message to the client, but also throws an error:
error
io.reactivex.exceptions.MissingBackpressureException: create: could not emit value due to lack of requests
at io.reactivex.internal.operators.flowable.FlowableCreate$ErrorAsyncEmitter.onOverflow(FlowableCreate.java:438)
at io.reactivex.internal.operators.flowable.FlowableCreate$NoOverflowBaseAsyncEmitter.onNext(FlowableCreate.java:406)
at io.micronaut.http.netty.websocket.NettyRxWebSocketSession.lambda$null$2(NettyRxWebSocketSession.java:191)
at io.netty.util.concurrent.DefaultPromise.notifyListener0(DefaultPromise.java:577)
What is the backpressure and values / requests this referring to, and what is the actual way I should process an async send of the message? I expect it to just send the single item I am trying to send..
The Micronaut API documentation mentions following the javax.websocket API closely but
the javax.websocket async API seems to make more sense in just providing a Future to listen to.
So the short question is, how to use the Micronaut Websocket API to send message with the async mode outside the Micronaut provided functions? Or am I doing it all wrong?
It seems like I might be making some general wrong assumption about this API, but cannot figure it out from docs and cannot find example.

Answer 1

I had the same problem with micronaut v1.3.2.
I managed to get it working with sendAsync/sendSync instead of send.
I had a look at the implementation of NettyRxWebSocketSession and it looks like send is implemented somewhat differently than sendAsync. Not clear if its due to configuration or just problem with the implementation of send.
I hope this helps.