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# Backoff for Node.js [![Build Status](https://secure.travis-ci.org/MathieuTurcotte/node-backoff.png?branch=master)](http://travis-ci.org/MathieuTurcotte/node-backoff) [![NPM version](https://badge.fury.io/js/backoff.png)](http://badge.fury.io/js/backoff) Fibonacci and exponential backoffs for Node.js. ## Installation ``` npm install backoff ``` ## Unit tests ``` npm test ``` ## Usage ### Object Oriented 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. ``` js 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.). If it fails // again then backoff, otherwise reset the backoff // instance. 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 the following. ``` 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". ### Functional It's also possible to avoid some boilerplate code when invoking an asynchronous function in a backoff loop by using `backoff.call(fn, [args, ...], callback)`. Typical usage looks like the following. ``` js var call = backoff.call(get, 'https://duplika.ca/', function(err, res) { console.log('Retries: ' + call.getResults().length); if (err) { console.log('Error: ' + err.message); } else { console.log('Status: ' + res.statusCode); } }); call.setStrategy(new backoff.ExponentialStrategy()); call.failAfter(10); call.start(); ``` ## API ### backoff.fibonacci([options]) Constructs a Fibonacci backoff (10, 10, 20, 30, 50, etc.). See bellow for options description. ### backoff.exponential([options]) Constructs an exponential backoff (10, 20, 40, 80, etc.). The options are the following. - 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. ### backoff.call(fn, [args, ...], callback) - fn: function to call in a backoff handler, i.e. the wrapped function - args: function's arguments - callback: function's callback accepting an error as its first argument Constructs a `FunctionCall` instance for the given function. The wrapped function will get retried until it succeds or reaches the maximum number of backoffs. In both cases, the callback function will be invoked with the last result returned by the wrapped function. It is the caller's responsability to initiate the call by invoking the `start` method on the returned `FunctionCall` instance. ### 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 `BackoffStrategy`interface 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([err]) Starts a backoff operation. If provided, the error parameter will be emitted as the last argument of the `backoff` and `fail` events to let the listeners know why the backoff operation was attempted. An error will be thrown 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 - err: optional error parameter passed to `backoff.backoff([err])` 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' - err: optional error parameter passed to `backoff.backoff([err])` 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 the following. - 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 the following. - randomisationFactor: defaults to 0, must be between 0 and 1 - initialDelay: defaults to 100 ms - maxDelay: defaults to 10000 ms ### Class FunctionCall This class manages the calling of an asynchronous function within a backoff loop. This class should rarely be instantiated directly since the factory method `backoff.call(fn, [args, ...], callback)` offers a more convenient and safer way to create `FunctionCall` instances. #### new FunctionCall(fn, args, callback) - fn: asynchronous function to call - args: an array containing fn's args - callback: fn's callback Constructs a function handler for the given asynchronous function. #### call.isPending() Returns whether the call is pending, i.e. hasn't been started. #### call.isRunning() Returns whether the call is in progress. #### call.isCompleted() Returns whether the call is completed. #### call.isAborted() Returns whether the call is aborted. #### call.setStrategy(strategy) - strategy: strategy instance to use, defaults to `FibonacciStrategy`. Sets the backoff strategy to use. This method should be called before `call.start()` otherwise an exception will be thrown. #### call.failAfter(maxNumberOfBackoffs) - maxNumberOfBackoffs: maximum number of backoffs before the call is aborted Sets the maximum number of backoffs before the call is aborted. By default, there is no limit on the number of backoffs that can be performed. This method should be called before `call.start()` otherwise an exception will be thrown.. #### call.getResults() Retrieves all intermediary results returned by the wrapped function. This method can be called at any point in time during the call life cycle, i.e. before, during and after the wrapped function invocation. Returns an array of arrays containing the results returned by the wrapped function for each call. For example, to get the error code returned by the second call, one would do the following. ``` js var results = call.getResults(); var error = results[1][0]; ``` #### call.start() Initiates the call the wrapped function. This method should only be called once otherwise an exception will be thrown. #### call.abort() Aborts the call. Past results can be retrieved using `call.getResults()`. This method can be called at any point in time during the call life cycle, i.e. before, during and after the wrapped function invocation. #### Event: 'call' - args: wrapped function's arguments Emitted each time the wrapped function is called. #### Event: 'callback' - results: wrapped function's return values Emitted each time the wrapped function invokes its callback. #### Event: 'backoff' - number: backoff number, starts at 0 - delay: backoff delay in milliseconds - err: the error that triggered the backoff operation Emitted each time a backoff operation is started. ## License This code is free to use under the terms of the [MIT license](http://mturcotte.mit-license.org/).