What is the WordPress object cache and how does a persistent object cache (Redis/Memcached) differ from the default in-memory cache?

Question

WordPress Fundamental — Hard

What is the WordPress object cache and how does a persistent object cache (Redis/Memcached) differ from the default in-memory cache?

Accepted Answer

The default WordPress object cache stores data in PHP arrays for the current request only, while a persistent cache like Redis/Memcached stores data externally, shared across requests and servers to reduce database queries.