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#65486 new enhancement

Performance: Avoid redundant cache invalidation in WP_REST_Meta_Fields during batch meta updates

Reported by:	spacedmonkey	Owned by:
Priority:	normal	Milestone:	Awaiting Review
Component:	REST API	Version:
Severity:	normal	Keywords:	good-first-bug has-patch has-unit-tests
Cc:		Focuses:	rest-api, performance

Description

The Problem

The WP_REST_Meta_Fields class handles updating, adding, and deleting metadata for various object types via the REST API. When a request modifies multiple meta keys at once, it iterates through them and performs the updates individually.

These operations trigger Core's cache invalidation hooks via the underlying added_*_meta, updated_*_meta, and deleted_*_meta hooks.

When a REST API request updates a large batch of meta fields for an object, the object's last_changed cache is cleared and reset repeatedly in the same request execution. This creates unnecessary overhead for database and object cache layers.

The Proposed Solution

We should temporarily unhook the _last_changed invalidation functions before processing multiple meta updates inside WP_REST_Meta_Fields, and re-hook them once the operations are complete.

To do this cleanly across different object types, each subclass extending WP_REST_Meta_Fields should define its specific cache invalidation callback function as a property. For example:

<?php
class WP_REST_Post_Meta_Fields extends WP_REST_Meta_Fields {
    // ...
    protected $cache_callback = 'wp_cache_set_posts_last_changed';
}

Then, in the base WP_REST_Meta_Fields class, we can dynamically target the hooks using get_meta_type() and the subclass property:

<?php
$meta_type = $this->get_meta_type(); // e.g., 'post', 'user', 'comment', 'term'
$hook_name = "updated_{$meta_type}_meta";

// Temporarily unhook the redundant cache invalidation callback
$has_action = false;
if ( ! empty( $this->cache_callback ) ) {
    $has_action = remove_action( $hook_name, $this->cache_callback );
}

// ... existing code that loops through and updates/deletes multiple meta keys ...

// Re-hook after the batch operations complete
if ( $has_action ) {
    add_action( $hook_name, $this->cache_callback );
}

Scope of Impact

WP_REST_Meta_Fields abstract class and its extending subclasses (WP_REST_Post_Meta_Fields, WP_REST_User_Meta_Fields, WP_REST_Comment_Meta_Fields, WP_REST_Term_Meta_Fields).
This optimization will significantly reduce object cache thrashing during complex REST API POST or PUT requests carrying heavy meta payloads.

Change History (1)

This ticket was mentioned in PR #12225 on WordPress/wordpress-develop by @sainathpoojary.

4 weeks ago #1

Keywords has-patch has-unit-tests added

Optimizes WP_REST_Meta_Fields::update_value() to prevent redundant object cache invalidations during batch meta updates. By temporarily unhooking the last_changed callbacks during the update loop and firing them only once at the end, this avoids unnecessary cache thrashing.

Trac ticket: #65486

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