Why does Akka TCP stream server disconnect client when there is no flow for the connection.handlewith? - akka-stream

Question

I am looking for an explanation for the behavior I see with the following code. When the
conn.handleWith is commented out, the TCP client connection that I make with netcat, connects, and in a couple of seconds shows disconnected by peer (i.e. the server disconnected the connection). When the conn.handleWith is present in the code I see no disconnection. I initially though it had to do with the idletimeout set up for the server, but that wasn't the case.
So why does the server disconnect the client when there is no flow to handle the connection?
package com.example;
import java.util.concurrent.CompletionStage;
import akka.Done;
import akka.NotUsed;
import akka.actor.typed.ActorSystem;
import akka.actor.typed.javadsl.Behaviors;
import akka.stream.javadsl.Sink;
import akka.stream.javadsl.Source;
import akka.stream.javadsl.Tcp;
import akka.stream.javadsl.Tcp.IncomingConnection;
import akka.stream.javadsl.Tcp.ServerBinding;
public class SimpleStream00 {
public static void main(String[] args) throws InterruptedException {
ActorSystem actorSystem = ActorSystem.create(Behaviors.empty(), "actorSystem");
final Sink<IncomingConnection, CompletionStage<Done>> handler = Sink.foreach(conn -> {
System.out.println("Client connected from: " + conn.remoteAddress());
// conn.handleWith(Flow.of(ByteString.class), actorSystem);
// Server does not drop the connection when previous line is uncommented
});
Source<IncomingConnection, CompletionStage<ServerBinding>> source = Tcp.get(actorSystem).bind("127.0.0.1",
8888); // .idleTimeout(Duration.ofSeconds(60));
CompletionStage<ServerBinding> bindingFuture = source.to(handler).run(actorSystem);
bindingFuture.handle((binding, throwable) -> {
if (binding != null) {
System.out.println("Server started, listening on: " + binding.localAddress());
} else {
System.err.println("Server could not bind to : " + throwable.getMessage());
actorSystem.terminate();
}
return NotUsed.getInstance();
});
}
}

Answer 1

A general principle in Akka Streams is that if there's no demand, the stream should consume as few resources as possible. Since without handleWith, your stream never signals demand for the ByteStrings from the connection, Akka's TCP layer disconnects the connection to save resources.