p-parallel

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Make promises run in parallel with a limit amount.

Start

npm install p-parallel --save

Usage

pParallel(functionArr, parallelLimit)

• functionArr：Array，an array of functions those return a promise.
• parallelLimit：Number，the max amount of running promises in parallel.

The method returns a single Promise. This promise will run all of the functions passed as functionArr in parallel asynchronously under the control of the max amount of running promises by parallelLimit. This promise fulfills when all of the promises returned by functions in functionArr have been fulfilled. It rejects with the reason of the first promise that rejects.

This method can be useful for do something in parallel with limited source, such as sending requests in parallel with a limit amount.

Different from Promise.all()

• While Promise.all() takes an iterable of promises as an input, while pParallel() takes an array of functions those return a promise.
• While Promise.all(promiseArr) runs all the promises in promiseArr in parallel asynchronously, pParallel(promiseArr, parallelLimit) runs under the control of the max amount of running promises by parallelLimit and in the order of FIFO.

Example

const pParallel = require('p-parallel');

function task(data, delay, success = true) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (success) {
        resolve(data);
      } else {
        reject(data);
      }
    }, delay);
  });
}

function test(functionArr) {
  pParallel(functionArr, 2).then(res => console.log('success:', res), res => console.log('fail:', res));
}

function success1() {
  return task('success1', 4000);
}
function success2() {
  return task('success2', 2000);
}
function success3() {
  return task('success3', 2000);
}
function fail1() {
  return task('fail1', 2000, false);
}
function fail2() {
  return task('fail2', 4000, false);
}

// this will print 'success: success1, success2, success3' after 4000ms
test([success1, success2, success3]);
// this will print 'fail: fail1' after 2000ms
test([success1, success2, fail]);

Tips

How to catch fail?

Sometimes, we need to catch exceptions to keep all promises in functionArr done. We can use .catch.

const pParallel = require('p-parallel');

function task(data, delay, success = true) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (success) {
        resolve(data);
      } else {
        reject(data)
      }
    }, delay);
  });
}

function test(functionArr) {
  pParallel(functionArr, 2).then(res => console.log('success:', res), res => console.log('fail:', res));
}

function success1() {
  return task('success1', 4000);
}
function success2() {
  return task('success2', 2000);
}
function fail1() {
  return task('fail1', 2000, false).catch(data => data);
}

// this will print 'success: success1, success2, fail1' after 4000ms
test([success1, success2, fail]);

A more elegant way to use p-parallel?

const pParallel = require('p-parallel');
// Personally, I think it's a more elegant way.
Promise.parallel = pParallel;