Undertow : use Hystrix Observable in Http handler - asynchronous

Question

I managed to setup an Hystrix Command to be called from an Undertow HTTP Handler:
public void handleRequest(HttpServerExchange exchange) throws Exception {
if (exchange.isInIoThread()) {
exchange.dispatch(this);
return;
}
RpcClient rpcClient = new RpcClient(/* ... */);
try {
byte[] response = new RpcCommand(rpcClient).execute();
// send the response
} catch (Exception e) {
// send an error
}
}
This works nice. But now, I would like to use the observable feature of Hystrix, calling observe instead of execute, making the code non-blocking.
public void handleRequest(HttpServerExchange exchange) throws Exception {
RpcClient rpcClient = new RpcClient(/* ... */);
new RpcCommand(rpcClient).observe().subscribe(new Observer<byte[]>(){
#Override
public void onCompleted() {
}
#Override
public void onError(Throwable throwable) {
exchange.setStatusCode(StatusCodes.INTERNAL_SERVER_ERROR);
exchange.endExchange();
}
#Override
public void onNext(byte[] body) {
exchange.getResponseHeaders().add(Headers.CONTENT_TYPE, "text/plain");
exchange.getResponseSender().send(ByteBuffer.wrap(body));
}
});
}
As expected (reading the doc), the handler returns immediately and as a consequence, the exchange is ended; when the onNext callback is executed, it fails with an exception:
Caused by: java.lang.IllegalStateException: UT000127: Response has already been sent
at io.undertow.io.AsyncSenderImpl.send(AsyncSenderImpl.java:122)
at io.undertow.io.AsyncSenderImpl.send(AsyncSenderImpl.java:272)
at com.xxx.poc.undertow.DiyServerBootstrap$1$1.onNext(DiyServerBootstrap.java:141)
at com.xxx.poc.undertow.DiyServerBootstrap$1$1.onNext(DiyServerBootstrap.java:115)
at rx.internal.util.ObserverSubscriber.onNext(ObserverSubscriber.java:34)
Is there a way to tell Undertow that the handler is doing IO asynchronously? I expect to use a lot of non-blocking code to access database and other services.
Thanks in advance!

Answer 1

You should dispatch() a Runnable to have the exchange not end when the handleRequest method returns. Since the creation of the client and subscription are pretty simple tasks, you can do it on the same thread with SameThreadExecutor.INSTANCE like this:
public void handleRequest(HttpServerExchange exchange) throws Exception {
exchange.dispatch(SameThreadExecutor.INSTANCE, () -> {
RpcClient rpcClient = new RpcClient(/* ... */);
new RpcCommand(rpcClient).observe().subscribe(new Observer<byte[]>(){
//...
});
});
}
(If you do not pass an executor to dispatch(), it will dispatch it to the XNIO worker thread pool. If you wish to do the client creation and subscription on your own executor, then you should pass that instead.)