[drupal-devel] [feature] Alternative caching for high traffic websites

Jeremy drupal-devel at drupal.org
Wed Mar 23 04:33:44 UTC 2005

Issue status update for http://drupal.org/node/19298

 Project:      Drupal
 Version:      cvs
 Component:    base system
 Category:     feature requests
 Priority:     normal
 Assigned to:  Jeremy at kerneltrap.org
 Reported by:  Jeremy at kerneltrap.org
 Updated by:   Jeremy at kerneltrap.org
 Status:       patch
 Attachment:   http://drupal.org/files/issues/system.module_5.patch (1.49 KB)

Attached is the patch for the system.module, adding the new cache
configuration option.

Jeremy at kerneltrap.org

Previous comments:

March 22, 2005 - 23:32 : Jeremy at kerneltrap.org

Attachment: http://drupal.org/files/issues/database.mysql_1.patch (382 bytes)

Drupal's existing caching mechanism doesn't perform well on highly
dynamic websites in which the cache is flushed frequently.  One example
is a site that is under attack by a spambot that is posting spam
comments every few seconds, causing all cached pages to be flushed
every few seconds.
The attached patches provide the following cache methods:
 1) CACHE_DISABLED:  no caching.  This is unchanged from before.
 2) CACHE_ENABLED_STRICT: This is Drupal's existing caching method, in
which the cache is flushed immediately whenever it contains invalid
 3) CACHE_ENABLED_LOOSE: This is the new caching method (feel free to
suggest a better name, this is the best I could come up with tonight). 
Essentially, it immediately flushes the cache only for specific users
who have modified cached data (whether or not they are logged in),
delaying the flushing of data for other users by several minutes.
This feature requires a new 'cache' field in the sessions table to
track the cache status for each user, whether or not they are logged
A one line change to the system.module updates the cache configuration
settings to include an additional option for loose caching.
The rest of the changes are in bootstrap.inc, as follows:
 - cache_clear_all: if strict caching is enabled, nothing is changed. 
If loose caching is enabled, we delay the flushing of the entire cache
for 5 minutes.  Instead, we update this user's cache field in the
sessions table so we know any cache pages created before the current
time are no longer valid for this user.  We only set the global
"cache_flush" variable if it's not already set.  If the global
"cache_flush" variable was set more than 5 minutes ago, we flush all
cache pages that were expired when the "cache_flush" variable was set.
 - cache_get:  if strict caching is enabled, nothing is changed.  If
loose caching is enabled, we first check the global "cache_flush"
variable and if older than 5 minutes we do a garbage collection call to
cache_clear_all.  Next, we see if there is a valid cache entry for us. 
If the cache entry is older than our session table's "cache" field, we
don't use it.  This will cause the cache to be regenerated by the
current user, and ultimately updated for all users.
Attached is the patch to database.mysql.  To add manually, execute the
ALTER TABLE sessions ADD cache int(11) NOT NULL default '0' AFTER

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