When loading many objects of a given type, such as posts in a WP_Query loop, it often makes sense to prime various metadata caches at the beginning of the loop. This means fetching all relevant metadata with a single database query, a technique that has the potential to improve performance dramatically in some cases.
In cases where the given metadata may not even be used in the loop, we can improve performance even more by only priming the metadata cache for affected items the first time a piece of metadata is requested - ie, by lazy-loading it. So, for example, comment meta may not be loaded into the cache in the comments section of a post until the first time get_comment_meta() is called in the context of the comment loop.
WP uses the WP_Metadata_Lazyloader class to queue objects for metadata cache priming. The class then detects the relevant get_*_meta() function call, and queries the metadata of all queued objects.
class WP_Metadata_Lazyloader {
/**
* Pending objects queue.
*
* @since 4.5.0
* @var array
*/
protected $pending_objects;
/**
* Settings for supported object types.
*
* @since 4.5.0
* @var array
*/
protected $settings = array();
/**
* Constructor.
*
* @since 4.5.0
*/
public function __construct() {
$this->settings = array(
'term' => array(
'filter' => 'get_term_metadata',
'callback' => array( $this, 'lazyload_meta_callback' ),
),
'comment' => array(
'filter' => 'get_comment_metadata',
'callback' => array( $this, 'lazyload_meta_callback' ),
),
'blog' => array(
'filter' => 'get_blog_metadata',
'callback' => array( $this, 'lazyload_meta_callback' ),
),
);
}
/**
* Adds objects to the metadata lazy-load queue.
*
* @since 4.5.0
*
* @param string $object_type Type of object whose meta is to be lazy-loaded. Accepts 'term' or 'comment'.
* @param array $object_ids Array of object IDs.
* @return void|WP_Error WP_Error on failure.
*/
public function queue_objects( $object_type, $object_ids ) {
if ( ! isset( $this->settings[ $object_type ] ) ) {
return new WP_Error( 'invalid_object_type', __( 'Invalid object type.' ) );
}
$type_settings = $this->settings[ $object_type ];
if ( ! isset( $this->pending_objects[ $object_type ] ) ) {
$this->pending_objects[ $object_type ] = array();
}
foreach ( $object_ids as $object_id ) {
// Keyed by ID for faster lookup.
if ( ! isset( $this->pending_objects[ $object_type ][ $object_id ] ) ) {
$this->pending_objects[ $object_type ][ $object_id ] = 1;
}
}
add_filter( $type_settings['filter'], $type_settings['callback'], 10, 5 );
/**
* Fires after objects are added to the metadata lazy-load queue.
*
* @since 4.5.0
*
* @param array $object_ids Array of object IDs.
* @param string $object_type Type of object being queued.
* @param WP_Metadata_Lazyloader $lazyloader The lazy-loader object.
*/
do_action( 'metadata_lazyloader_queued_objects', $object_ids, $object_type, $this );
}
/**
* Resets lazy-load queue for a given object type.
*
* @since 4.5.0
*
* @param string $object_type Object type. Accepts 'comment' or 'term'.
* @return void|WP_Error WP_Error on failure.
*/
public function reset_queue( $object_type ) {
if ( ! isset( $this->settings[ $object_type ] ) ) {
return new WP_Error( 'invalid_object_type', __( 'Invalid object type.' ) );
}
$type_settings = $this->settings[ $object_type ];
$this->pending_objects[ $object_type ] = array();
remove_filter( $type_settings['filter'], $type_settings['callback'] );
}
/**
* Lazy-loads term meta for queued terms.
*
* This method is public so that it can be used as a filter callback. As a rule, there
* is no need to invoke it directly.
*
* @since 4.5.0
* @deprecated 6.3.0 Use WP_Metadata_Lazyloader::lazyload_meta_callback() instead.
*
* @param mixed $check The `$check` param passed from the 'get_term_metadata' hook.
* @return mixed In order not to short-circuit `get_metadata()`. Generally, this is `null`, but it could be
* another value if filtered by a plugin.
*/
public function lazyload_term_meta( $check ) {
_deprecated_function( __METHOD__, '6.3.0', 'WP_Metadata_Lazyloader::lazyload_meta_callback' );
return $this->lazyload_meta_callback( $check, 0, '', false, 'term' );
}
/**
* Lazy-loads comment meta for queued comments.
*
* This method is public so that it can be used as a filter callback. As a rule, there is no need to invoke it
* directly, from either inside or outside the `WP_Query` object.
*
* @since 4.5.0
* @deprecated 6.3.0 Use WP_Metadata_Lazyloader::lazyload_meta_callback() instead.
*
* @param mixed $check The `$check` param passed from the {@see 'get_comment_metadata'} hook.
* @return mixed The original value of `$check`, so as not to short-circuit `get_comment_metadata()`.
*/
public function lazyload_comment_meta( $check ) {
_deprecated_function( __METHOD__, '6.3.0', 'WP_Metadata_Lazyloader::lazyload_meta_callback' );
return $this->lazyload_meta_callback( $check, 0, '', false, 'comment' );
}
/**
* Lazy-loads meta for queued objects.
*
* This method is public so that it can be used as a filter callback. As a rule, there
* is no need to invoke it directly.
*
* @since 6.3.0
*
* @param mixed $check The `$check` param passed from the 'get_*_metadata' hook.
* @param int $object_id ID of the object metadata is for.
* @param string $meta_key Unused.
* @param bool $single Unused.
* @param string $meta_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
* or any other object type with an associated meta table.
* @return mixed In order not to short-circuit `get_metadata()`. Generally, this is `null`, but it could be
* another value if filtered by a plugin.
*/
public function lazyload_meta_callback( $check, $object_id, $meta_key, $single, $meta_type ) {
if ( empty( $this->pending_objects[ $meta_type ] ) ) {
return $check;
}
$object_ids = array_keys( $this->pending_objects[ $meta_type ] );
if ( $object_id && ! in_array( $object_id, $object_ids, true ) ) {
$object_ids[] = $object_id;
}
update_meta_cache( $meta_type, $object_ids );
// No need to run again for this set of objects.
$this->reset_queue( $meta_type );
return $check;
}
}
