Object cache

Each WordPress environment is provisioned with its own siloed Memcached cluster for persistently caching application-level data. The WordPress object cache (and transients) is automatically configured to use these instances. This enables common operations to be automatically routed to memory instead of the database.

This also means that any application data can be easily cached in memory to reduce the cost of repetitive or expensive computations. The best candidates for caching are any data that will take longer to calculate (such as expensive database queries) than it takes to retrieve the data from the cache. Keep in mind that each object cache request is processed over a local network, so many cache lookups/sets can add processing time to the page.

• Cache entries must be kept under 1MB in size.
• Insights into the hit rate for an environment’s object cache, as well as for object cache queries by type, can be reviewed in the Insights & Metrics panel of the VIP Dashboard. 
• Statistics for a site’s object cache can be reviewed in Query Monitor, including the amount of time the object cache took to retrieve all requested data and the total size of all objects stored in memory.

Transients

Because transients are stored in the object cache on VIP, any plugins or custom code that query the database for transients will fail. Similarly, the wp transient list WP-CLI command will not work as expected because it performs a database query. The WP-CLI command will return a warning.

As an alternative, individual transients can be managed with the wp transient WP-CLI command if the key for the transient is known.

wp_cache functions

Memcached employs a Least Recently Used (LRU) algorithm. The LRU determines which object(s)—if any—should be evicted from the object cache when a new object is being added and the object cache is full.

WordPress Core wp_cache_* functions can be used to add, set, or replace objects in the cache. When using these functions, a specific TTL for a cached objects can be set by passing an integer value (that represents the number of seconds) to the $expire parameter. The default value of $expire is 0, which is no expiration. When an object has no expiration, it will persist indefinitely until evicted by the LRU algorithm, or if it is specifically deleted or updated by a wp_cache function.

For more information about object caching in WordPress, review WP Cache Functions in WordPress.org’s Developer Reference.