async-retry-ng

Retrying made simple, easy, and async.

Usage

// Packages
const retry = require('async-retry-ng')
const fetch = require('node-fetch')

await retry(async bail => {
  // if anything throws, we retry
  const res = await fetch('https://google.com')

  if (403 === res.status) {
    // don't retry upon 403
    bail(new Error('Unauthorized'))
    return
  }

  const data = await res.text()
  return data.substr(0, 500)
}, {
  retries: 5
})

API

retry(retrier : Function, opts : Object) => Promise

• The supplied function can be async or not. In other words, it can be a function that returns a Promise or a value.
• The supplied function receives two parameters
  1. A Function you can invoke to abort the retrying (bail)
  2. A Number identifying the attempt. The absolute first attempt (before any retries) is 1.
• The opts
  • retries: The maximum amount of times to retry the operation. Default is 10.
  • factor: The exponential factor to use. Default is 2.
  • minTimeout: The number of milliseconds before starting the first retry. Default is 1000.
  • maxTimeout: The maximum number of milliseconds between two retries. Default is Infinity.
  • randomize: Randomizes the timeouts by multiplying with a factor between 1 to 2. Default is true.
  • onRetry: an optional Function that is invoked after a new retry is performed. It's passed the Error that triggered it as a parameter.

All time values are in milliseconds.

Retries Explained

The formula used to calculate the individual timeouts is:

Math.min(random * minTimeout * Math.pow(factor, attempt), maxTimeout)

Have a look at this article for a better explanation of approach.

If you want to tune your factor / times settings to attempt the last retry after a certain amount of time, you can use wolfram alpha. For example in order to tune for 10 attempts in 5 minutes, you can use this equation:

Explaining the various values from left to right:

• k = 0 ... 9: The retries value (10)
• 1000: The minTimeout value in ms (1000)
• x^k: No need to change this, x will be your resulting factor
• 5 * 60 * 1000: The desired total amount of time for retrying in ms (5 minutes)

To make this a little easier for you, use wolfram alpha to do the calculations:

http://www.wolframalpha.com/input/?i=Sum[1000*x^k%2C+{k%2C+0%2C+9}]+%3D+5+*+60+*+1000

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