The Otto & Nacin Show
WordCamp San Francisco 2011
August 12, 2011

Nacin Otto
Contributing Developers to WordPress
Tech Ninjas at Audrey Capital
nacin@wordpress.org otto@wordpress.org
@nacin @otto42

Transients, Caching, and
the Complexities of
Multisite
(A brief overview.)

Basic Caching with
Transients

Transients are ideal for:
Temporary data that can
expire at will.

$value = get_transient( 'big_data' );
if ( false === $value ) {
// do something that takes a fair amount of time
$response = wp_remote_get( $url );
$value = wp_remote_retrieve_body( $response );
set_transient( 'big_data', $value, 60 * 60 * 24 );
}
echo $value;

$value = get_transient( 'big_data' );
if ( false === $value ) {
// do something that takes a fair amount of time
$response = wp_remote_get( $url );
$value = wp_remote_retrieve_body( $response );
set_transient( 'big_data', $value );
// this one doesn't expire, but it might anyway
}
echo $value;

Always persistent:
When there's no external
object cache, WordPress
uses the options table.

Object Caching

Object caching is ideal for:
Caching an object (often a
query result) to reduce
overhead.

$activity_object = wp_cache_get( $id, 'activity' );
if ( false === $activity_object ) {
// begrudgingly fetch our object
$activity_object = $wpdb->get_row(
$wpdb->prepare( "SELECT * FROM
$wpdb->activity WHERE ID = %d", $id ) );
wp_cache_set( $id, $activity_object , 'activity' );
}
// do something with $activity_object

Ideally persistent:
Use an object caching
backend like APC,
Memcached, or WinCache.

Object caching is not ideal if
you need both data
persistence and code
portability.
No knowledge of the server
setup? Use transients.

// Cache functions can take an expiration too
wp_cache_set( $id, $activity_object ,
'activity', $expires = 0 );

Enter Multisite
(Run.)

Site Transients!
Well, network-wide transients.
Pardon the terminology.

$value = get_site_transient( 'big_data' );
if ( false === $value ) {
// do something that takes a fair amount of time
$response = wp_remote_get( $url );
$value = wp_remote_retrieve_body( $response );
set_site_transient( 'big_data', $value, 86400 );
}
echo $value;

Always persistent:
When there's no external
object cache, WordPress
uses the sitemeta table.

It works in single-site!
When not running multisite,
it wraps *_transient().

Global Cache Groups
Cache groups ('activity') are site-
specific. Tell WordPress what isn't.

// WordPress core (simplified)
wp_cache_add_global_groups(
array(
'users', 'user_meta', 'site-transient',
'site-options', 'site-lookup',
'blog-lookup', 'blog-details',
)
);
// You (not simplified)
wp_cache_add_global_groups( 'activity' );

As you might expect, it works
just fine in single-site.

And then there's
switch_to_blog()
With great power comes
great responsibility.

Rule 1
Don't use it.
If you do, cache around it.

// Example is a widget with posts from another site:
$posts = wp_cache_get( 'recent_posts', $blog_id );
if ( false === $posts ) {
switch_to_blog( $blog_id );
$posts = new WP_Query( . . . )->posts;
wp_cache_set( $blog_id, $posts, 'recent_posts' );
restore_current_blog(); // OMG BBQ
}
// do something with $posts

// And for some extra credit:
add_action( 'init', function() {
wp_cache_add_global_groups( 'recent_posts' );
} );
add_action( 'save_post', function() {
global $blog_id;
if ( ! in_array( $blog_id, array( 1, 2 3, 4 ) )
return;
wp_cache_delete( $blog_id, 'recent_posts' );
// And for even more extra credit:
$posts = new WP_Query( . . . )->posts;
wp_cache_add( $blog_id, $posts, 'recent_posts' );
} );

Thanks! Questions?
@otto42 @nacin