Queue That

A queue managed in localStorage for async tasks that may run immediately before page unload. Queue That is built primarily to queue XHR requests for reporting.

How does it work?

• A queue of tasks is kept in localStorage.
• Tasks are batched in 100ms buckets.
• Batches are passed to the options.process function, which calls a callback (optionally with an error object) when the tasks have finished processing.
• On page load and when a task is queued, succeeds or fails there is a check to see if there are any pending tasks and if there are any other queues processing them, if there are no active queues then the queue will set a flag in localStorage and begin processing tasks.

Features

• When more than one tab is open, only one active queue will process tasks.
• If options.process calls back with an error, the tasks will be passed to options.process again after a second. This backoff time doubles for each sequential error. The backoff timer is stored in localStorage and so will not reset when Queue That is reinitialised.
• There is a 2 second timeout for the process function, after which the queue will back off and retry later, ignoring any success/error callback from the first options.process call.
• The batch array passed to options.process contains a containsRepeatedItems property, useful for deduplicating requests sent to a server.
• Tasks are batched with a default batch size of 20. The option batchSize can be used to change the batch size. Disable batching by setting batchSize to Infinity.
• Tasks are grouped in 100ms batches
• If a queue is not active for over 2.5 seconds, another tab's Queue may take over and become active (on an event such as initialization).
• Falls back to window.__queueThat__ variable if localStorage throws or doesn't work.
• Optional trim function that will be called as a setter every time the queue is set. This is good for when the queue is taking up a lot of localStorage or if JSON parsing/stringifying becomes slow.

API

queueThat

Receives an options argument and returns a function that can be used to queue tasks.

var queueThat = require('queue-that')
var post = require('post')
var when = require('when')

var q = queueThat({
  /**
   * This label is used to store the queue in localStorage.
   * If there are more than one queues, unique labels are required
   * to have them run without interference.
   *
   * @optional
   * @type {String}
   * @default 'Queue That'
   */
  label: 'Queue That',

  /**
   * Asynchronous function to process batches of tasks.
   * The queue will not process any more tasks until done is called
   * or the event times out.
   * 
   * @required
   * @type {Function}
   */
  process: function (batch, done) {
    var endpoint = 'https://somewhere.com/events' +
      (batch.containsRepeatedItems ? '?dedupe=true' : '?dedupe=false')
    when.all(
      batch.map(post.bind(null, 'https://somewhere.com/events'))
    ).then(done)
  },

  /**
   * Every time the queue is set, this setter will be called.
   * Good for reducing the queue size if it gets too long.
   * 
   * @optional
   * @type {Function}
   */
  trim: function (batch) {
    return batch.filter(function (task) {
      return task !== 'Low priority'
    })
  },

  /**
   * The time used to group tasks.
   * 
   * @optional
   * @type {Number}
   */
  queueGroupTime: 100,

  /**
   * The queue will wait this time when there is an
   * error before calling `options.process` again.
   * The time doubles on every error and resets on a success.
   * 
   * 
   * @optional
   * @type {Number}
   * @default 1000
   */
  backoffTime: 1000,

  /**
   * The queue will assume an error and backoff if a
   * task takes longer than this to process.
   * 
   * @optional
   * @type {Number}
   * @default 2000
   */
  processTimeout: 2000,

  /**
   * Another queue may become active if a queue hasn't
   * flagged itself as active for longer than this time.
   * This time must be longer than processTimeout.
   * 
   * @optional
   * @type {Number}
   * @default 2500
   */
  activeQueueTimeout: 2500
})

/**
 * This will usually fail because the page
 * will unload and cancel the XHR.
 */
post('https://somewhere.com/events', {
  name: 'Ronald',
  description: 'Clicked on a link'
})

/**
 * By queueing the event in localStorage, if the
 * window is closed, the event can be processed
 * later.
 */
q({
  name: 'Ronald',
  event: 'Clicked on a link'
})

flush

Triggers the queue to be processed on next tick. Usually called when an important task is added to the queue to ensure it is processed as soon as possible rather than waiting to be grouped with other tasks.

var q = queueThat({
  process: function (batch, done) {
    fetch('https://endpoint.com/stuff', { method: 'POST', body: JSON.stringify(batch) })
      .then(done)
  }
})

/**
 * Would usually be processed after
 * options.queueGroupTime (default 100ms)
 */
q({
  priority: 'low',
  data: 'OMG did you hear about that show?'
})

q({
  priority: 'high',
  data: 'Deathstar blueprints'
})

/**
 * Will cause all items to be processed on next tick.
 */
q.flush()

q({
  priority: 'high',
  data: 'Darth Vader\'s birthday plans'
})

/**
 * Calling flush more than once in a synchronous block of
 * execution is effectively a noop.
 */
q.flush()

/**
 * The following items will also be processed on next tick
 * because flush has been called.
 */
q({
  priority: 'medium',
  data: 'Storm trooper dress code'
})

q({
  priority: 'low',
  data: 'A new study shows this thing is bad for you!'
})

flushQueueCache

As a performance optimization, Queue That keeps an in-memory version of the queue which is flushed to localStorage on next tick. If the window is closed and a load of items are in the in-memory queue then the flush is likely to be abandoned by the browser. Calling flushQueueCache after queuing a bunch of items synchronously is advised to reduce the number of tasks that may be lost.

var q = queueThat({
  process: function (batch, done) {
    fetch('https://endpoint.com/stuff', { method: 'POST', body: JSON.stringify(batch) })
      .then(done)
  }
})

/**
 * Items are added to an in-memory queue.
 */
q({
  data: 'Little Bo Peep, went to sleep'
})

q({
  data: 'on the eve of a global invasion'
})

q({
  data: 'She woke up to find, all of mankind'
})

q({
  data: 'enslaved by an alien nation.'
})

/**
 * Flushing the in-memory cache will ensure that
 * the tasks are written to localStorage even if the
 * browser window is closed during execution.
 */
q.flushQueueCache()

Support

Supported and tested on

• IE8+
• Firefox
• Safari
• Opera
• Chrome