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What's the Scala way to implement a retry-able call like this one?

Asked 2011-10-28T14:45:33.607
66

Still the newbie in Scala and I'm now looking for a way to implement the following code on it:

@Override
public void store(InputStream source, String destination, long size) {

    ObjectMetadata metadata = new ObjectMetadata();
    metadata.setContentLength(size);
    final PutObjectRequest request = new PutObjectRequest(
            this.configuration.getBucket(), destination, source, metadata);

    new RetryableService(3) {

        @Override
        public void call() throws Exception {
            getClient().putObject(request);
        }
    };

}

What would be the best way to implement the same funcionality that RetryableService implements but in Scala?

It basically calls the call method N times, if all of them fail the exception is then raised, if they succeed it moves on. This one does not return anything but then I have another version that allows for returning a value (so, i have two classes in Java) and I believe I could do with a single class/function in Scala.

Any ideas?

EDIT

Current implementation in java is as follows:

public abstract class RetryableService {

private static final JobsLogger log = JobsLogger
        .getLogger(RetryableService.class);

private int times;

public RetryableService() {
    this(3);
}

public RetryableService(int times) {
    this.times = times;
    this.run();
}

private void run() {

    RuntimeException lastExceptionParent = null;

    int x = 0;

    for (; x < this.times; x++) {

        try {
            this.call();
            lastExceptionParent = null;
            break;
        } catch (Exception e) {
            lastExceptionParent = new RuntimeException(e);
            log.errorWithoutNotice( e, "Try %d caused exception %s", x, e.getMessage() );

            try {
                Thread.sleep( 5000 );
            } catch (InterruptedException e1) {
                log.errorWithoutNotice(
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1 Answer

1

If you want control of which exceptions you retry, you can use methods in scala.util.control.Exception:

import java.io._
import scala.util.control.Exception._

def ioretry[T](n: Int)(t: => T) = (
  Iterator.fill(n){ failing[T](classOf[IOException]){ Option(t) } } ++
  Iterator(Some(t))
).dropWhile(_.isEmpty).next.get

(As written, it will also retry on null; that's the Option(t) part. If you want nulls to be returned, use Some(t) inside the iterator fill instead.)

Let's try this out with

class IoEx(var n: Int) {
  def get = if (n>0) { n -= 1; throw new IOException } else 5
}
val ix = new IoEx(3)

Does it work?

scala> ioretry(4) { ix.get }
res0: Int = 5

scala> ix.n = 3

scala> ioretry(2) { ix.get }
java.io.IOException
    at IoEx.get(<console>:20)
    ...

scala> ioretry(4) { throw new Exception }
java.lang.Exception
    at $anonfun$1.apply(<console>:21)
    ...

Looks good!

answered 2011-10-28T18:32:44.250

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