Tiny Promise Pool

A tiny library to execute multiple promises in parallel, keeping not to execute more than N promises at any one time, using a promise.

Tiny Promise Pool is reviewed by
Tiny promise Pool is unit tested with

Install:

npm i tiny-promise-pool

Use with creator function:

var promisePool = require('tiny-promise-pool')

function nextPromise({index, data}) {
  if (index>=20) return null // no more
  return new Promise(function(res, rej) {
    res(index*2 + data)
  })
}

var all = promisePool({
  threads: 3,                 // maximum parallel promises
  promises: nextPromise,  // function to get/generate next promise
  context_data: 17       // user data for the next_promise function
})

all.then(function(result) {
  console.dir(result)         // after all promises are resolved
})

Result:

[
  { context: { index: 0, thread: 0, data: 17, ended: 10 }, promise: Promise { 17 }, result: 17 },
  { context: { index: 1, thread: 1, data: 17, ended: 1 }, promise: Promise { 19 }, result: 19 },
  { context: { index: 2, thread: 2, data: 17, ended: 7 }, promise: Promise { 21 }, result: 21 },
  { context: { index: 3, thread: 0, data: 17, ended: 6 }, promise: Promise { 23 }, result: 23 },
  { context: { index: 4, thread: 1, data: 17, ended: 2 }, promise: Promise { 25 }, result: 25 },
  { context: { index: 5, thread: 2, data: 17, ended: 3 }, promise: Promise { 27 }, result: 27 },
  { context: { index: 6, thread: 0, data: 17, ended: 4 }, promise: Promise { 29 }, result: 29 },
  { context: { index: 7, thread: 1, data: 17, ended: 5 }, promise: Promise { 31 }, result: 31 },
  ...
  { context: { index: 19, thread: 1, data: 17, ended: 19 }, promise: Promise { 55 }, result: 55 }
]

Use with array of promises:

function makePromise(i) {
  return new Promise(function (resolve, _reject) {
    resolve(i)
  })
}

const promiseList = [
  makePromise(0),
  makePromise(1)
]

promisePool({
  threads: 3,
  promises: promiseList,              // List of promises
  context_data: 'data for context'
}).then(function(result) {
  ...
})

Using generator to generate promises:

function *createPromiseMaker() {
  for (var i=0; i<10; i+=1) {
    yield new Promise(...)
  }
}

const pool = promisePool({
  threads: 3,
  promises: createPromiseMaker()
})

Nested promise pools:

const innerPool = promisePool({
  threads: 2,
  promises: [
    makePromise(2),
    makePromise(3),
    makePromise(4),
    makePromise(5)
  ],
  context_data: 'secondary promise pool'
})
const pool = promisePool({
  threads: 3,
  promises: [
    makePromise(0),
    makePromise(1),
    innerPool
  ],
  context_data: 'primary promise pool'
})
pool.then(function(result) {
  ...
}

Debugging tool:

promisePool = require('tiny-promise-pool')
visualize = require('tiny-promise-pool/promise-pool-visualize')
const pool = promisePool({
  ...
})
pool.then(visualize).then(result => {
   ...
})

TODO list:

API similar to Promise.all(): "Promise.pool([promise, ...], threads)"
options for behavior on reject:
- reject as soon as one is rejected (same as Promise.all)
- wait for all to resolve even if some are rejected (same as Promise.when)
support listeners for individual promise completions/rejections

migration from previous versions:

the following option keys are renamed:

next_promise_data -> context_data
next_promise -> promises
max_parallel -> threads

you can still use the older option keys, but they are deprecated

eetay/promise-pool-js

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