Scala: Read file as infinite/async stream of lines

Question

I have a file /files/somelog.log that is continuously written to by some other process. What I want to do is to read that file as a (never-ending) stream and respond to it whenever there is a new log line available.

val logStream: Stream[String] = readAsStream("/files/somelog.log")
logStream.foreach { line: String => println(line) }

So, immediately when the other process writes anther line to the logfile, I expect the println above to go off (and keep going off).

However, that doesn't seem to be how the default Stream-type works. Scala's Stream isn't async, it's just lazy. What I'm probably looking for is something similar to what akka streams offers (I think). But Akka is so huge (150MB!) - I don't want to pull in so many new things that I don't need just to do something this simple/basic. Is there really no other (tiny!) library or technique I can use?

github.com/ReactiveX/RxScala

Giovanni Caporaletti
– Giovanni Caporaletti

2016-03-25 23:46:29 +00:00
Commented Mar 25, 2016 at 23:46 — Giovanni Caporaletti
– Giovanni Caporaletti, Commented Mar 25, 2016 at 23:46

Sebastian · Accepted Answer · 2016-03-25 23:58:21Z

2

If you simply want to tail the file you could use Tailer from Apache Commons IO:

val l = new TailerListenerAdapter() {
    override def handle(line: String): Unit = println(line)
}

val tail = Tailer.create(new File("file.txt"), l)

answered Mar 25, 2016 at 23:58

Sebastian

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Comments

jwvh · Accepted Answer · 2016-03-26 08:42:01Z

0

I'm not actually recommending this, but you could roll your own.

def nextLine( itr: Iterator[String] ): String = {
  while (!itr.hasNext) Thread.sleep(2000)
  itr.next
}
. . .
val log = io.Source.fromFile("/files/somelog.log").getLines
val logTxt = nextLine( log )  // will block until next line is available
. . .

answered Mar 26, 2016 at 8:42

jwvh

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10 Comments

Łukasz Over a year ago

doesn't look very async to me

Dima Over a year ago

Not being async isn't the biggest problem with it. It actually doesn't work at all: once EOF is reached, the file is closed, and hasNext will never return true again.

kornfridge Over a year ago

It's an interesting suggestion. Too bad it won't work ^. You could always async'ify it manually by putting it in its own thread.

jwvh Over a year ago

@Dima, whadyameanitdosntwork?? All my test results are exactly as desired. It throws an error if the log file doesn't exist, but a more complete implementation would cover that. Try it. hasNext returns true after the next line is written to the log. [Wow, harsh audience.]

jwvh Over a year ago

@kornfridge, exactly! You can async'ify as desired.

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