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lib | ||
tests | ||
CHANGES.md | ||
index.js | ||
LICENSE | ||
package.json | ||
README.md |
Backoff for Node.js
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.
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.
var call = backoff.call(get, 'https://duplika.ca/', function(err, res) {
console.log('Num retries: ' + call.getNumRetries());
if (err) {
console.log('Error: ' + err.message);
} else {
console.log('Status: ' + res.statusCode);
}
});
call.retryIf(function(err) { return err.status == 503; });
call.setStrategy(new backoff.ExponentialStrategy());
call.failAfter(10);
call.start();
API
backoff.fibonacci([options])
Constructs a Fibonacci backoff (10, 10, 20, 30, 50, 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.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
- factor: defaults to 2, must be greater than 1
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 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
- factor: defaults to 2, must be greater than 1
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.retryIf(predicate)
- predicate: a function which takes in as its argument the error returned by the wrapped function and determines whether it is retriable.
Sets the predicate which will be invoked to determine whether a given error should be retried or not, e.g. a network error would be retriable while a type error would stop the function call. By default, all errors are considered to be retriable.
This method should be called before call.start()
otherwise an exception will
be thrown.
call.getLastResult()
Returns an array containing the last arguments passed to the completion callback of the wrapped function. For example, to get the error code returned by the last call, one would do the following.
var results = call.getLastResult();
// The error code is the first parameter of the callback.
var error = results[0];
Note that if the call was aborted, it will contain the abort error and not the last error returned by the wrapped function.
call.getNumRetries()
Returns the number of times the wrapped function call was retried. For a wrapped function that succeeded immediately, this would return 0. 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.
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 and causes the completion callback to be invoked with an abort error if the call was pending or running; does nothing otherwise. This method can safely be called mutliple times.
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.
Event: 'abort'
Emitted when a call is aborted.
Annotated source code
The annotated source code can be found at mathieuturcotte.github.io/node-backoff/docs.
License
This code is free to use under the terms of the MIT license.