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1# Async-Limiter
2
3A module for limiting concurrent asynchronous actions in flight. Forked from [queue](https://github.com/jessetane/queue).
4
5[![npm](http://img.shields.io/npm/v/async-limiter.svg?style=flat-square)](http://www.npmjs.org/async-limiter)
6[![tests](https://img.shields.io/travis/STRML/async-limiter.svg?style=flat-square&branch=master)](https://travis-ci.org/STRML/async-limiter)
7[![coverage](https://img.shields.io/coveralls/STRML/async-limiter.svg?style=flat-square&branch=master)](https://coveralls.io/r/STRML/async-limiter)
8
9This module exports a class `Limiter` that implements some of the `Array` API.
10Pass async functions (ones that accept a callback or return a promise) to an instance's additive array methods.
11
12## Motivation
13
14Certain functions, like `zlib`, have [undesirable behavior](https://github.com/nodejs/node/issues/8871#issuecomment-250915913) when
15run at infinite concurrency.
16
17In this case, it is actually faster, and takes far less memory, to limit concurrency.
18
19This module should do the absolute minimum work necessary to queue up functions. PRs are welcome that would
20make this module faster or lighter, but new functionality is not desired.
21
22Style should confirm to nodejs/node style.
23
24## Example
25
26``` javascript
27var Limiter = require('async-limiter')
28
29var t = new Limiter({concurrency: 2});
30var results = []
31
32// add jobs using the familiar Array API
33t.push(function (cb) {
34 results.push('two')
35 cb()
36})
37
38t.push(
39 function (cb) {
40 results.push('four')
41 cb()
42 },
43 function (cb) {
44 results.push('five')
45 cb()
46 }
47)
48
49t.unshift(function (cb) {
50 results.push('one')
51 cb()
52})
53
54t.splice(2, 0, function (cb) {
55 results.push('three')
56 cb()
57})
58
59// Jobs run automatically. If you want a callback when all are done,
60// call 'onDone()'.
61t.onDone(function () {
62 console.log('all done:', results)
63})
64```
65
66## Zlib Example
67
68```js
69const zlib = require('zlib');
70const Limiter = require('async-limiter');
71
72const message = {some: "data"};
73const payload = new Buffer(JSON.stringify(message));
74
75// Try with different concurrency values to see how this actually
76// slows significantly with higher concurrency!
77//
78// 5: 1398.607ms
79// 10: 1375.668ms
80// Infinity: 4423.300ms
81//
82const t = new Limiter({concurrency: 5});
83function deflate(payload, cb) {
84 t.push(function(done) {
85 zlib.deflate(payload, function(err, buffer) {
86 done();
87 cb(err, buffer);
88 });
89 });
90}
91
92console.time('deflate');
93for(let i = 0; i < 30000; ++i) {
94 deflate(payload, function (err, buffer) {});
95}
96t.onDone(function() {
97 console.timeEnd('deflate');
98});
99```
100
101## Install
102
103`npm install async-limiter`
104
105## Test
106
107`npm test`
108
109## API
110
111### `var t = new Limiter([opts])`
112Constructor. `opts` may contain inital values for:
113* `t.concurrency`
114
115## Instance methods
116
117### `t.onDone(fn)`
118`fn` will be called once and only once, when the queue is empty.
119
120## Instance methods mixed in from `Array`
121Mozilla has docs on how these methods work [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array).
122### `t.push(element1, ..., elementN)`
123### `t.unshift(element1, ..., elementN)`
124### `t.splice(index , howMany[, element1[, ...[, elementN]]])`
125
126## Properties
127### `t.concurrency`
128Max number of jobs the queue should process concurrently, defaults to `Infinity`.
129
130### `t.length`
131Jobs pending + jobs to process (readonly).
132
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