What is the best way to limit concurrency when using ES6's Promise.all()?

Solution 1:

P-Limit

I have compared promise concurrency limitation with a custom script, bluebird, es6-promise-pool, and p-limit. I believe that p-limit has the most simple, stripped down implementation for this need. See their documentation.

Requirements

To be compatible with async in example

• ECMAScript 2017 (version 8)
• Node version > 8.2.1

My Example

In this example, we need to run a function for every URL in the array (like, maybe an API request). Here this is called fetchData(). If we had an array of thousands of items to process, concurrency would definitely be useful to save on CPU and memory resources.

const pLimit = require('p-limit');

// Example Concurrency of 3 promise at once
const limit = pLimit(3);

let urls = [
    "http://www.exampleone.com/",
    "http://www.exampletwo.com/",
    "http://www.examplethree.com/",
    "http://www.examplefour.com/",
]

// Create an array of our promises using map (fetchData() returns a promise)
let promises = urls.map(url => {

    // wrap the function we are calling in the limit function we defined above
    return limit(() => fetchData(url));
});

(async () => {
    // Only three promises are run at once (as defined above)
    const result = await Promise.all(promises);
    console.log(result);
})();

The console log result is an array of your resolved promises response data.

Solution 2:

Note that Promise.all() doesn't trigger the promises to start their work, creating the promise itself does.

With that in mind, one solution would be to check whenever a promise is resolved whether a new promise should be started or whether you're already at the limit.

However, there is really no need to reinvent the wheel here. One library that you could use for this purpose is es6-promise-pool. From their examples:

// 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)
})