Attempt

This library exports a retry(...) function that can be used to invoke a function that returns a Promise multiple times until returned Promise is resolved or the max number of attempts is reached.

The delay between each attempt is configurable and allows multiple retry strategies.

The following features are supported:

• Fixed delay between attempts
• Exponential backoff
• Exponential backoff with jitter
• Abort retries early
• Abort due to timeout
• Error handler for each attempt

Installation

Using NPM:

npm install @lifeomic/attempt --save

Using Yarn:

yarn add @lifeomic/attempt

Usage

Node.js / CommonJS:

const retry = require('@lifeomic/attempt').retry;

ES6 / TypeScript

import { retry } from '@lifeomic/attempt';

try {
  const result = await retry(async (context) => {
    // some code that returns a promise or resolved value
  }, options);
} catch (err) {
  // If the max number of attempts was exceeded then `err`
  // will be the last error that was thrown.
  //
  // If error is due to timeout then `err.code` will be the
  // string `ATTEMPT_TIMEOUT`.
}

The options argument is optional, and when absent the default values are assigned. All times/durations are in milliseconds.

The following object shows the default options:

{
  delay: 200,
  maxAttempts: 3,
  initialDelay: 0,
  minDelay: 0,
  maxDelay: 0,
  factor: 0,
  timeout: 0,
  jitter: false,
  handleError: null,
  handleTimeout: null,
  beforeAttempt: null,
  calculateDelay: null
}

NOTE:

If you are using a JavaScript runtime that doesn't support modern JavaScript features such as async/await then you will need to use a transpiler such as babel to transpile the JavaScript code to your target environment.

Supported options:

• delay: Number

  The delay between each attempt in milliseconds. You can provide a factor to have the delay grow exponentially.

  (default: 200)

• initialDelay: Number

  The intialDelay is the amount of time to wait before making the first attempt. This option should typically be 0 since you typically want the first attempt to happen immediately.

  (default: 0)

• maxDelay: Number

  The maxDelay option is used to set an upper bound for the delay when factor is enabled. A value of 0 can be provided if there should be no upper bound when calculating delay.

  (default: 0)

• factor: Number

  The factor option is used to grow the delay exponentially. For example, a value of 2 will cause the delay to double each time. A value of 3 will cause the delay to triple each time. Fractional factors (e.g. 1.5) are also allowed.

  The following formula is used to calculate delay using the factor:

  delay = delay * Math.pow(factor, attemptNum)

  (default: 0)

• maxAttempts: Number

  The maximum number of attempts or 0 if there is no limit on number of attempts.

  (default: 3)

• timeout: Number

  A timeout in milliseconds. If timeout is non-zero then a timer is set using setTimeout. If the timeout is triggered then future attempts will be aborted.

  The handleTimeout function can be used to implement fallback functionality.

  (default: 0)

• jitter: Boolean

  If jitter is true then the calculated delay will be a random integer value between minDelay and the calculated delay for the current iteration.

  The following formula is used to calculate delay using jitter:

  delay = Math.random() * (delay - minDelay) + minDelay

  (default: false)

• minDelay: Number

  minDelay is used to set a lower bound of delay when jitter is enabled. This property has no effect if jitter is disabled.

  (default: 0)

• handleError: (err, context, options) => void

  handleError is a function that will be invoked when an error occurs for an attempt. The first argument is the error and the second argument is the context.

• handleTimeout: (context, options) => Promise | void

  handleTimeout is invoked if a timeout occurs when using a non-zero timeout. The handleTimeout function should return a Promise that will be the return value of the retry() function.

• beforeAttempt: (context, options) => void

  The beforeAttempt function is invoked before each attempt. Calling context.abort() will abort the attempt and stop retrying.

• calculateDelay: (context, options) => Number

  The calculateDelay function can be used to override the default delay calculation. Your provided function should return an integer value that is the calculated delay for a given attempt.

  Information in the provided context and options arguments should be used in the calculation.

  When calculateDelay is provided, any option that is used to calculate delay (delay, jitter, maxDelay, factor, etc.) will be ignored.

The context has the following properties:

• attemptNum: Number

  A zero-based index of the current attempt number (0, 1, 2, etc.).

• attemptsRemaining: Number

  The number of attempts remaining. The initial value is maxAttempts or -1 if maxAttempts is 0 (unbounded).

• abort: () => void

  The abort function can be called when handling an error via handleError or when beforeAttempt function is invoked. The abort function should be used to prevent any further attempts in cases when an error indicates that we should not retry.

  For example, an HTTP request that returns an HTTP error code of 400 (Bad Request) should not be retried because there is a problem with the input (and retrying will not fix this). However, a request that returns 504 (Gateway Timeout) should be retried because it might be a temporary problem.

Recipes

Retry with defaults

// Try the given operation up to 3 times with a delay of 200 between
// each attempt
const result = await retry(async function() {
  // do something that returns a promise
});

Stop retrying if an error indicates that we should not retry

// Try the given operation update to 4 times. The initial delay will be 0
// and subsequent delays will be 200, 400, 800
const result = await retry(async function() {
  // do something that returns a promise
}, {
  delay: 200,
  factor: 2,
  maxAttempts: 4,
  handleError (err, context) {
    if (err.retryable === false) {
      // We should abort because error indicates that request is not retryable
      context.abort();
    }
  }
});

Retry with exponential backoff

// Try the given operation update to 4 times. The initial delay will be 0
// and subsequent delays will be 200, 400, 800 (delay doubles each time due
// to factor of `2`)
const result = await retry(async function() {
  // do something that returns a promise
}, {
  delay: 200,
  factor: 2,
  maxAttempts: 4
});

Retry with exponential backoff and max delay

// Try the given operation up to 5 times. The initial delay will be 0
// and subsequent delays will be 200, 400, 500, 500 (capped at `maxDelay`)
const result = await retry(async function() {
  // do something that returns a promise
}, {
  delay: 200,
  factor: 2,
  maxAttempts: 5,
  maxDelay: 500
});

Retry with exponential backoff, jitter, min delay, and max delay

// Try the given operation 3 times. The initial delay will be 0
// and subsequent delays will be in the following range:
// - 100 to 200
// - 100 to 400
// - 100 to 500 (capped at `maxDelay`)
// - 100 to 500 (capped at `maxDelay`)
const result = await retry(async function() {
  // do something that returns a promise
}, {
  delay: 200,
  factor: 2,
  maxAttempts: 5,
  minDelay: 100,
  maxDelay: 500,
  jitter: true
});

Stop retrying if there is a timeout

// Try the given operation up to 5 times. The initial delay will be 0
// and subsequent delays will be 200, 400, 800, 1600.
//
// If an attempt fails to complete after 1 second then the retries
// are aborted and error with `code` `ATTEMPT_TIMEOUT` is thrown.
const result = await retry(async function() {
  // do something that returns a promise
}, {
  delay: 200,
  factor: 2,
  maxAttempts: 5,
  timeout: 1000
});

Stop retrying if there is a timeout but provide a fallback

// Try the given operation up to 5 times. The initial delay will be 0
// and subsequent delays will be 200, 400, 800, 1600.
//
// If an attempt fails to complete after 1 second then the retries
// are aborted and the `handleTimeout` implements some fallback logic.
const result = await retry(async function() {
  // do something that returns a promise
}, {
  delay: 200,
  factor: 2,
  maxAttempts: 5,
  timeout: 1000,
  async handleTimeout (context) {
    // do something that returns a promise or throw your own error
  }
});