back

A simple module to be used for creating exponentially weighted backoff attempts. Originally extracted from Primus.

NOTICE If you were a pre-1.0.0 back user, the API has changed to what is found below. If you do not like this slightly different abstraction and would prefer the former, slightly simpler API, it is still available with require('back/reconnect').

The API change thanks to a contribution from @Raynos makes things simpler as you don't have to manage the copying of the options object yourself in order to handle repeated backoff cases.

Example

1var http = require('http');
2var back = require('back');
3//
4// Options to use for backoff
5//
6// Remark: This object is modified so it should be cloned if you are dealing
7// with independent backoff attempts and want to use these values as a base.
8//
9var options = {
10  retries: 3,
11  minDelay: 1000, // Defaults to 500ms
12  maxDelay: 10000, // Defaults to infinity
13  // The following option is shown with its default value but you will most
14  // likely never define it as it creates the exponential curve.
15  factor: 2,
16};
17
18// Where we will store the backoff instance during a particular backoff attempt
19var attempt;
20
21function retry(err) {
22  var back = attempt || (attempt = new Back(options));
23  return back.backoff(function (fail) {
24    if (fail) {
25      // Oh noez we never reconnect :(
26      console.error('Retry failed with ' + err.message);
27      process.exit(1);
28    }
29    //
30    // Remark: .attempt and .timeout are added to this object internally
31    //
32    console.log('Retry attempt # ' + back.settings.attempt +
33                ' being made after ' + back.settings.timeout + 'ms');
34  request();
35  });
36}
37
38function request() {
39  http.get('http://localhost:9000', function (res) {
40    console.log('Successful Response that will not happen!');
41    //
42    // If we succeeded, we would set the current to null so the next error
43    // generates a new instance.
44    //
45    attempt = null;
46  }).on('error', retry);
47}
48
49request();

API

var back = new Back(backoffOpts);

The Back constructor function takes your backoff options and saves them as settings in the internal state of the back object.

back.backoff(callback)

The back instance has a backoff method that takes a callback that is executed after a setTimeout. The timeout is what is based on an exponential backoff of course! It will repeatedly all this callback based on the backoff options you passed to the back instance until it exhausts its efforts. When it has exhausted its attempts, it will return an error as the first argument to the callback.

back.close()

Clear backoff timer in cases where you want to dispose of the instance before the callback is executed.