File Upload and processing using akka-http websockets - scala

Question

I'm using some sample Scala code to make a server that receives a file over websocket, stores the file temporarily, runs a bash script on it, and then returns stdout by TextMessage.
Sample code was taken from this github project.
I edited the code slightly within echoService so that it runs another function that processes the temporary file.
object WebServer {
def main(args: Array[String]) {
implicit val actorSystem = ActorSystem("akka-system")
implicit val flowMaterializer = ActorMaterializer()
val interface = "localhost"
val port = 3000
import Directives._
val route = get {
pathEndOrSingleSlash {
complete("Welcome to websocket server")
}
} ~
path("upload") {
handleWebSocketMessages(echoService)
}
val binding = Http().bindAndHandle(route, interface, port)
println(s"Server is now online at http://$interface:$port\nPress RETURN to stop...")
StdIn.readLine()
binding.flatMap(_.unbind()).onComplete(_ => actorSystem.shutdown())
println("Server is down...")
}
implicit val actorSystem = ActorSystem("akka-system")
implicit val flowMaterializer = ActorMaterializer()
val echoService: Flow[Message, Message, _] = Flow[Message].mapConcat {
case BinaryMessage.Strict(msg) => {
val decoded: Array[Byte] = msg.toArray
val imgOutFile = new File("/tmp/" + "filename")
val fileOuputStream = new FileOutputStream(imgOutFile)
fileOuputStream.write(decoded)
fileOuputStream.close()
TextMessage(analyze(imgOutFile))
}
case BinaryMessage.Streamed(stream) => {
stream
.limit(Int.MaxValue) // Max frames we are willing to wait for
.completionTimeout(50 seconds) // Max time until last frame
.runFold(ByteString(""))(_ ++ _) // Merges the frames
.flatMap { (msg: ByteString) =>
val decoded: Array[Byte] = msg.toArray
val imgOutFile = new File("/tmp/" + "filename")
val fileOuputStream = new FileOutputStream(imgOutFile)
fileOuputStream.write(decoded)
fileOuputStream.close()
Future(Source.single(""))
}
TextMessage(analyze(imgOutFile))
}
private def analyze(imgfile: File): String = {
val p = Runtime.getRuntime.exec(Array("./run-vision.sh", imgfile.toString))
val br = new BufferedReader(new InputStreamReader(p.getInputStream, StandardCharsets.UTF_8))
try {
val result = Stream
.continually(br.readLine())
.takeWhile(_ ne null)
.mkString
result
} finally {
br.close()
}
}
}
}
During testing using Dark WebSocket Terminal, case BinaryMessage.Strict works fine.
Problem: However, case BinaryMessage.Streaming doesn't finish writing the file before running the analyze function, resulting in a blank response from the server.
I'm trying to wrap my head around how Futures are being used here with the Flows in Akka-HTTP, but I'm not having much luck outside trying to get through all the official documentation.
Currently, .mapAsync seems promising, or basically finding a way to chain futures.
I'd really appreciate some insight.

Answer 1

Yes, mapAsync will help you in this occasion. It is a combinator to execute Futures (potentially in parallel) in your stream, and present their results on the output side.
In your case to make things homogenous and make the type checker happy, you'll need to wrap the result of the Strict case into a Future.successful.
A quick fix for your code could be:
val echoService: Flow[Message, Message, _] = Flow[Message].mapAsync(parallelism = 5) {
case BinaryMessage.Strict(msg) => {
val decoded: Array[Byte] = msg.toArray
val imgOutFile = new File("/tmp/" + "filename")
val fileOuputStream = new FileOutputStream(imgOutFile)
fileOuputStream.write(decoded)
fileOuputStream.close()
Future.successful(TextMessage(analyze(imgOutFile)))
}
case BinaryMessage.Streamed(stream) =>
stream
.limit(Int.MaxValue) // Max frames we are willing to wait for
.completionTimeout(50 seconds) // Max time until last frame
.runFold(ByteString(""))(_ ++ _) // Merges the frames
.flatMap { (msg: ByteString) =>
val decoded: Array[Byte] = msg.toArray
val imgOutFile = new File("/tmp/" + "filename")
val fileOuputStream = new FileOutputStream(imgOutFile)
fileOuputStream.write(decoded)
fileOuputStream.close()
Future.successful(TextMessage(analyze(imgOutFile)))
}
}