[akka-streams] Trying to mimic socket level programming , connect client and then send data if connection is available

[murtuza chhil]

Hello,

I am a newbie trying to understand Akka Streams.

Coming from regular socket programming I am trying to mimic a client connection and have the following runnable graph.

        Flow<ByteString, ByteString, CompletionStage<OutgoingConnection>> flow = Tcp
                .get(system).outgoingConnection("127.0.0.1", 6000);
        Source<ByteString, NotUsed> clientSource = Source.single(ByteString
                .fromString("XYZ"));

        clientSource.via(flow).to(Sink.ignore()).run(mat);

​

This works fine when used with the samples in the doc, where the source is attached that has a list and there is a server listening and we run the source->flow->sink through a materializer.

But this is not a typical use case, typically one would attempt to connect to a server,  and either it connects or fails. If connected send data that will be made available and if connection fails, have the ability to backoff.

Currently I don't know how to access the error when a server is not available and want to establish a connection before data is available.

Don't know how to prevent connections every time data is sent, I understand why it happens, but don't know how to reuse a materialized flow that has already connected.

What I am trying to get my head around is how does one run a graph without any source data and get a completable future that tells me if it succeeded or not.

Then I want to use the same connection to send data later using the source.

I have attempted with a runfold and that give me the akka TCP Exception when the server is not availble, but I simply want to forward the data from the source without manipulation.

There was another thread that I had started https://groups.google.com/d/msg/akka-user/1ztizMy9FnI/U9toR-cOBgAJ, would appreciate if someone responds to it. 

Any pointers or snippets would be greatly appreciated. Yes I am not used to this paradigm of programming but trying to figure it out.

-chhil

[Johan Andrén]

Hi Chhil,

The flow returned by outgoingConnection materializes into a CompletionStage<OutgoingConnection> which will be failed if the connection fails, so that is where you can implement your error handling for the initial connection. You would have to keep the materialized value when you construct the flow though, so that it is returned from run(). 

You can read more about materialized values here: http://doc.akka.io/docs/akka/2.4/java/stream/stream-composition.html#materialized-values and TCP with streams here: http://doc.akka.io/docs/akka/2.4/java/stream/stream-io.html#stream-io-java  

In general, read as much of the streams docs as you can to get a good understanding of how Akka Streams work, trial and error with streams + tcp will hurt!

--

Johan

Akka Team

[murtuza chhil]

Hi Johan,

Thank you very much. Yes indeed stream + tcp is hurting me for sure and I have tried reading the docs and sample code over and over.

So yes, I made some progress. Please ignore the verbosity of the code, I just need to familiarize myself with return values and look at the source of those easily via eclipse.

        final ActorSystem system = ActorSystem.create("Client");
        final ActorMaterializer mat = ActorMaterializer.create(system);

        Tcp tcp = Tcp.get(system);
        Flow<ByteString, ByteString, CompletionStage<OutgoingConnection>> outgoingConnection = tcp
                .outgoingConnection("127.0.0.1", 6000);

        Source<ByteString, NotUsed> clientSource = Source.empty(); 

        CompletionStage<Done> cs = clientSource.via(outgoingConnection)
                .runWith(Sink.ignore(), mat);

        cs.exceptionally(ex -> {
            ex.printStackTrace();
            return null;
        });

        Source<ByteString, NotUsed> clientSource1 = Source.single(ByteString
                .fromString("Chhil")); 

        NotUsed var = clientSource1.via(outgoingConnection).to(Sink.ignore())
                .run(mat);

​

Managed to get a connection made usingSource.empty()​ for the initial connect.
The CompletionStage kicks in nicely. You mentioned I should be able to get  CompletionStage<OutgoingConnection> to determine problems with the connection, this I don't know how or when it gets returned when the graph is run. Please do correct me if understanding is wrong.
I use the materialized value from the connect and that works fine, as I don't see a new connect after the initial connect with the Source.empty(). I assume this is the right way to get the initial connection and then change the source for regular communication.

Now the problem I have run into is, that after sending data, the channel disconnects. What must I do to keep it open?

-chhil

[Konrad Malawski]

Source.single completes the stream (closes the connection) once it has signalled the data.

Simply put a Source.maybe behind it (a.concat(b)) or use a different source, like unfold or an iterator).

-- 
Konrad `ktoso` Malawski
Akka @ Lightbend

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[murtuza chhil]

Hi Konrad,

The  Source<ByteString, CompletableFuture<Optional<ByteString>>> clientSource = Source.<ByteString>maybe(); does the trick, don't see a disconnect on the netcat server send.

Also figured, I shouldn't be running the graph multiple times once to connect and the second time to send data. Once its run, any data fed by the source will be sent out.

This was the reason I was seeing multiple connections.

Thank you. Appreciate the help.

-chhil