ES6 Promise Pool

Runs Promises in a pool that limits their maximum concurrency.

Motivation

An ECMAScript 6 Promise is a great way of handling 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. On modern platforms, you can also use ES6 generator functions for this.

Compatibility

This module can be used both under Node.js (version 0.10 and up) and on the Web. If your platform does not have a Promise implementation, it will be polyfilled by ES6-Promise.

Installation

npm install --save es6-promise-pool

bower install --save es6-promise-pool

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

Usage

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

var PromisePool = promisePool.PromisePool

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)
})

Producers

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)
  })
}

Function

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')
  })

Generator

We can achieve the same result with ECMAScript 6 generator functions.

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

const pool = new PromisePool(promiseProducer, 3)

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

Events

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')
  })

Alternatives

• Vilic Vane's Promise Pool offers a similar API.
• Bluebird includes Promise.map(), which takes a concurrency option.
• With Q, you can use qlimit.
• Async does not use promises, but offers a queue() function.

Author

Tim De Pauw

License

MIT