Backoff for Node.js

Fibonacci and exponential backoffs for Node.js.

Installation

npm install backoff

Unit tests

npm test

Usage

The usual way to instantiate a new Backoff object is to use one predefined factory method: backoff.fibonacci([options]), backoff.exponential([options]).

Backoff inherits from EventEmitter. When a backoff starts, a backoff event is emitted and, when a backoff ends, a ready event is emitted. Handlers for these two events are called with the current backoff number and delay.

var backoff = require('backoff');

var fibonacciBackoff = backoff.fibonacci({
    randomisationFactor: 0,
    initialDelay: 10,
    maxDelay: 300
});

fibonacciBackoff.failAfter(10);

fibonacciBackoff.on('backoff', function(number, delay) {
    // Do something when backoff starts, e.g. show to the
    // user the delay before next reconnection attempt.
    console.log(number + ' ' + delay + 'ms');
});

fibonacciBackoff.on('ready', function(number, delay) {
    // Do something when backoff ends, e.g. retry a failed
    // operation (DNS lookup, API call, etc.).
    fibonacciBackoff.backoff();
});

fibonacciBackoff.on('fail', function() {
    // Do something when the maximum number of backoffs is
    // reached, e.g. ask the user to check its connection.
    console.log('fail');
});

fibonacciBackoff.backoff();

The previous example would print:

0 10ms
1 10ms
2 20ms
3 30ms
4 50ms
5 80ms
6 130ms
7 210ms
8 300ms
9 300ms
fail

Note that Backoff objects are meant to be instantiated once and reused several times by calling reset after a successful "retry".

API

backoff.fibonacci([options])

Constructs a Fibonacci backoff (10, 10, 20, 30, 50, etc.).

See bellow for the options description.

backoff.exponential([options])

Constructs an exponential backoff (10, 20, 40, 80, etc.).

The options are:

• randomisationFactor: defaults to 0, must be between 0 and 1
• initialDelay: defaults to 100 ms
• maxDelay: defaults to 10000 ms

With these values, the backoff delay will increase from 100 ms to 10000 ms. The randomisation factor controls the range of randomness and must be between 0 and 1. By default, no randomisation is applied on the backoff delay.

Class Backoff

new Backoff(strategy)

• strategy: the backoff strategy to use

Constructs a new backoff object from a specific backoff strategy. The backoff strategy must implement the BackoffStrategyinterface defined bellow.

backoff.failAfter(numberOfBackoffs)

• numberOfBackoffs: maximum number of backoffs before the fail event gets emitted, must be greater than 0

Sets a limit on the maximum number of backoffs that can be performed before a fail event gets emitted and the backoff instance is reset. By default, there is no limit on the number of backoffs that can be performed.

backoff.backoff()

Starts a backoff operation. Will throw an error if a backoff operation is already in progress.

In practice, this method should be called after a failed attempt to perform a sensitive operation (connecting to a database, downloading a resource over the network, etc.).

backoff.reset()

Resets the backoff delay to the initial backoff delay and stop any backoff operation in progress. After reset, a backoff instance can and should be reused.

In practice, this method should be called after having successfully completed the sensitive operation guarded by the backoff instance or if the client code request to stop any reconnection attempt.

Event: 'backoff'

• number: number of backoffs since last reset, starting at 0
• delay: backoff delay in milliseconds

Emitted when a backoff operation is started. Signals to the client how long the next backoff delay will be.

Event: 'ready'

• number: number of backoffs since last reset, starting at 0
• delay: backoff delay in milliseconds

Emitted when a backoff operation is done. Signals that the failing operation should be retried.

Event: 'fail'

Emitted when the maximum number of backoffs is reached. This event will only be emitted if the client has set a limit on the number of backoffs by calling backoff.failAfter(numberOfBackoffs). The backoff instance is automatically reset after this event is emitted.

Interface BackoffStrategy

A backoff strategy must provide the following methods.

strategy.next()

Computes and returns the next backoff delay.

strategy.reset()

Resets the backoff delay to its initial value.

Class ExponentialStrategy

Exponential (10, 20, 40, 80, etc.) backoff strategy implementation.

new ExponentialStrategy([options])

The options are:

• randomisationFactor: defaults to 0, must be between 0 and 1
• initialDelay: defaults to 100 ms
• maxDelay: defaults to 10000 ms

Class FibonacciStrategy

Fibonnaci (10, 10, 20, 30, 50, etc.) backoff strategy implementation.

new FibonacciStrategy([options])

The options are:

• randomisationFactor: defaults to 0, must be between 0 and 1
• initialDelay: defaults to 100 ms
• maxDelay: defaults to 10000 ms

License

This code is free to use under the terms of the MIT license.