WP_Object_Cache

Core class that implements an object cache.


Description Description

The WordPress Object Cache is used to save on trips to the database. The Object Cache stores all of the cache data to memory and makes the cache contents available by using a key, which is used to name and later retrieve the cache contents.

The Object Cache can be replaced by other caching mechanisms by placing files in the wp-content folder which is looked at in wp-settings. If that file exists, then this file will not be included.


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More Information More Information

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Role of the WordPress Object Cache Role of the WordPress Object Cache

WP_Object_Cache is WordPress’ class for caching data which may be computationally expensive to regenerate, such as the result of complex database queries. The object cache is defined in wp-includes/cache.php.

Do not use the class directly in your code when writing plugins, but use the wp_cache functions listed below.

By default, the object cache is non-persistent. This means that data stored in the cache resides in memory only and only for the duration of the request. Cached data will not be stored persistently across page loads unless you install a persistent caching plugin.

Use the Transients API instead of these functions if you need to guarantee that your data will be cached. If persistent caching is configured, then the transients functions will use the wp_cache functions described in this document. However if persistent caching has not been enabled, then the data will instead be cached to the options table.

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wp_cache functions wp_cache functions

Most of these functions take a:

  • $key: the key to indicate the value.
  • $data: the value you want to store.
  • $group: (optional) this is a way of grouping data within the cache. Allows you to use the same key across different groups.
  • $expire: (optional) this defines how many seconds to keep the cache for. Only applicable to some functions. Defaults to 0 (as long as possible).

wp_cache_add

wp_cache_add( $key, $data, $group, $expire )

This function adds data to the cache if the cache key doesn’t already exist. If it does exist, the data is not added and the function returns false.

wp_cache_set

wp_cache_set( $key, $data, $group, $expire )

Adds data to the cache. If the cache key already exists, then it will be overwritten; if not then it will be created.

wp_cache_get

wp_cache_get( $key, $group )
wp_cache_get( $key, $group = '', $force = false, $found = null )

Returns the value of the cached object, or false if the cache key doesn’t exist.

To disambiguate a cached false from a non-existing key, you should do absolute testing of $found, which is passed by reference, against false: if $found === false, the key does not exist.

wp_cache_delete

wp_cache_delete( $key, $group )

Clears data from the cache for the given key.

wp_cache_replace

wp_cache_replace( $key, $data, $group, $expire )

Replaces the given cache if it exists, returns false otherwise. This is similar to wp_cache_set() except the cache object is not added if it doesn’t already exist.

wp_cache_flush

wp_cache_flush()

Clears all cached data.

wp_cache_add_non_persistent_groups

wp_cache_add_non_persistent_groups($groups)

Hints to the object cache that the group or list of groups should not be cached in persistent storage. This is useful when adding items to the cache that should only be available for the duration of a script session, and not beyond. $groups can be an array of strings, or a single group name. NB: only some caching plugins implement this function!

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Persistent Caching Persistent Caching

Prior to WordPress 2.5, data stored using the wp_cache functions was stored persistently if you added define('WP_CACHE', true) to your wp-config.php file.

This is no longer the case, and adding the define will have no effect unless you install a persistent cache plugin (see list below).

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Persistent Cache Plugins Persistent Cache Plugins

  • Memcached Object Cache provides a persistent backend for the WordPress object cache. A memcached server and the PECL memcached extension are required.
  • APCu Object Cache provides a persistent backend for the WordPress object cache using APC / APCu, a PHP key/value store saved in memory.
  • Redis Object Cache supports the use of Predis (PHP client library), HHVM’s Redis extension and the PECL Redis extension to provide a persistent backend for the WordPress object cache. Redis is required.
  • W3 Total Cache provides object level caching using disk, opcode or memcache(d) memory stores. W3TC also provides: browser, page and database caching, in addition to Content Delivery Network Support, Mobile Support, Minification and more.
  • LiteSpeed Cache supports the use of Redis, Memcached, and LSMCD object caches. LSCWP also provides server-level full-page caching, support for browser cache and CDN, and other optimization features. LiteSpeed Web Server required.

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The Transients_API provides persistent but temporary data caching by giving it a custom name and a timeframe after which it will be expired and regenerated.

Note: Transients only get deleted when a request is made. So, until someone visits your page and calls up the Transient, it will stay in the DB. In short: It’s not a real persistent cache and not equal to stuff running on cron jobs.

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Resources Resources

  • The Debug Bar plugin helps you analyze the object cache.
  • Scott Taylor’s article WordPress + Memcached explains various cache plugins and concepts.

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Source Source

File: wp-includes/class-wp-object-cache.php

class WP_Object_Cache {

	/**
	 * Holds the cached objects.
	 *
	 * @since 2.0.0
	 * @var array
	 */
	private $cache = array();

	/**
	 * The amount of times the cache data was already stored in the cache.
	 *
	 * @since 2.5.0
	 * @var int
	 */
	public $cache_hits = 0;

	/**
	 * Amount of times the cache did not have the request in cache.
	 *
	 * @since 2.0.0
	 * @var int
	 */
	public $cache_misses = 0;

	/**
	 * List of global cache groups.
	 *
	 * @since 3.0.0
	 * @var array
	 */
	protected $global_groups = array();

	/**
	 * The blog prefix to prepend to keys in non-global groups.
	 *
	 * @since 3.5.0
	 * @var string
	 */
	private $blog_prefix;

	/**
	 * Holds the value of is_multisite().
	 *
	 * @since 3.5.0
	 * @var bool
	 */
	private $multisite;

	/**
	 * Sets up object properties; PHP 5 style constructor.
	 *
	 * @since 2.0.8
	 */
	public function __construct() {
		$this->multisite   = is_multisite();
		$this->blog_prefix = $this->multisite ? get_current_blog_id() . ':' : '';
	}

	/**
	 * Makes private properties readable for backward compatibility.
	 *
	 * @since 4.0.0
	 *
	 * @param string $name Property to get.
	 * @return mixed Property.
	 */
	public function __get( $name ) {
		return $this->$name;
	}

	/**
	 * Makes private properties settable for backward compatibility.
	 *
	 * @since 4.0.0
	 *
	 * @param string $name  Property to set.
	 * @param mixed  $value Property value.
	 * @return mixed Newly-set property.
	 */
	public function __set( $name, $value ) {
		return $this->$name = $value;
	}

	/**
	 * Makes private properties checkable for backward compatibility.
	 *
	 * @since 4.0.0
	 *
	 * @param string $name Property to check if set.
	 * @return bool Whether the property is set.
	 */
	public function __isset( $name ) {
		return isset( $this->$name );
	}

	/**
	 * Makes private properties un-settable for backward compatibility.
	 *
	 * @since 4.0.0
	 *
	 * @param string $name Property to unset.
	 */
	public function __unset( $name ) {
		unset( $this->$name );
	}

	/**
	 * Adds data to the cache if it doesn't already exist.
	 *
	 * @since 2.0.0
	 *
	 * @uses WP_Object_Cache::_exists() Checks to see if the cache already has data.
	 * @uses WP_Object_Cache::set()     Sets the data after the checking the cache
	 *                                  contents existence.
	 *
	 * @param int|string $key    What to call the contents in the cache.
	 * @param mixed      $data   The contents to store in the cache.
	 * @param string     $group  Optional. Where to group the cache contents. Default 'default'.
	 * @param int        $expire Optional. When to expire the cache contents. Default 0 (no expiration).
	 * @return bool True on success, false if cache key and group already exist.
	 */
	public function add( $key, $data, $group = 'default', $expire = 0 ) {
		if ( wp_suspend_cache_addition() ) {
			return false;
		}

		if ( empty( $group ) ) {
			$group = 'default';
		}

		$id = $key;
		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$id = $this->blog_prefix . $key;
		}

		if ( $this->_exists( $id, $group ) ) {
			return false;
		}

		return $this->set( $key, $data, $group, (int) $expire );
	}

	/**
	 * Sets the list of global cache groups.
	 *
	 * @since 3.0.0
	 *
	 * @param array $groups List of groups that are global.
	 */
	public function add_global_groups( $groups ) {
		$groups = (array) $groups;

		$groups              = array_fill_keys( $groups, true );
		$this->global_groups = array_merge( $this->global_groups, $groups );
	}

	/**
	 * Decrements numeric cache item's value.
	 *
	 * @since 3.3.0
	 *
	 * @param int|string $key    The cache key to decrement.
	 * @param int        $offset Optional. The amount by which to decrement the item's value. Default 1.
	 * @param string     $group  Optional. The group the key is in. Default 'default'.
	 * @return int|false The item's new value on success, false on failure.
	 */
	public function decr( $key, $offset = 1, $group = 'default' ) {
		if ( empty( $group ) ) {
			$group = 'default';
		}

		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$key = $this->blog_prefix . $key;
		}

		if ( ! $this->_exists( $key, $group ) ) {
			return false;
		}

		if ( ! is_numeric( $this->cache[ $group ][ $key ] ) ) {
			$this->cache[ $group ][ $key ] = 0;
		}

		$offset = (int) $offset;

		$this->cache[ $group ][ $key ] -= $offset;

		if ( $this->cache[ $group ][ $key ] < 0 ) {
			$this->cache[ $group ][ $key ] = 0;
		}

		return $this->cache[ $group ][ $key ];
	}

	/**
	 * Removes the contents of the cache key in the group.
	 *
	 * If the cache key does not exist in the group, then nothing will happen.
	 *
	 * @since 2.0.0
	 *
	 * @param int|string $key        What the contents in the cache are called.
	 * @param string     $group      Optional. Where the cache contents are grouped. Default 'default'.
	 * @param bool       $deprecated Optional. Unused. Default false.
	 * @return bool False if the contents weren't deleted and true on success.
	 */
	public function delete( $key, $group = 'default', $deprecated = false ) {
		if ( empty( $group ) ) {
			$group = 'default';
		}

		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$key = $this->blog_prefix . $key;
		}

		if ( ! $this->_exists( $key, $group ) ) {
			return false;
		}

		unset( $this->cache[ $group ][ $key ] );
		return true;
	}

	/**
	 * Clears the object cache of all data.
	 *
	 * @since 2.0.0
	 *
	 * @return true Always returns true.
	 */
	public function flush() {
		$this->cache = array();

		return true;
	}

	/**
	 * Retrieves the cache contents, if it exists.
	 *
	 * The contents will be first attempted to be retrieved by searching by the
	 * key in the cache group. If the cache is hit (success) then the contents
	 * are returned.
	 *
	 * On failure, the number of cache misses will be incremented.
	 *
	 * @since 2.0.0
	 *
	 * @param int|string $key   The key under which the cache contents are stored.
	 * @param string     $group Optional. Where the cache contents are grouped. Default 'default'.
	 * @param bool       $force Optional. Unused. Whether to force an update of the local cache
	 *                          from the persistent cache. Default false.
	 * @param bool       $found Optional. Whether the key was found in the cache (passed by reference).
	 *                          Disambiguates a return of false, a storable value. Default null.
	 * @return mixed|false The cache contents on success, false on failure to retrieve contents.
	 */
	public function get( $key, $group = 'default', $force = false, &$found = null ) {
		if ( empty( $group ) ) {
			$group = 'default';
		}

		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$key = $this->blog_prefix . $key;
		}

		if ( $this->_exists( $key, $group ) ) {
			$found             = true;
			$this->cache_hits += 1;
			if ( is_object( $this->cache[ $group ][ $key ] ) ) {
				return clone $this->cache[ $group ][ $key ];
			} else {
				return $this->cache[ $group ][ $key ];
			}
		}

		$found               = false;
		$this->cache_misses += 1;
		return false;
	}

	/**
	 * Retrieves multiple values from the cache in one call.
	 *
	 * @since 5.5.0
	 *
	 * @param array  $keys  Array of keys under which the cache contents are stored.
	 * @param string $group Optional. Where the cache contents are grouped. Default 'default'.
	 * @param bool   $force Optional. Whether to force an update of the local cache
	 *                      from the persistent cache. Default false.
	 * @return array Array of values organized into groups.
	 */
	public function get_multiple( $keys, $group = 'default', $force = false ) {
		$values = array();

		foreach ( $keys as $key ) {
			$values[ $key ] = $this->get( $key, $group, $force );
		}

		return $values;
	}

	/**
	 * Increments numeric cache item's value.
	 *
	 * @since 3.3.0
	 *
	 * @param int|string $key    The cache key to increment
	 * @param int        $offset Optional. The amount by which to increment the item's value. Default 1.
	 * @param string     $group  Optional. The group the key is in. Default 'default'.
	 * @return int|false The item's new value on success, false on failure.
	 */
	public function incr( $key, $offset = 1, $group = 'default' ) {
		if ( empty( $group ) ) {
			$group = 'default';
		}

		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$key = $this->blog_prefix . $key;
		}

		if ( ! $this->_exists( $key, $group ) ) {
			return false;
		}

		if ( ! is_numeric( $this->cache[ $group ][ $key ] ) ) {
			$this->cache[ $group ][ $key ] = 0;
		}

		$offset = (int) $offset;

		$this->cache[ $group ][ $key ] += $offset;

		if ( $this->cache[ $group ][ $key ] < 0 ) {
			$this->cache[ $group ][ $key ] = 0;
		}

		return $this->cache[ $group ][ $key ];
	}

	/**
	 * Replaces the contents in the cache, if contents already exist.
	 *
	 * @since 2.0.0
	 *
	 * @see WP_Object_Cache::set()
	 *
	 * @param int|string $key    What to call the contents in the cache.
	 * @param mixed      $data   The contents to store in the cache.
	 * @param string     $group  Optional. Where to group the cache contents. Default 'default'.
	 * @param int        $expire Optional. When to expire the cache contents. Default 0 (no expiration).
	 * @return bool False if not exists, true if contents were replaced.
	 */
	public function replace( $key, $data, $group = 'default', $expire = 0 ) {
		if ( empty( $group ) ) {
			$group = 'default';
		}

		$id = $key;
		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$id = $this->blog_prefix . $key;
		}

		if ( ! $this->_exists( $id, $group ) ) {
			return false;
		}

		return $this->set( $key, $data, $group, (int) $expire );
	}

	/**
	 * Resets cache keys.
	 *
	 * @since 3.0.0
	 *
	 * @deprecated 3.5.0 Use switch_to_blog()
	 * @see switch_to_blog()
	 */
	public function reset() {
		_deprecated_function( __FUNCTION__, '3.5.0', 'switch_to_blog()' );

		// Clear out non-global caches since the blog ID has changed.
		foreach ( array_keys( $this->cache ) as $group ) {
			if ( ! isset( $this->global_groups[ $group ] ) ) {
				unset( $this->cache[ $group ] );
			}
		}
	}

	/**
	 * Sets the data contents into the cache.
	 *
	 * The cache contents are grouped by the $group parameter followed by the
	 * $key. This allows for duplicate IDs in unique groups. Therefore, naming of
	 * the group should be used with care and should follow normal function
	 * naming guidelines outside of core WordPress usage.
	 *
	 * The $expire parameter is not used, because the cache will automatically
	 * expire for each time a page is accessed and PHP finishes. The method is
	 * more for cache plugins which use files.
	 *
	 * @since 2.0.0
	 *
	 * @param int|string $key    What to call the contents in the cache.
	 * @param mixed      $data   The contents to store in the cache.
	 * @param string     $group  Optional. Where to group the cache contents. Default 'default'.
	 * @param int        $expire Not Used.
	 * @return true Always returns true.
	 */
	public function set( $key, $data, $group = 'default', $expire = 0 ) {
		if ( empty( $group ) ) {
			$group = 'default';
		}

		if ( $this->multisite && ! isset( $this->global_groups[ $group ] ) ) {
			$key = $this->blog_prefix . $key;
		}

		if ( is_object( $data ) ) {
			$data = clone $data;
		}

		$this->cache[ $group ][ $key ] = $data;
		return true;
	}

	/**
	 * Echoes the stats of the caching.
	 *
	 * Gives the cache hits, and cache misses. Also prints every cached group,
	 * key and the data.
	 *
	 * @since 2.0.0
	 */
	public function stats() {
		echo '<p>';
		echo "<strong>Cache Hits:</strong> {$this->cache_hits}<br />";
		echo "<strong>Cache Misses:</strong> {$this->cache_misses}<br />";
		echo '</p>';
		echo '<ul>';
		foreach ( $this->cache as $group => $cache ) {
			echo '<li><strong>Group:</strong> ' . esc_html( $group ) . ' - ( ' . number_format( strlen( serialize( $cache ) ) / KB_IN_BYTES, 2 ) . 'k )</li>';
		}
		echo '</ul>';
	}

	/**
	 * Switches the internal blog ID.
	 *
	 * This changes the blog ID used to create keys in blog specific groups.
	 *
	 * @since 3.5.0
	 *
	 * @param int $blog_id Blog ID.
	 */
	public function switch_to_blog( $blog_id ) {
		$blog_id           = (int) $blog_id;
		$this->blog_prefix = $this->multisite ? $blog_id . ':' : '';
	}

	/**
	 * Serves as a utility function to determine whether a key exists in the cache.
	 *
	 * @since 3.4.0
	 *
	 * @param int|string $key   Cache key to check for existence.
	 * @param string     $group Cache group for the key existence check.
	 * @return bool Whether the key exists in the cache for the given group.
	 */
	protected function _exists( $key, $group ) {
		return isset( $this->cache[ $group ] ) && ( isset( $this->cache[ $group ][ $key ] ) || array_key_exists( $key, $this->cache[ $group ] ) );
	}
}

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Methods Methods

  • __construct — Sets up object properties; PHP 5 style constructor.
  • __destruct — Saves the object cache before object is completely destroyed.
  • __get — Makes private properties readable for backward compatibility.
  • __isset — Makes private properties checkable for backward compatibility.
  • __set — Makes private properties settable for backward compatibility.
  • __unset — Makes private properties un-settable for backward compatibility.
  • _exists — Serves as a utility function to determine whether a key exists in the cache.
  • add — Adds data to the cache if it doesn't already exist.
  • add_global_groups — Sets the list of global cache groups.
  • decr — Decrements numeric cache item's value.
  • delete — Removes the contents of the cache key in the group.
  • flush — Clears the object cache of all data.
  • get — Retrieves the cache contents, if it exists.
  • get_multiple — Retrieves multiple values from the cache in one call.
  • incr — Increments numeric cache item's value.
  • replace — Replaces the contents in the cache, if contents already exist.
  • reset — Resets cache keys. — deprecated
  • set — Sets the data contents into the cache.
  • stats — Echoes the stats of the caching.
  • switch_to_blog — Switches the internal blog ID.

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Changelog Changelog

Changelog
Version Description
2.0.0 Introduced.

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User Contributed Notes User Contributed Notes

  1. Skip to note 1 content
    Contributed by Priyanka Behera

    The object cache is used for caching the results of expensive SQL queries. So they’re not performed multiple times within a page load. In the below example, imagine the $query variable is an expensive SQL query.

    $result = wp_cache_get( 'my_result' );
    if ( false === $result ) {
    	$result = $wpdb->get_results( $query );
    	wp_cache_set( 'my_result', $result );
    } 
    // Do something with $result;
    

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