Runs Promises in a pool that limits their concurrency.

An ECMAScript 2015 Promise is a great way to handle asynchronous operations. The Promise.all function provides an easy interface to let a bunch of promises settle concurrently.

However, it's an all-or-nothing approach: all your promises get created simultaneously. If you have a ton of operations that you want to run with some concurrency, Promise.all is no good.

Instead, you probably want to limit the maximum number of simultaneous operations. That's where this module comes in. It provides an easy way of waiting for any number of promises to settle, while imposing an upper bound on the number of simultaneously executing promises.

The promises can be created in a just-in-time fashion. You essentially pass a function that produces a new promise every time it is called. Alternatively, you can pass an ES2015 iterator, meaning you can also use generator functions.

This module can be used both under Node.js and on the Web. If your platform does not have a native Promise implementation, you can use a polyfill such as ES6-Promise.

npm install --save es6-promise-pool

bower install --save es6-promise-pool

<script src="es6-promise-pool.js"></script>

// On the Web, leave out this line and use the script tag above instead.
var PromisePool = require('es6-promise-pool')

var promiseProducer = function () {
  // Your code goes here.
  // If there is work left to be done, return the next work item as a promise.
  // Otherwise, return null to indicate that all promises have been created.
  // Scroll down for an example.
}

// The number of promises to process simultaneously.
var concurrency = 3

// Create a pool.
var pool = new PromisePool(promiseProducer, concurrency)

// Start the pool.
var poolPromise = pool.start()

// Wait for the pool to settle.
poolPromise.then(function () {
  console.log('All promises fulfilled')
}, function (error) {
  console.log('Some promise rejected: ' + error.message)
})

The PromisePool constructor takes a Promise-producing function as its first argument. Let's first assume that we have this helper function that returns a promise for the given value after time milliseconds:

var delayValue = function (value, time) {
  return new Promise(function (resolve, reject) {
    console.log('Resolving ' + value + ' in ' + time + ' ms')
    setTimeout(function () {
      console.log('Resolving: ' + value)
      resolve(value)
    }, time)
  })
}

Now, let's use the helper function above to create five such promises, which are each fulfilled after a second. Because of the concurrency of 3, the first three promises will be fulfilled after one second. Then, the remaining two will be processed and fulfilled after another second.

var count = 0
var promiseProducer = function () {
  if (count < 5) {
    count++
    return delayValue(count, 1000)
  } else {
    return null
  }
}

var pool = new PromisePool(promiseProducer, 3)

pool.start()
  .then(function () {
    console.log('Complete')
  })

We can achieve the same result with ECMAScript 2015 iterators. Since ES2015 generator functions return such an iterator, we can make the example above a lot prettier:

const generatePromises = function * () {
  for (let count = 1; count <= 5; count++) {
    yield delayValue(count, 1000)
  }
}

const promiseIterator = generatePromises()
const pool = new PromisePool(promiseIterator, 3)

pool.start()
  .then(() => console.log('Complete'))

It's also possible to pass a generator function directly. In that case, it will be invoked with no arguments and the resulting iterator will be used. This feature will however be removed in version 3.

We can also ask the promise pool to notify us when an individual promise is fulfilled or rejected. The pool fires fulfilled and rejected events exactly for this purpose.

var pool = new PromisePool(promiseProducer, concurrency)

pool.addEventListener('fulfilled', function (event) {
  // The event contains:
  // - target:    the PromisePool itself
  // - data:
  //   - promise: the Promise that got fulfilled
  //   - result:  the result of that Promise
  console.log('Fulfilled: ' + event.data.result)
})

pool.addEventListener('rejected', function (event) {
  // The event contains:
  // - target:    the PromisePool itself
  // - data:
  //   - promise: the Promise that got rejected
  //   - error:   the Error for the rejection
  console.log('Rejected: ' + event.data.error.message)
})

pool.start()
  .then(function () {
    console.log('Complete')
  })

Since version 2.0.0, this module does not depend on ES6-Promise anymore. If you want to support platforms without a native Promise implementation, please load a polyfill first.

If you prefer not to polyfill the global Promise for whatever reason, you can also pass your Promise class as an option instead:

var ES6Promise = require('es6-promise').Promise // or another implementation
var pool = new PromisePool(promiseProducer, concurrency, {promise: ES6Promise})

• Vilic Vane's Promise Pool offers a similar API.
• Bluebird includes Promise.map(), which takes a concurrency option.
• Similarly, λ (a.k.a. contra) has λ.concurrent() with the optional cap parameter.
• With Q, you can use qlimit.
• Async does not use promises, but offers a queue() function.

Tim De Pauw

MIT