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Motivation

Need ability to invoke tasks asynchronously at the bottom of core.async. And fast.

Goals

  • Executor interface
    • Need ability to invoke tasks asynchronously
      • Effectively ExecutorService.submit(Runnable)
    • May want ability to shutdown the executor?
  • Executor implementation(s)
    • Need some implementation to cover JDK 1.6, 1.7, 1.8
    • Possible implementations
      • ThreadPoolExecutor
      • ForkJoin 
    • Executor library version
      • Provide implementation without external dependency
        • Use what's in the JDK or
        • Repackage jsr166 into our own package (jsr166 is public domain)
      • Allow path for users to use a bleeding edge version
    • Configuration 
      • Both ThreadPoolExecutor and ForkJoin have a number of of possible configuration parameters
      • Provide good defaults but also let user change them

Non-goals

  • Do not need to provide general access to ForkJoin (fork, join, recursive computation, etc)
  • Do not need access to futures for tasks
  • Do not need task cancellation

Proposed solution

  • Define protocol for async task execution
    • (defprotocol Executor (exec `[e task]))
  • Provide several implementations
    • for ThreadPoolExecutor
    • for ForkJoinPool 
      • Repackage jsr166y into our own packages (put in Clojure 1.6)
      • Allow user to use jsr166e instead?
      • Use built-in JDK version
  • Choose default implementation based on environment
    • (maybe?)  Check for jsr166e.ForkJoinPool (external JDK version - override path for the future?)
    • Check for clojure.concurrent.ForkJoinPool (Clojure repackaged jsr166y) - Clojure 1.6+, JDK 1.6+
    • Check for java.util.concurrent.ForkJoinPool (JDK built-in version) - Clojure 1.5, JDK 1.7+
    • Check for jsr166y.ForkJoinPool (external JDK version) - Clojure 1.5, external jsr166 jar
    • Fallback to Executor version - Clojure 1.5, no external jar
  • Allow user to override the implementation we chose with their own custom executor that does whatever they want

Background / Reference

Background:

  • JSR 166 
    • jsr166x - included in JDK 1.6, does not contain ForkJoin
    • jsr166y - compiled with JDK 1.6, included in JDK 1.7. jsr166y jar has fixes past what is in the JDK 1.6.
    • jsr166e - compiled with JDK 1.8, to be included in JDK 1.8. jsr166e jar has fixes past what is in JDK 1.7. 
  • Scala solution
    • Repackage known version of jsr166y as scala.concurrent and include in the Scala language (Akka makes use of it from there)
      • Covers JDK 1.6+ (good) but does not contain the very latest updates for JDK 1.8 that are in jsr166e (bad)
      • When they have a version of Scala that requires JDK 1.7, they will presumably update to jsr166e
    • Akka has pluggable executor providers and using a custom one allows you to make use of jsr166e if you need it
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