How to combine alpakka kafka with akka stream websocket

Question

I am using https://doc.akka.io/docs/alpakka-kafka/current/consumer.html to consume data from kafka as the following:

implicit val system: ActorSystem = ActorSystem("SAPEVENTBUS")
implicit val materializer: Materializer = ActorMaterializer()

val config = system.settings.config.getConfig("akka.kafka.consumer")
val consumerSettings =
  ConsumerSettings(config, new StringDeserializer, new StringDeserializer)
    .withBootstrapServers("localhost:9092")
    .withGroupId("SAP-BUS")
    .withProperty(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest")
    .withProperty(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "true")
    .withProperty(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, "5000")

val kafkaConsumer =  Consumer
  .plainSource(
    consumerSettings,
    Subscriptions.topics("SAPEVENTBUS"))
  .toMat(Sink.foreach(println))(Keep.both)
  .mapMaterializedValue(DrainingControl.apply)  

Next, I will forward the received result to the webserver through the akka http websocket client

Here how a websocket client can be build:

implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()
import system.dispatcher

// print each incoming strict text message
val printSink: Sink[Message, Future[Done]] =
  Sink.foreach {
    case message: TextMessage.Strict =>
      println(message.text)
  }

val helloSource: Source[Message, NotUsed] =
  Source.single(TextMessage("hello world!"))

// the Future[Done] is the materialized value of Sink.foreach
// and it is completed when the stream completes
val flow: Flow[Message, Message, Future[Done]] =
  Flow.fromSinkAndSourceMat(printSink, helloSource)(Keep.left)

// upgradeResponse is a Future[WebSocketUpgradeResponse] that
// completes or fails when the connection succeeds or fails
// and closed is a Future[Done] representing the stream completion from above
val (upgradeResponse, closed) =
  Http().singleWebSocketRequest(WebSocketRequest("ws://echo.websocket.org"), flow)

val connected = upgradeResponse.map { upgrade =>
  // just like a regular http request we can access response status which is available via upgrade.response.status
  // status code 101 (Switching Protocols) indicates that server support WebSockets
  if (upgrade.response.status == StatusCodes.SwitchingProtocols) {
    Done
  } else {
    throw new RuntimeException(s"Connection failed: ${upgrade.response.status}")
  }
}

I have two questions:

1. How to combine the consumer and websocket client into one stream and let it send the message to the webserver.

2. I would like to broadcast the received message from the webserver into two Sinks depending on the content.

How to build such a graph?

Answer 1

If you're planning to push all Kafka messages into web socket without response handling you should write web socket's message handler in a true bi-directional scenario where input and output aren’t logically connected:

//Kafka reading logic
val kafkaSource: Source[ConsumerRecord[String, String], Consumer.Control] = Consumer
    .plainSource(consumerSettings, Subscriptions.topics("SAPEVENTBUS"))
//kafka message serialization logic
val kafkaRecordToMessageTransform: Flow[ConsumerRecord[String, String], Message, NotUsed] =
    Flow[ConsumerRecord[String, String]].map[Message](consumerRecord => {
        TextMessage.Strict(s"${consumerRecord.key} - ${consumerRecord.value}")
    })

//web socket's messages sending logic
val webSocketWriteLogic: Source[Message, Consumer.Control] =
    kafkaSource.via(kafkaRecordToMessageTransform)

//web socket's messages receiving logic
val webSocketReadLogic: Sink[Message, NotUsed] = Flow[Message].mapAsync[String](1)({
    case textMessage: TextMessage =>
        textMessage.toStrict(collectTimeout).map(_.text)
    case binaryMessage: BinaryMessage =>
        binaryMessage.toStrict(collectTimeout).map(_.data.toString())
}).to(Sink.foreach[String](messageText => println(s"received $messageText")))

//web socket's logic
val webSocketLogic: Flow[Message, Message, Consumer.Control] =
    Flow.fromSinkAndSourceMat(webSocketReadLogic, webSocketWriteLogic)(Keep.right)

You can broadcast stream messages into multiple Sinks depending on some condition by partition stage. Also, you can check this explanation.