Caching framework architecture¶
The caching framework can handle multiple caches with different configurations. A single cache consists of any number of cache entries.
A single cache entry is defined by these fields:
A string as unique identifier within this cache. It is used to store and retrieve entries.
The data to be cached.
A lifetime in seconds of this cache entry. An entry can not be retrieved from cache, if the lifetime is expired.
Additional tags (an array of strings) assigned to the entry. It is used to remove specific cache entries.
The difference between identifier and tags is quite simple: an identifier uniquely identifies a cache entry, and a tag is additional data applied to an entry (used for cache eviction). Thus, an identifier refers to a single cache entry to store and retrieve an entry, and a tag can refer to multiple cache entries.
About the identifier¶
The identifier is used to store ("set") and retrieve ("get") entries from the cache and holds all information to differentiate entries from each other. For performance reasons, it should be quick to calculate.
Suppose a resource-intensive extension is added as a plugin on two different pages. The calculated content depends on the page on which it is inserted and if a user is logged in or not. So, the plugin creates at maximum four different ontent outputs, which can be cached in four different cache entries:
page 1, no user logged in
page 1, a user is logged in
page 2, no user logged in
page 2, a user is logged in
To differentiate all entries from each other, the identifier is built from the page ID where the plugin is located, combined with the information whether a user is logged in. These are concatenated and hashed. In PHP this could look like this:
$identifier = sha1((string)$this->getPageUid() . (string)$this->isUserLoggedIn());
sha1 is a good hash algorithm in this case, as collisions are extremely unlikely. It scales O(n) with the input length.
When the plugin is accessed, the identifier is calculated early in the program flow. Next, the plugin looks up for a cache entry with this identifier. If such an entry exists, the plugin can return the cached content, else it calculates the content and stores a new cache entry with this identifier.
In general, the identifier is constructed from all dependencies which specify a unique set of data. The identifier should be based on information which already exist in the system at the point of its calculation. In the above scenario the page ID and whether or not a user is logged in are already determined during the frontend bootstrap and can be retrieved from the system quickly.
Caches in the TYPO3 Core¶
The TYPO3 Core defines and uses several caching framework caches by default. This section gives an overview of default caches, its usage and behaviour. If not stated otherwise, the default database backend with variable frontend is used.
The various caches are organized in groups. Currently, the following groups exist:
System caches. Flushing system caches should be avoided as much as possible, as rebuilding them requires significant resources.
Low-level caches. Flushing low-level caches manually should be avoided completely.
All other caches.
Cache clearing commands can be issued to target a particular group. If a cache does not belong to a group, it will be flushed when the "all" group is flushed, but such caches should normally be transient anyway.
There are TSconfig options for permissions corresponding to each group.
The following caches exist in the TYPO3 Core:
Core cache for compiled PHP code. It should not be used by extensions.
This cache is instantiated very early during bootstrap and can not be re-configured by instance-specific
Cache entries are located in directory
var/cache/code/core/(for Composer-based installations) and
typo3temp/var/cache/code/core/(for legacy installations). The full directory and any file in this directory can be safely removed and will be re-created upon next request. This is especially useful during development
groups: all, pages
Stores several key-value based cache entries, mostly used during frontend rendering.
groups: all, pages
The frontend page cache. It stores full frontend pages.
The content is compressed by default to reduce database memory and storage overhead.
Runtime cache to store data specific for current request.
Used by several Core parts during rendering to re-use already calculated data.
Valid for one request only.
Can be re-used by extensions that have similar caching needs.
groups: all, pages
Cache for rootline calculations.
A quick and simple cache dedicated for Core usage, it should not be re-used by extensions.
Cache for assets.
Cache for the localized labels.
Cache for Fluid templates.
Contains detailed information about a class' member variables and methods.
Cache for TypoScript.
New in version 12.4.2.
Cache for database schema information.
Contains the contents of RSS feeds retrieved by RSS widgets on the dashboard.
This cache can be used by extension authors for their own RSS widgets.
In rare cases, for example, when classes that are required during the
bootstrap process are introduced (usually when working on the TYPO3 Core),
cache clearings requests themselves might throw fatal errors.
The solution here is to manually remove the cache files from
var/cache/code/ (for Composer-based installations) or
typo3temp/var/cache/code/ (for legacy installations).
Garbage collection task¶
The Core system provides a scheduler task to collect the garbage of all cache backends. This is important for backends like the database backend that do not remove old cache entries and tags internally. It is highly recommended to add this scheduler task and run it once in a while (maybe once a day at night) for all used backends that do not delete entries which exceeded their lifetime on their own to free up memory or hard disk space.
The caching framework architecture is based on the following classes:
Main interface to handle cache entries of a specific cache. Different frontends and further interfaces exist to handle different data types.
Main interface that every valid storage backend must implement. Several backends and further interfaces exist to specify specific backend capabilities. Some frontends require backends to implement additional interfaces.
\TYPO3\CMS\Core\Cache\CacheManager was used before TYPO3 v10.1 to
retrieve an object implementing
FrontendInterface. It is now
recommended to use dependency injection
to retrieve this object and no longer use the
Do not use the
ext_localconf.php - instead
load caches on demand at the place where they are needed.