Strategies to implement selective loading of plugins

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I’m trying to figure out some kind of mechanism to load plugins on demand, depending on the page url, to improve performance.

My primary concern is that I have some admin-ajax.php calls that are recurring while the user is active on the page. I tried profiling these calls, and discovered that the majority of plugins loaded for the ajax call were unnecessary.

While I could use is_admin() to identify an ajax call, I wish to leave WordPress and plugin code intact. Plus, is_admin() does not differentiate between an ajax call and admin panel.

I’ve been looking into the plugin loading procedure. I only saw 'muplugins_loaded', 'plugins_loaded' actions, no filters to tap into the loading process. I checked wp_get_active_and_valid_plugins and it appears the only way to change plugin loading is to update_option('active_plugins') explicitly, which is not a viable option. Other things are hard coded.

(I’d like to mention that this plugin http://wordpress.org/extend/plugins/selective-loading/ use the update_option method, which in my opinion is only suitable in a non-ajax environment, or otherwise the option change wouldn’t be atomic.)

I’m wondering if there was still some way to implement selective plugin loading?

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Filter option_active_plugins. You can change the result of get_option() here without actually changing the database.

if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
    add_filter( 'option_active_plugins', 'disable_plugins_temporary' );

function disable_plugins_temporary( $plugins )
{
    // unset plugins you don't need, then

    return $plugins;
}

Background

wp_get_active_and_valid_plugins() calls get_option( 'active_plugins', array() ) to get the active plugins. In get_option( $option, $default = false ) we find this filter:

return apply_filters( 'option_' . $option, maybe_unserialize( $value ) );

So the resulting name for our filter is option_active_plugins.

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