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    <p>Very important thing, though usually not described in tutorials:</p> <p><code>Runnables to be executed on an ExecutorService should not block</code>. This is because each blocking switches off a working thread, and if ExecutorService has limited number of working threads, there is a risk to fall into deadlock (thread starvation), and if ExecutorService has unlimited number of working threads, then there is a risk to run out of memory. Blocking operations in the tasks simply destroy all advantages of ExecutorService, so use blocking operations on usual threads only.</p> <p><code>FutureTask.get()</code> is blocking operation, so can be used on ordinary threads and not from an ExecutorService task. That is, it cannot serve as a building block, but only to deliver result of execution to the master thread.</p> <p>Right approach to build execution from tasks is to start next task when all input data for the next task is ready, so that the task do not have to block waiting for input data. So you need a kind of a gate which stores intermediate results and starts new task when all arguments have arrived. Thus tasks do not bother explicitly to start other tasks. So a gate, which consists of input sockets for arguments and a Runnable to compute them, can be considered as a right building block for computations on ExcutorServices.</p> <p>This approach is called dataflow or workflow (if gates cannot be created dynamically).</p> <p>Actor frameworks like Akka use this approach but are limited in the fact that an actor is a gate with single input socket. </p> <p>I have written a true dataflow library published at <a href="https://github.com/rfqu/df4j" rel="noreferrer">https://github.com/rfqu/df4j</a>.</p>
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    1. COThanks for that explanation. It basically points out my problem very well. The thing is if you have some sort of task you want to reuse, then the logic what to do after cannot be included in that task and the Future interface is missing async callbacks. I recently came across https://code.google.com/p/guava-libraries/wiki/ListenableFutureExplained which also tries to solve this. But I usually want to find out how its meant to work, and basically, if i do not assume they did not think about this designing the interfaces, I still dont get it.
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    2. COThe best way to thank is to upvote the answer. Df4j has several kinds of building blocks, including `CallbackPromise` which is close to guava's `ForwardingCheckedFuture`. If you'll find that some other kinds of building blocks are worth to include in the library, simply file an issue at https://github.com/rfqu/df4j/issues
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    3. COYou seem misinformed, Akka dataflow does not use actors: http://doc.akka.io/docs/akka/2.1.0/scala/dataflow.html, Akka Futures (now in Scala Standard Library, doesn't use Actors either: http://doc.akka.io/docs/akka/2.1.0/scala/futures.html)
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      1. USViktor Klang
 

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