Convert HttpEntity.Chunked to Array[String]

Question

I have the following problem. I am querying a server for some data and getting it back as HttpEntity.Chunked. The response String looks like this with up to 10.000.000 lines like this:

[{"name":"param1","value":122343,"time":45435345},
{"name":"param2","value":243,"time":4325435},
......]

Now I want to get the incoming data into and Array[String] where each String is a line from the response, because later on it should be imported into an apache spark dataframe. Currently I am doing it likes this:

//For the http request
trait StartHttpRequest {
  implicit val system: ActorSystem
  implicit val materializer: ActorMaterializer

  def httpRequest(data: String, path: String, targetPort: Int, host: String): Future[HttpResponse] = {
    val connectionFlow: Flow[HttpRequest, HttpResponse, Future[OutgoingConnection]] = {
      Http().outgoingConnection(host, port = targetPort)
    }
    val responseFuture: Future[HttpResponse] =
      Source.single(RequestBuilding.Post(uri = path, entity = HttpEntity(ContentTypes.`application/json`, data)))
        .via(connectionFlow)
        .runWith(Sink.head)
    responseFuture
  }
}

//result of the request
val responseFuture: Future[HttpResponse] = httpRequest(.....)

//convert to string
responseFuture.flatMap { response =>
        response.status match {
          case StatusCodes.OK =>
            Unmarshal(response.entity).to[String]
    }
}

//and then something like this, but with even more stupid stuff
responseFuture.onSuccess { str:String =>
    masterActor! str.split("""\},\{""")
}

My question is, what would be a better way to get the result into an array? How can I unmarshall the response entity directly? Because .to[Array[String]] for example did not work. And because there are so many lines coming, could I do it with a stream, to be more efficent?

Answer 1

Answering your questions out of order:

How can I unmarshall the response entity directly?

There is an existing question & answer related to unmarshalling an Array of case classes.

what would be a better way to get the result into an array?

I would take advantage of the Chunked nature and use streams. This allows you to do string processing and json parsing concurrently.

First you need a container class and parser:

case class Data(name : String, value : Int, time : Long)

object MyJsonProtocol extends DefaultJsonProtocol {
  implicit val dataFormat = jsonFormat3(Data)
}

Then you have to do some manipulations to get the json objects to look right:

//Drops the '[' and the ']' characters
val dropArrayMarkers = 
  Flow[ByteString].map(_.filterNot(b => b == '['.toByte || b == ']'.toByte))

val preppendBrace = 
  Flow[String].map(s => if(!s.startsWith("{")) "{" + s else s)

val appendBrace = 
  Flow[String].map(s => if(!s.endsWith("}")) s + "}" else s)

val parseJson = 
  Flow[String].map(_.parseJson.convertTo[Data])

Finally, combine these Flows to convert a Source of ByteString into a Source of Data objects:

def strSourceToDataSource(source : Source[ByteString,_]) : Source[Data, _] = 
  source.via(dropArrayMarkers)
        .via(Framing.delimiter(ByteString("},{"), 256, true))
        .map(_.utf8String)
        .via(prependBrace)
        .via(appendBrace)
        .via(parseJson)

This source can then be drained into an Seq of Data objects:

val dataSeq : Future[Seq[Data]] = 
  responseFuture flatMap { response =>
    response.status match {
      case StatusCodes.OK =>
        strSourceToDataSource(response.entity.dataBytes).runWith(Sink.seq)
    }
  }