promise-poller

promise-poller

A basic poller built on top of promises.

Sometimes, you may perform asynchronous operations that may fail. In many of those cases, you want to retry these operations one or more times before giving up. promise-poller handles this elegantly using promises.

Usage

Basic usage

The core of promise-poller is a task function. This is simply a function that starts your asynchronous task and returns a promise. If the task function does not return a promise, it will be wrapped in a promise. To start polling, pass your task function to the promisePoller function:

import promisePoller from 'promise-poller';

function myTask() {
  // do some async stuff that returns a promise
  return promise;
}

var poller = promisePoller({
  taskFn: myTask
});

The promisePoller function will return a "master promise". This promise will be resolved when your task succeeds, or rejected if your task fails and no retries remain.

The master promise will be resolved with the value that your task promise is resolved with. If the poll fails, the master promise will be rejected with an array of all the rejection reasons for each poll attempt.

promise-poller will attempt your task by calling the function and waiting on the returned promise. If the promise is rejected, promise-poller will wait one second and try again. It will attempt to execute your task 3 times before rejecting the master promise.

Use in non-ES2015 environments

promise-poller is written using ES2015 and transpiled with Babel. The main promisePoller function is the default export. If you are using promise-poller in an ES5 environment, you will have to specify the default property when requiring the library in:

var promisePoller = require('promise-poller').default;

Specify polling options

You can specify a different polling interval or number of retries:

var poller = promisePoller({
  taskFn: myTask,
  interval: 500, // milliseconds
  retries: 5
});

Specify timeout

If you want each poll attempt to reject after a certain timeout has passed, use the timeout option:

var poller = promisePoller({
  taskFn: myTask,
  interval: 500,
  timeout: 2000
});

In the above example, the poll is considered failed if it isn't resolved after 2 seconds. If there are retries remaining, it will retry the poll as usual.

Specify "master timeout"

Instead of timing out each poll attempt, you can set a timeout for the entire master polling operation:

var poller = promisePoller({
  taskFn: myTask,
  interval: 500,
  retries: 10,
  masterTimeout: 2000
});

In the above example, the entire poll operation will fail if there is not a successful poll within 2 seconds. This will reject the master promise.

Cancel polling

You may want to cancel the polling early. For example, if the poll fails because of an invalid password, that's not likely to change, so it would be a waste of time to continue to poll. To cancel polling early, return false from the task function instead of a promise.

The shouldContinue function

You can specify an optional shouldContinue function that takes two arguments. The first argument is a rejection reason when a poll fails, and the second argument is the resolved value when a poll succeeds. If the poll attempt failed, and you want to abort further polling, return false from this function. On the other hand, if your poll resolved to a value but you want to keep polling, return true from this function.

Select polling strategy

By default, promise-poller will use a fixed interval between each poll attempt. For example, with an interval option of 500, the poller will poll approximately every 500 milliseconds. This is the fixed-interval strategy. There are two other strategies available that may better suit your use case. To select a polling strategy, specify the strategy option, e.g.:

promisePoller({
  taskFn: myTask,
  strategy: 'linear-backoff'
});

Linear backoff (linear-backoff)

Options:

• start - The starting value to use for the polling interval (default = 1000)
• increment - The amount to increase the interval by on each poll attempt.

Linear backoff will increase the interval linearly by some constant amount for each poll attempt. For example, using the default options, the first retry will wait 1000 milliseconds. Each successive retry will wait an additional 1000 milliseconds: 1000, 2000, 3000, 4000, etc.

Exponential backoff with jitter (exponential-backoff)

Options:

• min - The minimum interval amount to use (default = 1000)
• max - The maximum interval amount to use (default = 30000)

Exponential backoff increases the poll interval by a power of two for each poll attempt. promise-poller uses exponential backoff with jitter. Jitter takes a random value between min and 2^n on the nth polling interval, not to exceed max.

For more information about exponential backoff with jitter, and its advantages, see https://www.awsarchitectureblog.com/2015/03/backoff.html.

Progress notification

You can also specify a progress callback function. Each time the task fails, the progress callback will be called with the number of retries remaining and the error that occurred (the value that the task promise was rejected with):

function progress(retriesRemaining, error) {
  // log the error?
}

var poller = promisePoller({
  taskFn: myTask,
  interval: 500,
  retries: 5,
  progressCallback: progress
});

Debugging

promise-poller uses the debug library. The debug name is promisePoller. To run your program with debug output for the promise-poller, set the DEBUG environment variable accordingly:

% DEBUG=promisePoller node path/to/app.js

If you have more than one poller active at a time, and you need to differentiate between them in debug output, you can give the promisePoller options a name property:

var poller = promisePoller({
  taskFn: myTask,
  interval: 500,
  retries: 5,
  name: 'App Server Poller'
});

When this poller prints debug messages, the poller name will be included:

promisePoller (App Server Poller) Poll failed. 1 retries remaining. +504ms

Contributors

• Joe Attardi
• /u/jcready
• Jason Stitt

License

The MIT License (MIT)

Copyright (c) 2016 Joe Attardi [e-mail]

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.