How to retrieve messages from Alpakka Mqtt Streaming client?

Question

I was following document for writing a Mqtt client subscriber using alpakka.

https://doc.akka.io/docs/alpakka/3.0.4/mqtt-streaming.html?_ga=2.247958340.274298740.1642514263-524322027.1627936487

After the code marked in bold, I’m not sure how could I retrieve/interact with subscribed messages. Any lead?

Pair<SourceQueueWithComplete<Command>, CompletionStage> run =
Source.<Command>queue(3, OverflowStrategy.fail())
.via(mqttFlow)
.collect(
new JavaPartialFunction<DecodeErrorOrEvent, Publish>() {
@Override
public Publish apply(DecodeErrorOrEvent x, boolean isCheck) {
if (x.getEvent().isPresent() && x.getEvent().get().event() instanceof Publish)
return (Publish) x.getEvent().get().event();
else throw noMatch();
}
})
.toMat(Sink.head(), Keep.both())
.run(system);

SourceQueueWithComplete<Command> commands = run.first();
commands.offer(new Command<>(new Connect(clientId, ConnectFlags.CleanSession())));
commands.offer(new Command<>(new Subscribe(topic)));
session.tell(
new Command<>(
new Publish(
ControlPacketFlags.RETAIN() | ControlPacketFlags.QoSAtLeastOnceDelivery(),
topic,
ByteString.fromString(“ohi”))));

// for shutting down properly
commands.complete();
commands.watchCompletion().thenAccept(done → session.shutdown());

Also, in the following example, it shows how to subscribe to the client but nothing about how to get messages after the subscription.

https://github.com/pbernet/akka_streams_tutorial/blob/master/src/main/scala/alpakka/mqtt/MqttEcho.scala

Will be grateful if anyone knows the solution or can point to any resource which uses the same connector as mqtt client and can retrieve messages.

For some reason the code doesn't get bold. Below is the one that I was talking about. commands.offer(new Command<>(new Subscribe(topic))); — AAA, Jan 21 '22 at 21:37

earthling paul · Answer 1 · 2022-01-28T14:07:28.650

0

The code to retrieve messages for the subscriber is hidden in the client method which is used for both publisher and subscriber:

     ...
     //Only the Publish events are interesting for the subscriber
    .collect { case Right(Event(p: Publish, _)) => p }
    .wireTap(event => logger.info(s"Client: $connectionId received: ${event.payload.utf8String}"))
    .toMat(Sink.ignore)(Keep.both)
    .run()

https://github.com/pbernet/akka_streams_tutorial/blob/3e4484c5356e55522366e65e42e1741c18830a18/src/main/scala/alpakka/mqtt/MqttEcho.scala#L136

I was struggling with this connector and then tried an example with the one based on Eclipse Paho, which in the end looks better:

https://github.com/pbernet/akka_streams_tutorial/blob/3e4484c5356e55522366e65e42e1741c18830a18/src/main/scala/alpakka/mqtt/MqttPahoEcho.scala#L41

Paul

edited Jan 28 '22 at 14:07

answered Jan 28 '22 at 13:59

earthling paul

503
4
8

Hey Paul, thanks very much for your response. Actually I just figured it out few days ago and can see the subscribing messages coming through :-) – AAA Jan 28 '22 at 18:34
Hey Paul, it seems like subscriber is able to receive messages from the broker but seems like there's a big delay after the messages are being published by the publisher. Any idea? – AAA Feb 03 '22 at 21:10
Found out that it has to do with the ControlPacketFlags. If it is, while creating the publish message, supplied as QoSAtMostOnceDelivery and retain using bitwise | operator then it would work fine. – AAA Feb 04 '22 at 14:24

How to retrieve messages from Alpakka Mqtt Streaming client?

1 Answers1