MDL-36052 cache: Fixed up the cache README file.
[moodle.git] / cache / README.md
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1Moodle Universal Cache / Cache API
2==================================
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3
4Sample code snippets
5--------------------
6
7A definition:
8
9 $definitions = array(
4075433b 10 'string' => array( // Required, unique to the component
8139ad13 11 'mode' => cache_store::MODE_APPLICATION, // Required
8139ad13 12 'requireidentifiers' => array( // Optional
4075433b 13 'lang'
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14 ),
15 'requiredataguarantee' => false, // Optional
16 'requiremultipleidentifiers' => false, // Optional
17 'overrideclass' => null, // Optional
18 'overrideclassfile' => null, // Optional
19 'datasource' => null, // Optional
20 'datasourcefile' => null, // Optional
21 'persistent' => false, // Optional
22 'ttl' => 0, // Optional
23 'mappingsonly' => false // Optional
24 'invalidationevents' => array( // Optional
25 'contextmarkeddirty'
26 ),
27 )
28 );
29
30Getting a something from a cache using the definition:
31
32 $cache = cache::make('core', 'string');
33 if (!$component = $cache->get('component')) {
34 // get returns false if its not there and can't be loaded.
35 $component = generate_data();
36 $cache->set($component);
37 }
38
39The same thing but from using params:
40
41 $cache = cache::make_from_params(cache_store::MODE_APPLICATION, 'core', 'string');
42 if (!$component = $cache->get('component')) {
43 // get returns false if its not there and can't be loaded.
44 $component = generate_data();
45 $cache->set($component);
46 }
47
48If a data source had been specified in the definition the following would be all that was needed.
49
50 $cache = cache::make('core', 'string');
51 $component = $cache->get('component');
52
53The bits that make up the cache API
54-----------------------------------
55
4075433b 56There are several parts that make up the Cache API.
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57
58### Loader
59The loader is central to the whole thing.
60It is used by the end developer to get an object that handles caching.
6190% of end developers will not need to know or use anything else about the cache API.
62In order to get a loader you must use one of two static methods, make, or make_with_params.
63To the end developer interacting with the loader is simple and is dictated by the cache_loader interface.
64Internally there is lots of magic going on. The important parts to know about are:
65* There are two ways to get with a loader, the first with a definition (discussed below) the second with params. When params are used they are turned into an adhoc definition with default params.
66* A loader get passed three things when being constructed, a definition, a store, and another loader or datasource if there is either.
67* If a loader is the third arg then requests will be chained to provide redundancy.
68* If a data source is provided then requests for an item that is not cached will be passed to the data source and that will be expected to load the data. If it loads data that data is stored in each store on its way back to the user.
69* There are three core loaders. One for each application, session, and request.
70* A custom loader can be used. It will be provided by the definition (thus cannot be used with adhoc definitions) and must override the appropriate core loader
71* The loader handles ttl for stores that don't natively support ttl.
72* The application loader handles locking for stores that don't natively support locking.
73
74### Store
75The store is the bridge between the cache API and a cache solution.
76Cache store plugins exist within moodle/cache/store.
77The administrator of a site can configure multiple instances of each plugin, the configuration gets initialised as a store for the loader when required in code (during construction of the loader).
78The following points highlight things you should know about stores.
79* A cache_store interface is used to define the requirements of a store plugin.
80* The store plugin can inherit the cache_is_lockable interface to handle its own locking.
81* The store plugin can inherit the cache_is_key_aware interface to handle is own has checks.
82* Store plugins inform the cache API about the things they support. Features can be required by a definition.
83** Data guarantee - Data is guaranteed to exist in the cache once it is set there. It is never cleaned up to free space or because it has not been recently used.
84** Multiple identifiers - Rather than a single string key, the parts that make up the key are passed as an array.
85** Native TTL support - When required the store supports native ttl and doesn't require the cache API to manage ttl of things given to the store.
86
87### Definition
88_Definitions were not a part of the previous proposal._
89Definitions are cache definitions. They will be located within a new file for each component/plugin at **db/caches.php**.
90They can be used to set all of the requirements of a cache instance and are used to ensure that a cache can only be interacted with in the same way no matter where it is being used.
91It also ensure that caches are easy to use, the config is stored in the definition and the developer using the cache does not need to know anything about it.
92When getting a loader you can either provide a definition name, or a set or params.
93* If you provide a definition name then the matching definition is found and used to construct a loader for you.
94* If you provide params then an adhoc definition is created. It will have defaults and will not have any special requirements or options set.
95
96Definitions are designed to be used in situations where things are more than basic.
97
98The following settings are required for a definition:
99* name - Identifies the definition and must be unique.
100* mode - Application, session, request.
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101
102The following optional settings can also be defined:
103* requireidentifiers - Any identifiers the definition requires. Must be provided when creating the loader.
104* requiredataguarantee - If set to true then only stores that support data guarantee will be used.
105* requiremultipleidentifiers - If set to true then only stores that support multiple identifiers will be used.
106* overrideclass - If provided this class will be used for the loader. It must extend on of the core loader classes (based upon mode).
107* overrideclassfile - Included if required when using the overrideclass param.
108* datasource - If provided this class will be used as a data source for the definition. It must implement the cache_data_source interface.
109* datasourcefile - Included if required when using the datasource param.
110* persistent - If set to true the loader will be stored when first created and provided to subsequent requests. More on this later.
111* ttl - Can be used to set a ttl value for data being set for this cache.
112* mappingsonly - This definition can only be used if there is a store mapping for it. More on this later.
113* invalidationevents - An array of events that should trigger this cache to invalidate.
114
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115Its important to note that internally the definition is also aware of the component. This is picked up when the definition is read based upon the location of the caches.php file.
116
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117The persist option.
118As noted the persist option causes the loader generated for this definition to be stored when first created. Subsequent requests for this definition will be given the original loader instance.
119Data passed to or retrieved from the loader and its chained loaders gets cached by the instance.
120This option should be used when you know you will require the loader several times and perhaps in different areas of code.
121Because it caches key=>value data it avoids the need to re-fetch things from stores after the first request. Its good for performance, bad for memory.
122It should be used sparingly.
123
124The mappingsonly option.
125The administrator of a site can create mappings between stores and definitions. Allowing them to designate stores for specific definitions (caches).
126Setting this option to true means that the definition can only be used if a mapping has been made for it.
127Normally if no mappings exist then the default store for the definition mode is used.
128
129### Data source
130Data sources allow cache _misses_ (requests for a key that doesn't exist) to be handled and loaded internally.
131The loader gets used as the last resort if provided and means that code using the cache doesn't need to handle the situation that information isn't cached.
132They can be specified in a cache definition and must implement the cache_data_source interface.
133
134### How it all chains together.
135Consider the following if you can:
136
137Basic request for information (no frills):
138
139 => Code calls get
140 => Loader handles get, passes the request to its store
141 <= Memcache doesn't have the data. sorry.
142 <= Loader returns the result.
143 |= Code couldn't get the data from the cache. It must generate it and then ask the loader to cache it.
144
145Advanced initial request for information not already cached (has chained stores and data source):
146
147 => Code calls get
148 => Loader handles get, passes the request to its store
149 => Memcache handles request, doesn't have it passes it to the chained store
150 => File (default store) doesn't have it requests it from the loader
151 => Data source - makes required db calls, processes information
152 ...database calls...
153 ...processing and moulding...
154 <= Data source returns the information
155 <= File caches the information on its way back through
156 <= Memcache caches the information on its way back through
157 <= Loader returns the data to the user.
158 |= Code the code now has the data.
159
160Subsequent request for information:
161
162 => Code calls get
163 => Loader handles get, passes the request to its store
164 <= Store returns the data
165 <= Loader returns the data
166 |= Code has the data
167
168Other internal magic you should be aware of
169-------------------------------------------
170The following should fill you in on a bit more of the behind the scenes stuff for the cache API.
171
172### Helper class
173There is a helper class called cache_helper which is abstract with static methods.
174This class handles much of the internal generation and initialisation requirements.
175In normal use this class will not be needed outside of the API (mostly internal use only)
176
177### Configuration
178There are two configuration classes cache_config and cache_config_writer.
179The reader class is used for every request, the writer is only used when modifying the configuration.
180Because the cache API is designed to cache database configuration and meta data it must be able to operate prior to database configuration being loaded.
181To get around this we store the configuration information in a file in the dataroot.
182The configuration file contains information on the configured store instances, definitions collected from definition files, and mappings.
183That information is stored and loaded in the same way we work with the lang string files.
184This means that we use the cache API as soon as it has been included.
185
186### Invalidation
187Cache information can be invalidated in two ways.
1881. pass a definition name and the keys to be invalidated (or none to invalidate the whole cache).
1892. pass an event and the keys to be invalidated.
190
191The first method is designed to be used when you have a single known definition you want to invalidate entries from within.
192The second method is a lot more intensive for the system. There are defined invalidation events that definitions can "subscribe" to (through the definitions invalidationevents option).
193When you invalidate by event the cache API finds all of the definitions that subscribe to the event, it then loads the stores for each of those definitions and purges the keys from each store.
4075433b 194This is obviously a recursive and therefor intense process.