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Cache Collector

Coding Standards Testing Suite

Dynamic cache key collector for easy purging.

Background

One common problem with large WordPress sites that utilize Memcache is the problems that arise when trying to purge cache keys that are dynamically generated. For example, if a cache key is the hash of a remote request. You would need to calculate the hashed cache key to properly purge it from the cache. Another common problem would be trying to purge all the cache keys in a specific group (Memcache doesn't support group purging).

Cache Collector solves this by storing cache/transient keys in collections. These collections can then be purged in a single command. Here's a real-world use case:

When viewing a post, the post's related posts are fetched from a remote source and displayed to the user. This operation is expensive due to the remote request and needs to be cached. When the post is updated, the related post cache needs to be flushed as well.

Enter Cache Collector. When the post is updated, the related post cache key is added to a collection. When the post is updated, the cache key that is connected to the post will automatically be purged.

To flip this around, say the remote data source is having issues and you need to flush all the related post cache keys. You can do this by purging the "related posts" cache collection. This stores all the cache keys for all related posts. In one command you can purge an entire cache group without having to calculate the cache key for each.

Installation

You can install the package via composer:

composer require alleyinteractive/cache-collector

Usage

Activate the plugin in WordPress and use the below methods to interface with the cache collector.

Registering Keys

One important note when registering cache keys: registering a key should only be done when the cache/transient is stored. When the key is stored the system will set an expiration date for the key to be eventually purged from the collection if unused. To prevent continually updating the keys in the collection and degrading site performance, the key should only be registered when the cache/transient is stored. The cache collector will eventually remove the key from the collection when it expires.

TL;DR: Register the key only when the cache/transient is stored. Don't register it on every page load.

Register a Key in a Cache Collection

cache_collector_register_key( string $collection, string $key, string $group = '', int $ttl = 0, string $type = 'cache' );

// Example using named arguments.
cache_collector_register_key(
	collection: 'related_posts',
	key: $related_posts_cache_key,
	group: 'related_posts',
	ttl: 3600,
	type: 'cache',
);

The plugin also provides cache_collector_register_transient_key() and cache_collector_register_cache_key() to make it easier to register keys for a transient/cache without having to specify the $type argument.

cache_collector_register_transient_key( string $collection, string $transient, int $ttl = 0 );
cache_collector_register_cache_key( string $collection, string $key, string $group = '', int $ttl = 0 );

Purging a Cache Collection

Purge all the cache entries in a collection.

cache_collector_purge( string $collection );

Registering a Key Related to a Post

A post cache collection is a collection of cache keys related to a post. When a post is updated, the post's cache collection is automatically purged. This allows you to purge all of the cache keys related to a post at once.

cache_collector_register_post_key( \WP_Post|int $post, string $key, string $group = '', string $type = 'cache' );

// Example using named arguments.
cache_collector_register_post_key(
	post: $post,
	key: $related_posts_cache_key,
	group: 'related_posts',
	type: 'cache',
);

Purging a Post's Cache Collection

Purge a cache collection related to a post.

cache_collector_purge_post( \WP_Post|int $post_id );

Registering a Key Related to a Term

cache_collector_register_term_key( \WP_Term|int $term, string $key, string $group = '', string $type = 'cache' );

// Example using named arguments.
cache_collector_register_term_key(
	term: $term,
	key: $related_posts_cache_key,
	group: 'related_posts',
	type: 'cache',
);

Purging a Term's Cache Collection

cache_collector_purge_term( \WP_Term|int $term );

Full Example

The following example shows how to use the cache collector to register a cache key for future purging.

$arguments = [
	'limit' => 100,
	'term' => '...',
	'fields' => [ ... ],
];

$data = wp_cache_get( md5( $arguments ), 'my_cache_group' );

if ( false === $data ) {
	$data = remote_data_fetch( $arguments );
	wp_cache_set( md5( $arguments ), $data, 'my_cache_group' );

	// Register the cache key for the collection.
	cache_collector_register_cache_key( 'my_collection', md5( $arguments ), 'my_cache_group' );
}

Now we can purge the cache collection whenever we need to:

cache_collector_purge( 'my_collection' );

We can also purge this with the WP-CLI command included with the plugin:

wp cache-collector purge my_collection

Testing

Run composer test to run tests against PHPUnit and the PHP code in the plugin.

Changelog

Please see CHANGELOG for more information on what has changed recently.

Credits

This project is actively maintained by Alley Interactive. Like what you see? Come work with us.

License

The GNU General Public License (GPL) license. Please see License File for more information.