[concurrency-interest] Composing multiple tasks into a single Future/FutureTask

√iktor Ҡlang viktor.klang at gmail.com
Tue Apr 17 18:44:43 EDT 2012

Hi Ariel,

Akka's got composable Futures:

Essentiall, what you're looking for is the "andThen" operation:


val resultingFuture = Future { compressAndChecksum(payload) } andThen { _
=> fsync() }


On Tue, Apr 17, 2012 at 5:39 PM, Ariel Weisberg <ariel at weisberg.ws> wrote:

> Hi all,
> I have been looking for the right way to build an API that returns a
> Future that is composed of tasks that will be performed in multiple
> stages, potentially from more than one thread.
> My use cases is a disk based persistence store. The initial put API
> call creates a task that compresses and checksums the payload. The
> resulting Future is bound in a Runnable and submitted to a single
> thread ExecutorService that timestamps the entry and appends it to a
> file and it is the Future of the second task that is currently returned
> by the API.
>  The trick is that the Future that is returned by the put call should
>  not complete until the write is durable. Writes are made durable by
>  periodically invoking fsync from a dedicated thread or by rotating
>  write threads as each blocks on fsync/force. This allows for group
>  commit at an interval the IO device can handle without killing
>  throughput.
> Ideally I could create a FutureTask that is composed of several other
> Futures, some of which will not exist when the FutureTask is initially
> created and returned by the API. I do know how many dependencies there
> will be
> expected up front.
> If I had such a thing I would create a task to do the fsync and add it
> as the last dependency to every Future read by the put API call.
> This would be a lot easier if I were willing to have many threads
> standing around blocking on Futures, but that rubs me the wrong way. It
> isn't fun to jstack and see all the noise.
> I suspect that there is something in Guava
> com.google.common.util.concurrent.Futures to handle this case, but it
> isn't jumping out at me.
> Thanks,
> Ariel Weisberg
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Viktor Klang

Akka Tech Lead
Typesafe <http://www.typesafe.com/> - The software stack for applications
that scale

Twitter: @viktorklang
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