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82 lines
3.1 KiB
Markdown
82 lines
3.1 KiB
Markdown
5 years ago
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# ASAP
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This `asap` CommonJS package contains a single `asap` module that
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exports a single `asap` function that executes a function **as soon as
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possible**.
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```javascript
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asap(function () {
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// ...
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});
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```
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More formally, ASAP provides a fast event queue that will execute tasks
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until it is empty before yielding to the JavaScript engine's underlying
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event-loop. When the event queue becomes non-empty, ASAP schedules a
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flush event, preferring for that event to occur before the JavaScript
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engine has an opportunity to perform IO tasks or rendering, thus making
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the first task and subsequent tasks semantically indistinguishable.
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ASAP uses a variety of techniques to preserve this invariant on
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different versions of browsers and NodeJS.
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By design, ASAP can starve the event loop on the theory that, if there
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is enough work to be done synchronously, albeit in separate events, long
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enough to starve input or output, it is a strong indicator that the
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program needs to push back on scheduling more work.
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Take care. ASAP can sustain infinite recursive calls indefinitely
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without warning. This is behaviorally equivalent to an infinite loop.
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It will not halt from a stack overflow, but it *will* chew through
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memory (which is an oddity I cannot explain at this time). Just as with
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infinite loops, you can monitor a Node process for this behavior with a
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heart-beat signal. As with infinite loops, a very small amount of
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caution goes a long way to avoiding problems.
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```javascript
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function loop() {
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asap(loop);
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}
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loop();
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```
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ASAP is distinct from `setImmediate` in that it does not suffer the
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overhead of returning a handle and being possible to cancel. For a
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`setImmediate` shim, consider [setImmediate][].
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[setImmediate]: https://github.com/noblejs/setimmediate
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If a task throws an exception, it will not interrupt the flushing of
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high-priority tasks. The exception will be postponed to a later,
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low-priority event to avoid slow-downs, when the underlying JavaScript
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engine will treat it as it does any unhandled exception.
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## Heritage
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ASAP has been factored out of the [Q][] asynchronous promise library.
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It originally had a naïve implementation in terms of `setTimeout`, but
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[Malte Ubl][NonBlocking] provided an insight that `postMessage` might be
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useful for creating a high-priority, no-delay event dispatch hack.
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Since then, Internet Explorer proposed and implemented `setImmediate`.
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Robert Kratić began contributing to Q by measuring the performance of
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the internal implementation of `asap`, paying particular attention to
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error recovery. Domenic, Robert, and I collectively settled on the
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current strategy of unrolling the high-priority event queue internally
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regardless of what strategy we used to dispatch the potentially
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lower-priority flush event. Domenic went on to make ASAP cooperate with
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NodeJS domains.
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[Q]: https://github.com/kriskowal/q
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[NonBlocking]: http://www.nonblocking.io/2011/06/windownexttick.html
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For further reading, Nicholas Zakas provided a thorough article on [The
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Case for setImmediate][NCZ].
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[NCZ]: http://www.nczonline.net/blog/2013/07/09/the-case-for-setimmediate/
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## License
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Copyright 2009-2013 by Contributors
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MIT License (enclosed)
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