[documentation] Drupal guide to caching

[Peter Wolanin]

Here's a kind of hands-on article from Lullabot (though maybe some D5
specifics): http://www.lullabot.com/articles/a_beginners_guide_to_caching_data

Also, the cache_menu description seems more related to D5 than D6.  In
D6 we cache the router plus trees of menu links.  Due to recent
patches the caching of links is actually via an additional level of
indirection to avoid caching lots of redundant data.  More details on
request...

-Peter

> ---------- Forwarded message ----------
> From: "Steve Dondley" <sdondley at gmail.com>
> To: documentation at drupal.org
> Date: Sat, 29 Mar 2008 21:33:12 -0400
> Subject: [documentation] Drupal guide to caching
> I'm writing a drupal guide to caching. If you know something about
>  caching and are so inclined, please add your two cents about what I
>  have so far:
>
>  THE DRUPAL 6 GUIDE TO CACHING
>
>  This guide is an introduction to the various caching mechanims Drupal
>  uses to speed the site. It's primarily designed for developers but a
>  lot of it should be understandable by Drupal site administrators who
>  are familiar with the basics of how Drupal delivers content.
>
>  The Basics
>  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 aren't. 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
>  it's 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.
>
>  Just like style sheets, javascript files can also be consolidated.
>  However, these files are not compressed.
>
>  How to turn on stylesheet and javascript file caching
>  ======================================================
>  First, be sure you have your "Download method" is set to "Public" (set
>  at admin/settings/file-system). You will not be able to use stylesheet
>  and javascript caching when your "Download method" is set to
>  "Private."
>
>  Next, in the navigation menu, select "Administer -> Site Configuration
>  -> Performance" (admin/settings/performance) and scroll down to the
>  "Bandwidth optimizations" fieldset and check off "Enabled" for both
>  CSS files and Javascript files.
>
>  Note that turning on stylesheet and javascript caching can interfere
>  with theme development and should only be enabled in a production
>  environment.
>
>  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:
>
>  cache
>  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, it's
>  regenerated from ???. A visit to the admin/build/modules page will
>  trigger a cache refresh.
>
>  cache_block
>  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
>  block's 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 block's 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.
>
>  cache_filter
>  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.
>
>  Question: What is the thinking having the cache_filter refereshed on cron jobs?
>
>  AUTHOR'S NOTE: The items below have to be researched more to determine
>  what trigges them to refresh and are unfinished.
>
>  cache_form
>  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.
>
>  cache_menu
>  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.
>
>  cache_page
>  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).
>
>  cache_update
>  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 what's available for download on
>  drupal.org. When this cache goes stale, it is refresed from the data
>  in the system table.
>
>  --
>  Prometheus Labor Communications, Inc.
>  http://prometheuslabor.com
>  413-572-1300
>
>  Communicate or Die: American Labor Unions and the Internet
>  http://communicateordie.com
>
>