Caching in Drupal 6
Caches are used to improve the performance of your Drupal site. Rather than extracting the same data over and over again every time a page is loaded, caching stores frequently accessed and relatively static data
in a convenient place and format.
Caching has a drawback in that it can lead to "stale" data. This means that the website outputs old data or content from the cache even though newer stuff exists somewhere else. This problem can be particularly troublesome for developers who can get confused as to why changes they expect to see happen arent. Hopefully, by reading this document, you'll have a more pleasant and less confusing Drupal experience.
What gets cached, where it gets cached, and how There are two different ways Drupal stores cached data:
1) Using files
Drupal can consolidate all the css files your site delivers on each page load and place them into a fewer number of files. The resulting files are also compressed. This is important for Drupal sites where its not unusual to have a dozen or more stylesheets associated with each page, depending on how many modules are enabled. Having so many stylesheets will increase page load times because the browser has to make several round trips to the server to download all the stylesheet files. By using the css caching feature, you can consolidate these files into fewer larger files and decrease page load time significantly.
2) In your database
The main location where Drupal stores cached data is in special tables in your database. Drupal core sets up seven tables for caching data. Other modules will add additional cache tables to the database as needed. The seven tables set up by core are:
An "all purpose" table that can be shared by various modules. This table is designed for modules that need to store only a few rows of data. Drupal core uses this table to store the following data:
* Variable data. These are variables that are set with the variable_set() function and retrieved with the variable_get() function. When this cache goes stale, it is refereshed by data from the variable table. A call to the variable_set function will trigger a cache refresh.
* Theme registry data. This registry is a listing of all the themes that can be overridden by theme developers. The existance of the theme registry makes it easy to overide themes by allowing a user to simply
drop a tpl.php file in the theme's directory. When this cache goes stale, it's regenerated from the function definitions contained in module and theme files.
* Schema data. This data contains information about the table structure of the database. When this cache goes stale, its regenerated from ???. A visit to the admin/build/modules page will trigger a cache refresh.
A table for storing content generated by your blocks. This saves Drupal from having to repeatedly query the database for unchanged block content. When this cache goes stale, it is refreshed by data from the boxes table where block content is stored. Any update to a blocks content will trigger a cache refresh for that block. The
entire cache is refreshed when a node, comment, user, or taxonomy term is added or updated. Module developers have the option of gaining more control over when a particular blocks cache is refreshed using cache
granularity settings for their blocks. Refer to the constants defined at the top of the block.module for further details.
The block cache can be turned on an off at "Administer -> Site Configuration -> Performance" (admin/settings/performance).
Note that block caching is inactive when modules defining content access restrictions are enabled. For example, if organic groups, content access, taxonomy access modules or other modules that restrict
access to certain kinds of content are turned on, block caching will be turned off.
A table for storing filtered pieces of content. This saves Drupal from having to run the same expensive regular expression operations on unchanged content that gets run through the input filters. When this
cache goes stale, it is refreshed by the data in the node_revisions table which contains node content. Cron jobs, updates to nodes, and updates to filter formats will trigger cache refreshes.
A table for storing forms generated by the forms api. This saves Drupal from having to rebuild a unchanged forms. When this cache goes stale, it is refreshed by output from the form module.
A table for storing the menu items and menu item hierarchies. This saves Drupal from having to regenerate the data structures needed to define the menu items and their hiearchies each page load. When the data goes stale, the cache is refreshed from the data contained in the menu table.
A table for storing pages for anonymous users. This saves Drupal from making dozens or even hundreds of expensive queries needed to generate a page. When the cache for a particular page goes stale, it gets refreshed by the html output for that page. This cache can be turned on an off under "Administer -> Site Configuration -> Performance" (admin/settings/performance).
A table used to store information about installed modules and themes. This saves Drupal from having to perform two very expensive operations for listing the installed modules and themes and the status of these modules and releases compared to whats available for download on drupal.org. When this cache goes stale, it is refresed from the data in the system table.