Best practice to make cleanup after Scala Future is complete

Question

I want to make some cleanup (like close db connection) after a Future is complete.
Currently I achieve it this way:

Future { ... } onComplete {
  case Success(v) =>
    // ...
    conn.close()
  case Failure(ex) =>
    // ...
    conn.close()
}

There's duplicate code and it's also tedious.
Is there any best practice to this?

Alyssaalyssum

Answer 1

Since the same action conn.close() is performed on both success and failure, consider executing it as a side-effect using andThen like so

Future { ... } andThen { _ => conn.close() }

Similarly, using onComplete we could do

Future { ... } onComplete { _ => conn.close() }

The difference between andThen and onComplete is that latter will return Unit, that is, discard the returned value of the Future.

Medicate

Answer 2

The sad truth is that Scala Futures don't have this basic feature built in. I would therefore strongly suggest using a modern effect system like ZIO or cats-effect, both of which solve this problem and a myriad of others that Futures have. The easiest way to do what you want is to use the bracket method:

https://zio.dev/docs/overview/overview_handling_resources

Now bracket works great, but there's a way that usually works even better: the Managed type. It's virtually impossible to write code that leaks resources if you consistently use Managed when acquiring resources:

https://zio.dev/docs/datatypes/datatypes_managed

That said,if you absolutely must use Futures, you'll have to write your own try-finally equivalent. Or you can use mine:

  def tryFinally[A](tryy: => Future[A])(finallyy: => Future[Any])(
  implicit ec: ExecutionContext): Future[A] =
    Future.fromTry(Try(tryy)).flatten.transformWith { t =>
      finallyy.flatMap((_: Any) => Future.fromTry(t))
    }

Prescript

Answer 3

I like to use map, you can do something like this:

val mapped: Future[String] = future.map(_ => "OK").recover{case _ => "KO"}

Supersonics

Using two calls when one would do is not good practice. Use a single call like onComplete, andThen or transform – Choplogic