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Last Updated: February 25, 2016
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· mortimerpa
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Retry a fail-able block in Scala

I have seen a few implementations of this online, in particular Retrying with style, which is a two years old post and has a few things that bothered me:

  1. it's blocking, Futures where not in scala at the time,
  2. it uses mutable variables, which are not intrinsically bad in such a contained context; but my editor highlights them in red and I don't like having red bits in my code :D

I have thus created a new implementation available at this gist.

The idea is that you could have a block of code that fails for any kind of reasons which are non deterministic. For instance, a call to the DB could fail because the connection queue is full, or a request to a webservice could fail because the "internet is busy".

Instead of writting a loop and a lot of code relevant to the retry part, you can write a block wrapper in scala that will take care of everything in the background for you:

val myResult = retry(10) {
   ...
   makeWSCall()
   ...
}

This is what the little bit of code that I have put in this gist does. I won't copy all of it here as it's too long, but here are some comments on how it works:

  • I use a Promise to eventually embed a value in a Future. The future of that promise is returned directly and the tries are executed asynchronously.
  • if an exception is caught, it tries again, unless it has exhausted the given number of retries, in which case it fails the future.
  • as soon as the result can be computed without exceptions, it is returned in the future.

Because the block returns a Future, the retries are executed asynchronously, hopefully not blocking your data flow. You can use the Future API to manipulate the value eventually returned:

myResult.map(_ * 2)

You can also use the built-in recovery methods of Future to deal with a failed result (too many retries):

val myResult = retry(10) {
   ...
   makeWSCall()
   ...
} recover {
  case t: Throwable => 0
}

The retry function also provides some advanced optional parameters:

  • in addition to the mandatory maximum retry count, you can define an optional deadline after which it should just give up:
val myResult = retry(10, Some(10 seconds fromNow)) {
   ...
   makeWSCall()
   ...
}
  • by default, the block will be retried with an exponential back-off, as it is often used to deal with resources that don't like to be overloaded, you can change this default if you want
val myResult = retry(10, backoff = (r) => 100 milliseconds) {
   ...
   makeWSCall()
   ...
}
  • if you want the block to fail without retry for particular exceptions, you can specify a filtering function with the ignoreThrowable optional parameter
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