Side-effect caching support

Posted by David Strauss on February 5, 2007 at 2:04am

Issue Summary

Currently, caching has limited easy use because side-effecting operations are not repeated when the cached item is loaded.

For example, a function building the front page might return the content but also add a stylesheet and an RSS feed header. If only the results of the front-page-building function are cached, loading the cached version requires either manually repeating the side effects or awkwardly storing the side-effects in the cache for replay. As function calls get deeper, predicting and replaying these side effects becomes very difficult.

We could improve such caching by having a hook_side_effect that is called whenever side-effecting operations run. hook_side_effect would be called with the function name (e.g. 'drupal_add_js') and the array of arguments the function received. Caching modules could implement the hook to record and replay side effects when the cached item is retrieved.

Comments

#1

Posted by David Strauss on February 6, 2007 at 6:48am

Title:

Side-effect function hooks for caching

» The addition

Add this:

module_invoke_all('side_effect', __FUNCTION__, func_get_args());

to following are side-effecting functions:

drupal_add_css
drupal_add_feed
drupal_add_js
drupal_add_link

This will allow caching modules to log and replay side-effects on cache restoration.

#2

Posted by David Strauss on February 6, 2007 at 6:51am

Title:

The addition

» Side-effect caching support

Whoops, didn't notice that the title changed the entire bug/feature report. Should have read the descriptive text...

#3

Posted by David Strauss on February 6, 2007 at 7:05am

Assigned to:	Anonymous	» David Strauss
Status:	active	» needs review

Here's a patch to a current HEAD checkout. I tried to minimize the hook calls to when actual effects happen. I'm sure this could be improved further.

Attachment	Size	Status	Test result	Operations
side_effect_cache.patch	1.19 KB	Ignored: Check issue status.	None	None

#4

Posted by David Strauss on February 6, 2007 at 7:08am

I'd like feedback over whether its better to implement this as a hook or to simply compare the arrays before a function call and after to identify additions. The latter would be more complicated but possibly faster in the common case (no cache).

#5

Posted by David Strauss on February 6, 2007 at 7:29am

Given the arbitrary values for $scope for drupal_get_js() and the inability to simply load the whole array for all scopes, it seems impossible to determine what core side-effects a function has used by probing for changes.

#6

Posted by David Strauss on February 6, 2007 at 7:48am

Here's an alternative patch that at least allows retrieving the entire array of JavaScript for all scopes. This has a clear performance advantage over adding hook_side_effect, but it's not as extensible. hook_side_effect could be used to record arbitrary side-effecting calls. This alternative patch would require the cache to probe for side effects in specific places.

Of course, a hybrid approach is possible, too. The cache could probe core modules for side effects but still implement hook_side_effect for arbitrary additional effects.