multi-promise

Run multiple promises in parallel with a strategy

Motivation

Assume you want to run multiple promises in parallel, how would you go about it?

There's Promise.all but this will bail out on the first error, which might not be what you want.

In case you're only interested in one of the results you could consider Promise.race, but you can't choose if you want a fulfilled or rejected promise.

To add another requirement to the mix: How about canceling all other promises when one is finished?

MultiPromise is a higher level abstraction that makes handling a bunch of promises more fun to work with.

Features

• Run multiple promises in parallel with a custom strategy
  • Built in: firstSuccess, firstError, waitForAll, bailOnError
• Optional: determineSuccessFn to check if fulfilled promises are really a success
• Optional: transformResultFn to return exactly the result you need
• Optional: cleanupFn to cancel/abort other promises
• Optional: timeout to define a global timeout for all tasks
• Optional: Add an id to each task to make later business logic easier
• Generic: Use any kind of promises (no assumptions are being made)
• Written in Typescript but works with JS (Node, Browser) as well
• Lightweight, fast, no dependencies

Installation

yarn add multi-promise

or

npm install --save multi-promise

Usage

// TypeScript
import { MultiPromise } from 'multi-promise'
 
// ES6 JavaScript
import { MultiPromise } from 'multi-promise'
 
// Legacy JavaScript
const MultiPromise = require('multi-promise').MultiPromise

Example

Let's say you want to issue 5 requests in parallel and get the first successful result. You also want some say in what success means, as a non-rejected promise (e.g. statusCode 200) could still mean failure in your business logic. Once a successful request has been made all other running requests should be aborted (no one likes sockets being kept open, right?)

const { MultiPromise } = require('multi-promise')
const rp = require('request-promise')
 
// define defaults for the request library
const rpx = rp.defaults({
  timeout: 20 * 1000,
  resolveWithFullResponse: true,
  time: true,
  simple: false,
  json: true
})
 
async function main() {
  const mp = new MultiPromise()
 
  // add tasks with an optional id
  // note the use of a wrapper function to not prematurely run the promises
  mp.add(task => rpx({ uri: 'https://invalid.ripe' }), { id: 'ripe' })
  mp.add(task => rpx({ uri: 'https://invalid2.ripe' }))
  mp.add(task => rpx({ uri: 'https://httpbin.org/delay/3' }), { id: 'foobar1' })
  mp.add(task => rpx({ uri: 'https://example.com' }), { id: 'example' })
  mp.add(task => rpx({ uri: 'https://httpbin.org/delay/9' }), { id: 'foobar2' })
 
  // optional: specify a success callback to conditionally reject promises
  // the example.com request will succeed first, but we don't like it :-)
  // we can either throw or return false here to reject a fulfilled promise
  mp.determineSuccessFn = (task, response) => {
    if (response.body.toString().includes('example'))
      throw new Error('Example is bad')
  }
 
  // optional: transform the result property of a fulfilled promise
  // the first argument is always the task itself,
  // the following arguments are specific to your promises
  mp.transformResultFn = (task, response) => ({
    statusCode: response.statusCode,
    headers: response.headers,
    body: response.body
  })
 
  // optional: clean up once we're finished
  // in this case we call `.cancel()` on the request promise
  // for all requests that are not finished yet (this will release the sockets)
  mp.cleanupFn = tasks => {
    tasks.filter(task => !task.isFinished).forEach(task => { task.handle.cancel()) })
  }
 
  // run all promises in parallel with a strategy
  // in this case return the first successful promise
  const results = await mp.firstSuccess()
  console.log(results)
  // => [ { id: 'foobar1', result: { statusCode: 200, headers: [Object], body: [Object] } } ]
}
main()

Strategies

.firstSuccess()

Return first successful promise, don't mind erroneous ones

.firstError()

Return first erroneous promise, ignore successful ones

.waitForAll() (default)

Return all promises, don't stop on errors

.bailOnError()

Stop on erroneous promises (Promise.all behaviour)

Custom strategy

Example:

const firstSuccessStrategy = (task, tasks) => {
  if (task.isError) task.addToResults = false
  if (task.isSuccess) return true
}
 
const mp = new MultiPromise()
const results = await mp.run(firstSuccessStrategy)

The strategy function is being called whenever a promise returns. By default all promise results are added to the final results array. Use task.addToResults = false to exclude a result. If the strategy function returns true then the existing results will be returned and the execution finished.

Options

const mp = new MultiProxy({
  timeout?: number // optional global timeout
  returnUnfinished?: boolean // default: false, will add all promises to results
  strategyFn?: MultiPromiseStrategy 
  determineSuccessFn?: Function 
  transformResultFn?: Function 
  cleanupFn?: Function 
})

Task object

An array of task objects will be returned.

interface MultiPromiseTask {
  /** User chosen task id of any type */
  id?: any
  /** Numerical task index, auto-generated */
  index?: number
  /** Anonymous task function returning a promise */
  fn: (dummy?: any) => any
  /** Promise handle */
  handle?: Promise<any>
  isFinished?: boolean
  isSuccess?: boolean
  isError?: boolean
  addToResults?: boolean
  result?: any
  error?: any
}

Successful (fulfilled) promises contain a result property, erroneous (rejected) promises an error property.

In addition an optional id and a couple of booleanes are included.

Debug

DEBUG=multi-promise node ...

License

MIT