Please hold: your call is in a queue. An introduction to queuing in Drupal. My talk from Drupalcamp London 1st-3rd March 2013. A version of this talk with minor modifications was also given at the Drupal Learning April 2013 meetup in London (http://www.meetup.com/Learning-Drupal-Meetup/events/111107222/). It's not just the Brits that love queues, Drupal loves them too. Maybe your website is making requests to another internal or third party system. Maybe a third party is sending messages to you. Either way, as you begin to grow your services are increasingly likely to run into problems completing these requests in a predictable amount of time and with guaranteed success. Queueing is one solution to this, balancing load, ensuring jobs are processed successfully - but the uses don't end there. This session will provide an overview of queueing patterns, discuss some scenarios where you may want to implement queues, and look at how Drupal implements queueing - as well as some of the gotchas in implementing queue processing at scale.

1. PLEASE HOLD: YOUR
CALL IS IN A QUEUE
An introduction to queueing in Drupal
by Tom Phethean
Sunday, 3 March 13

2. QUEUING IN 30 MINUTES...
•I hate waiting in line...
• Drupal Queue API
• Beyond Core
• Queue jumpers and other gotchas
• Questions
Sunday, 3 March 13

3. I HATE WAITING IN LINE...
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4. Sunday, 3 March 13

5. Sunday, 3 March 13

6. Sunday, 3 March 13

7. Sunday, 3 March 13

8. QUEUES WILL FORM
WHEN PROCESSES WITH VARIABILITY
ARE LOADED TO HIGH LEVELS
OF UTILISATION
WITHOUT CONSTRAINT
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9. SOMETHING WILL QUEUE
SOMEWHERE
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10. SOMETHING WILL QUEUE
SOMEWHERE
Whether you like it or not
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11. DO YOU WANT TO BE IN
CONTROL?
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12. <silly_question>
DO YOU WANT TO BE IN
CONTROL?
</silly_question>
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13. DRUPAL TO THE
RESCUE!
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14. CORE QUEUE API
• Two kinds of queue: reliable and non-reliable
• Provides interface to common queue functions
• MemoryQueue implements QueueInterface
• SystemQueue implements ReliableQueueInterface
• In Drupal 7 (and 8), Batch API extends SystemQueue
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15. WHERE CAN I FIND QUEUE
API?
• Drupal 7:
• ./modules/system/system.queue.inc
• Drupal 8:
• ./core/lib/Drupal/Core/Queue
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16. QUEUE API METHODS
• createQueue() • numberOfItems()
• createItem() • deleteItem()
• claimItem() • deleteQueue()
• releaseItem()
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17. Code time.
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18. /**
* Create a queue.
*/
function queues_create_queue() {
$queue = DrupalQueue::get('my_queue');
$queue->createQueue();
}
Sunday, 3 March 13

19. /**
* Add an item to a queue.
*/
function queues_create_queueitem() {
$queue = DrupalQueue::get('my_queue');
$queue_data = array (
'stuff' => 'test',
);
return $queue->createItem($queue_data);
}
/**
* Check number of items on a queue.
*/
function queues_count_queueitems() {
$queue = DrupalQueue::get('my_queue');
return $queue->numberOfItems();
}
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20. /**
* Process a queue item
*/
function queues_process_queueitem() {
$queue = DrupalQueue::get('my_queue');
$item = $queue->claimItem(30);
if ($item && $item->data['stuff'] == 'test') {
// Success - we don't need this item anymore.
$queue->deleteItem($item);
}
else {
// Something unexpected happened, we still need
this item.
$queue->releaseItem($item);
}
}
Sunday, 3 March 13

21. /**
* Implements hook_cron_queue_info().
*/
function queues_cron_queue_info() {
$queues['my_queue'] = array(
'worker callback' => 'queues_process_queueitem',
'time' => 60,
// Time is NOT the lease item.
// Time is the amount of time cron will spend
processing this queue.
);
return $queues;
}
Sunday, 3 March 13

22. /**
* Implements hook_queue_info() from queue_ui module.
*/
function queues_queue_info() {
return array(
'my_queue' => array(
'title' => t('My test queue'),
'batch' => array(
'operations' =>
array(array('queues_batch_process', array())),
'finished' => 'queues_batch_finished',
'title' => t('Processing my test queue'),
),
'cron' => array(
'callback' =>'queues_process_queueitem',
),
),
);
}
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23. That’s it!
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24. BEYOND CORE
• Drupal Queue (backport to D6) - http://drupal.org/project/drupal_queue
• Queue UI - http://drupal.org/project/queue_ui
• Examples - http://drupal.org/project/examples
• Queue backends:
• Beanstalkd - http://drupal.org/project/beanstalkd
• STOMP - http://drupal.org/project/stomp
• Redis - http://drupal.org/project/redis_queue
Sunday, 3 March 13

25. CUSTOM BACKENDS
• Default queue class:
• $conf['queue_default_class'] = 'SystemQueue';
• $conf[‘queue_default_reliable_class’] = ‘SystemQueue’;
• Override queue classes:
• $conf['queue_class_{queue name}'] = 'StompQueue';
Sunday, 3 March 13

26. CUSTOM BACKENDS
• Default queue class:
• $conf['queue_default_class'] = 'SystemQueue';
• $conf[‘queue_default_reliable_class’] = ‘SystemQueue’;
• Override queue classes:
• $conf['queue_class_{queue name}'] = 'RedisQueue';
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27. QUEUES AND DRUSH
• drush queue-list
• drush queue-run [queue name]
• ...use with caution: queue-run will always delete your queue
item
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28. WHEN TO QUEUE
• Remote service call triggered by user interface
• Complex business rules around data processing
• Need guaranteed success
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29. SOME EXAMPLES
• Publishing user information to a remote CRM
• Backend order fulfilment
• Email sending (see notifications module)
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30. GOTCHAS
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31. MULTIPLE CONSUMERS
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32. MULTIPLE CONSUMERS
• Identify what it is you’re processing
• Use Lock API to secure a lock on it:
• lock_acquire(‘my_id’)
• lock_release(‘my_id’)
• Check the “thing” is in a state you expect it to be
• Release or Delete from queue as appropriate
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33. DEBUGGING YOUR QUEUE
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34. DEBUGGING YOUR QUEUE
• watchdog() is your friend
• Log EVERYTHING
• Abstract it so you can toggle verbose logging
• Use Graylog, LogStash etc to save your database from death
by logs
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35. THINGS CAN GO WRONG...
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36. THINGS CAN GO WRONG...
• Monitor your queue depths with Munin or Nagios
• Alert if they get too deep
• Know what the alert means (and listen to it)!
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37. THINGS BREAK.
BUT WHEN THEY DO, KNOW THAT
EVERY ITEM IN YOUR QUEUE
IS A REQUEST WHICH WOULD
OTHERWISE HAVE BEEN LOST.
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38. IMPLEMENTING QUEUEING IN YOUR APPLICATION
SMOOTHS THE PEAKS,
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39. IMPLEMENTING QUEUEING IN YOUR APPLICATION
SMOOTHS THE PEAKS,
MAKE THROUGHPUT PREDICTABLE,
Sunday, 3 March 13

40. IMPLEMENTING QUEUEING IN YOUR APPLICATION
SMOOTHS THE PEAKS,
MAKE THROUGHPUT PREDICTABLE,
REMOVES PROCESSING TIME FROM
THE USER EXPERIENCE
Sunday, 3 March 13

41. IMPLEMENTING QUEUEING IN YOUR APPLICATION
SMOOTHS THE PEAKS,
MAKE THROUGHPUT PREDICTABLE,
REMOVES PROCESSING TIME FROM
THE USER EXPERIENCE
AND ENSURES THAT PROBLEMS ARE
HANDLED GRACEFULLY
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42. IN OTHER WORDS...
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43. Sunday, 3 March 13

44. QUESTIONS?
Tweet: @tsphethean
Drupal.org: tsphethean
IRC: tsphethean
Sunday, 3 March 13

45. IMAGE ATTRIBUTIONS
• Image of supermarket queue - http://petoneponderings.blogspot.co.uk/2012_12_01_archive.html
• Post Office queue - http://www.flickr.com/photos/welshkaren/5367517662/
• Apple Store queueing http://www.telegraph.co.uk/technology/apple/7854982/Apple-iPhone-4-more-
than-a-quarter-of-users-think-video-calling-is-a-gimmick.html
• Supermarket crush - http://www.dailymail.co.uk/news/article-2078597/Boxing-Day-sales-Record-
numbers-shops-open-80-push-grab-shoppers.html
• Drupal Superhero - http://www.origineight.net/blog-post/web-sites-you-manage
• Lots of Shoppers - http://www.dailymail.co.uk/news/article-2253523/United-Nations-Bargain-Hunters-
Britons-the-queue-China-Middle-East-lead-rush-designer-brands.html
• Confused baby - http://thementalpausechronicles.blogspot.co.uk/2008/08/head-scratching.html
• Mark Sonnabaum - http://www.whatwouldmarksonnabaumdo.com
Sunday, 3 March 13