An internal presentation I gave at the OU about Moodle Caching.

1. MUC
Moodle Universal Cache
Tim Hunt

2. Introduction

3. What is a cache
A place to keep data that would take time to re-compute
A cache stores values identified by a key
E.g. list of activities in a course, stored by course id
Caches may get purged at any time
Cannot store real data there, just derived values for
performance

4. Why have a cache system?
Developers want to do stuff with data, not re-implement
caching
Want to be able to use different back-end stores
Disc, Memcache, REDIS, …
cachestore_ is a plugin type
Admins should control which data goes in which store

5. Cache performance
Typical call to cache::get For a typical page
• 0.5 ms data fetched from Memcache 150
• 0.05 ms data in static acceleration 700
For comparison: typical DB query
• 3 ms 40

6. Using caches

7. Typical use
$cache = cache::make('core', 'string');
$strings = $cache->get($component);
if ($strings === false) {
$strings = load_strings($component);
$cache->set($component, $strings);
}

8. Defining your cache
In db/caches.php in your plugin. E.g. from lib/db/caches.php
$definitions = array(
// Used to store processed lang files.
// Keys used are revision, lang and component of the string file.
// Static acceleration size is based on student access of the site.
'string' => array(
'mode' => cache_store::MODE_APPLICATION,
'simplekeys' => true,
'simpledata' => true,
'staticacceleration' => true, // Metadata helps cache
'staticaccelerationsize' => 30, // system handle the data
'canuselocalstore' => true, // efficiently.
),
);
$string['cachedef_string'] = 'Language string cache';

9. Types of cache
Application – Data that applies throughout Moodle
E.g. list of activities in each course / language strings
Session – Data that is relevant while a user is logged in
E.g. list of all course categories this user can access
Request – Data within the handling of one request
E.g. list of temporary tables

10. Clearing the cache
$cache = cache::make('core', 'questiondata');
$cache->delete($questionid);
cache::make('core', 'questiondata')->purge();
Recommended: just clear out what has changed
Dangerous
Also not recommended: time-to-live in cache definition

11. Bulk actions
$cache->get_many(['key1', 'key2']);
// ['key1' => 'value1', 'key2' => 'value2']
$cache->set_many(
['key1' => 'value1', 'key2' => 'value2']);
$cache->delete_many(['key1', 'key2']);

12. Advanced use: data source

13. Data source: why?
Avoids the normal
“is this in the cache? Yes: good; No, re-compute”
Tell the cache where to get the data if it is not yet stored

14. Data source: definition
$definitions = array(
// Cache for question definitions. This is used by the question
// bank class. Users probably do not need to know about this cache.
// They will just call question_bank::load_question.
'questiondata' => array(
'mode' => cache_store::MODE_APPLICATION,
'simplekeys' => true, // The id of the question is used.
'requiredataguarantee' => false,
'datasource' => 'question_finder',
'datasourcefile' => 'question/engine/bank.php',
),
)

15. class question_finder implements cache_data_source {
public static function get_instance_for_cache(cache_definition $definition) {
return self::get_instance(); // Singleton pattern.
}
public function load_for_cache($questionid) {
global $DB;
$questiondata = $DB->get_record_sql('SELECT q.*, qc.contextid
FROM {question} q
JOIN {question_categories} qc ON q.category = qc.id
WHERE q.id = :id', array('id' => $questionid), MUST_EXIST);
get_question_options($questiondata);
return $questiondata;
}
}
Data source: use
$cache = cache::make('core', 'questiondata');
$cache->get($questionid);

16. Data source: implementation
public function get($key, $strictness = IGNORE_MISSING) {
// ...
$result = $this->store->get($key);
// ...
if ($result === false) {
if ($this->loader !== false) {
$result = $this->loader->get($key);
} else {
if ($this->datasource !== false) {
$result = $this->datasource->load_for_cache($key);
}
}
}
// ...
}

17. Cache administration

18. Cache administration
E.g. https://learn2acct.open.ac.uk/cache/admin.php

19. Cache administration: continued

20. Cache administration:
continued

21. Cache administration: continued

22. Issues

23. Memcache purging
MUC lets you store many different caches in one backend
cache::purge(); should just clear one of those caches
With Memcache, cache::purge() wipes all caches
This is one of the reasons the VLE crashed in October
Moodle 3.2 now has REDIS cache store in core
We should consider switching

24. Complex keys
MUC tries to support keys that are not just ints or strings
However this hurts performance – best not to use it
This probably applies to other advanced MUC features
Caching is about performance
Simple is good

25. Automated tests
MUC caches are automatically cleared between each unit
test or Behat test

26. Useful links
https://docs.moodle.org/dev/Cache_API
https://docs.moodle.org/dev/Cache_API_-_Quick_reference
cache/README.md in the code
https://docs.moodle.org/dev/The_Moodle_Universal_Cache_(MUC)