retry

Abstraction for exponential and custom retry strategies for failed operations.

Installation

npm install retry

Current Status

This module has been tested and is ready to be used.

Tutorial

The example below will retry a potentially failing dns.resolve operation 10 times using an exponential backoff strategy. With the default settings, this means the last attempt is made after 17 minutes and 3 seconds.

var dns = require('dns');
var retry = require('retry');

function faultTolerantResolve(address, cb) {
  var operation = retry.operation();

  operation.attempt(function(currentAttempt) {
    dns.resolve(address, function(err, addresses) {
      if (operation.retry(err)) {
        return;
      }

      cb(err ? operation.mainError() : null, addresses);
    });
  });
}

faultTolerantResolve('nodejs.org', function(err, addresses) {
  console.log(err, addresses);
});

Of course you can also configure the factors that go into the exponential backoff. See the API documentation below for all available settings. currentAttempt is an int representing the number of attempts so far.

var operation = retry.operation({
  retries: 5,
  factor: 3,
  minTimeout: 1 * 1000,
  maxTimeout: 60 * 1000,
  randomize: true,
});

API

retry.operation([options])

Creates a new RetryOperation object. options is the same as retry.createTimeouts()'s options, with three additions:

• forever: Whether to retry forever, defaults to false.
• unref: Whether to unref the setTimeout's, defaults to false.
• maxRetryTime: The maximum time (in milliseconds) that the retried operation is allowed to run. Default is Infinity.

retry.createTimeouts([options])

Returns an array of timeouts. All time options and return values are in milliseconds. If options is an array, a copy of that array is returned.

options is a JS object that can contain any of the following keys:

• retries: The maximum amount of times to retry the operation. Default is 10. Seting this to 1 means do it once, then retry it once.
• 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 false.

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%5B1000*x^k%2C+{k%2C+0%2C+9}%5D+%3D+5+*+60+*+1000

retry.createTimeout(attempt, opts)

Returns a new timeout (integer in milliseconds) based on the given parameters.

attempt is an integer representing for which retry the timeout should be calculated. If your retry operation was executed 4 times you had one attempt and 3 retries. If you then want to calculate a new timeout, you should set attempt to 4 (attempts are zero-indexed).

opts can include factor, minTimeout, randomize (boolean) and maxTimeout. They are documented above.

retry.createTimeout() is used internally by retry.createTimeouts() and is public for you to be able to create your own timeouts for reinserting an item, see issue #13.

new RetryOperation(timeouts, [options])

Creates a new RetryOperation where timeouts is an array where each value is a timeout given in milliseconds.

Available options:

• forever: Whether to retry forever, defaults to false.
• unref: Wether to unref the setTimeout's, defaults to false.

If forever is true, the following changes happen:

• RetryOperation.errors() will only output an array of one item: the last error.
• RetryOperation will repeatedly use the timeouts array. Once all of its timeouts have been used up, it restarts with the first timeout, then uses the second and so on.

retryOperation.errors()

Returns an array of all errors that have been passed to retryOperation.retry() so far. The returning array has the errors ordered chronologically based on when they were passed to retryOperation.retry(), which means the first passed error is at index zero and the last is at the last index.

retryOperation.mainError()

A reference to the error object that occured most frequently. Errors are compared using the error.message property.

If multiple error messages occured the same amount of time, the last error object with that message is returned.

If no errors occured so far, the value is null.

retryOperation.attempt(fn)

Defines the function fn that is to be retried and executes it for the first time right away. The fn function can receive an optional currentAttempt callback that represents the number of attempts to execute fn so far.

retryOperation.retry(error)

Returns false when no error value is given, or the maximum amount of retries has been reached.

Otherwise it returns true, and retries the operation after the timeout for the current attempt number.

retryOperation.stop()

Allows you to stop the operation being retried. Useful for aborting the operation on a fatal error etc.

retryOperation.reset()

Resets the internal state of the operation object, so that you can call attempt() again as if this was a new operation object.

retryOperation.attempts()

Returns an int representing the number of attempts it took to call fn before it was successful.

License

retry is licensed under the MIT license.

Changelog

0.10.0 Adding stop functionality, thanks to @maxnachlinger.

0.9.0 Adding unref functionality, thanks to @satazor.

0.8.0 Implementing retry.wrap.

0.7.0 Some bug fixes and made retry.createTimeout() public. Fixed issues #10, #12, and #13.

0.6.0 Introduced optional timeOps parameter for the attempt() function which is an object having a property timeout in milliseconds and a property cb callback function. Whenever your retry operation takes longer than timeout to execute, the timeout callback function cb is called.

0.5.0 Some minor refactoring.

0.4.0 Changed retryOperation.try() to retryOperation.attempt(). Deprecated the aliases start() and try() for it.

0.3.0 Added retryOperation.start() which is an alias for retryOperation.try().

0.2.0 Added attempts() function and parameter to retryOperation.try() representing the number of attempts it took to call fn().