Promise Pool

Runs Promises in a pool that limits their maximum concurrency.

Motivation

An ES6 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.

Demo

Node.js

npm install
node demo.js

Add --harmony for generator support if you have Node.js version 0.11.

Web

See demo.html.

Basic Usage

// On the Web, just leave out this line.
var promisePool = require('es6-promise-pool');

// Can also be a generator. See below.
var promiseProducer = function() {
  // There's a 10% chance that we return null, indicating that there are no
  // more promises left to process.
  if (Math.floor(Math.random() * 10) === 0) {
    console.log('No more promises in pool');
    return null;
  }
  // If we didn't return null, we pass a new promise to the pool.
  var promisedValue = new Date().getTime();
  console.log('Creating new promise for ' + promisedValue);
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      console.log('Resolving promise: ' + promisedValue);
      // See below for instructions on how to retrieve the value.
      resolve(promisedValue);
    }, 1000);
  });
};

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

// See below.
var options = {};

// Create a pool promise and wait for it to settle.
promisePool(promiseProducer, concurrency, options)
.then(function() {
  console.log('All promises resolved');
}, function(error) {
  console.log('Some promise rejected: ' + error.message);
});

Producers

The promisePool function takes a variety of Promise-producing objects. Let's first assume 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 each resolve after a second. Because of the concurrency of 3, the first three promises will resolve after a second. Then, the remaining two will be processed and resolve after another second.

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

promisePool(promiseProducer, 3)
.then(function() {
  console.log('Complete');
});

Generator

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

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

promisePool(promiseProducer, 3)
.then(function() {
  console.log('Complete');
});

Options

The options object lets us provide additional callback functions to listen for promise progress.

When a promise settles, either options.onresolve or options.onreject will be called. Both functions receive the pool promise (as returned by promisePool), the promise that settled, and either the resolved value or the Error that caused the rejection.

var options = {};

options.onresolve = function(poolPromise, promise, result) {
  console.log('Resolved: ' + result);
};

options.onreject = function(poolPromise, promise, error) {
  console.log('Rejected: ' + error.message);
};

promisePool(promiseProducer, concurrency, options);

Alternatives

• Async.js
• Promise Pool
• qlimit

Author

Tim De Pauw

License

Copyright © 2015 Tim De Pauw

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.