Akka streams: Reading multiple files

Question

I have a list of files. I want:

1. To read from all of them as a single Source.
2. Files should be read sequentially, in-order. (no round-robin)
3. At no point should any file be required to be entirely in memory.
4. An error reading from a file should collapse the stream.

It felt like this should work: (Scala, akka-streams v2.4.7)

val sources = Seq("file1", "file2").map(new File(_)).map(f => FileIO.fromPath(f.toPath)
    .via(Framing.delimiter(ByteString(System.lineSeparator), 10000, allowTruncation = true))
    .map(bs => bs.utf8String)
  )
val source = sources.reduce( (a, b) => Source.combine(a, b)(MergePreferred(_)) )
source.map(_ => 1).runWith(Sink.reduce[Int](_ + _)) // counting lines

But that results in a compile error since FileIO has a materialized value associated with it, and Source.combine doesn't support that.

Mapping the materialized value away makes me wonder how file-read errors get handled, but does compile:

val sources = Seq("file1", "file2").map(new File(_)).map(f => FileIO.fromPath(f.toPath)
    .via(Framing.delimiter(ByteString(System.lineSeparator), 10000, allowTruncation = true))
    .map(bs => bs.utf8String)
    .mapMaterializedValue(f => NotUsed.getInstance())
  )
val source = sources.reduce( (a, b) => Source.combine(a, b)(MergePreferred(_)) )
source.map(_ => 1).runWith(Sink.reduce[Int](_ + _))  // counting lines

But throws an IllegalArgumentException at runtime:

java.lang.IllegalArgumentException: requirement failed: The inlets [] and outlets [MergePreferred.out] must correspond to the inlets [MergePreferred.preferred] and outlets [MergePreferred.out]

Answer 1

The code below is not as terse as it could be, in order to clearly modularize the different concerns.

// Given a stream of bytestrings delimited by the system line separator we can get lines represented as Strings
val lines = Framing.delimiter(ByteString(System.lineSeparator), 10000, allowTruncation = true).map(bs => bs.utf8String)

// given as stream of Paths we read those files and count the number of lines
val lineCounter = Flow[Path].flatMapConcat(path => FileIO.fromPath(path).via(lines)).fold(0l)((count, line) => count + 1).toMat(Sink.head)(Keep.right)

// Here's our test data source (replace paths with real paths)
val testFiles = Source(List("somePathToFile1", "somePathToFile2").map(new File(_).toPath))

// Runs the line counter over the test files, returns a Future, which contains the number of lines, which we then print out to the console when it completes
testFiles runWith lineCounter foreach println

Answer 2

Update Oh, I didn't see the accepted answer because I didn't refresh the page >_<. I'll leave this here anyway since I've also added some notes about error handling.

I believe the following program does what you want:

import akka.NotUsed
import akka.actor.ActorSystem
import akka.stream.{ActorMaterializer, IOResult}
import akka.stream.scaladsl.{FileIO, Flow, Framing, Keep, Sink, Source}
import akka.util.ByteString
import scala.concurrent.{Await, Future}
import scala.util.{Failure, Success}
import scala.util.control.NonFatal
import java.nio.file.Paths
import scala.concurrent.duration._

object TestMain extends App {
  implicit val actorSystem = ActorSystem("test")
  implicit val materializer = ActorMaterializer()
  implicit def ec = actorSystem.dispatcher

  val sources = Vector("build.sbt", ".gitignore")
    .map(Paths.get(_))
    .map(p =>
      FileIO.fromPath(p)
        .viaMat(Framing.delimiter(ByteString(System.lineSeparator()), Int.MaxValue, allowTruncation = true))(Keep.left)
        .mapMaterializedValue { f =>
          f.onComplete {
            case Success(r) if r.wasSuccessful => println(s"Read ${r.count} bytes from $p")
            case Success(r) => println(s"Something went wrong when reading $p: ${r.getError}")
            case Failure(NonFatal(e)) => println(s"Something went wrong when reading $p: $e")
          }
          NotUsed
        }
    )
  val finalSource = Source(sources).flatMapConcat(identity)

  val result = finalSource.map(_ => 1).runWith(Sink.reduce[Int](_ + _))
  result.onComplete {
    case Success(n) => println(s"Read $n lines total")
    case Failure(e) => println(s"Reading failed: $e")
  }
  Await.ready(result, 10.seconds)

  actorSystem.terminate()
}

The key here is the flatMapConcat() method: it transforms each element of a stream into a source and returns a stream of elements yielded by these sources if they are run sequentially.

As for handling errors, you can either add a handler to the future in the mapMaterializedValue argument, or you can handle the final error of the running stream by putting a handler on the Sink.foreach materialized future value. I did both in the example above, and if you test it, say, on a nonexisting file, you'll see that the same error message will be printed twice. Unfortunately, flatMapConcat() does not collect materialized values, and frankly I can't see the way it could do it sanely, therefore you have to handle them separately, if necessary.