WordPress Object Cache system allows transients to be stored in object cache backend, such as APC, Memcached or Redis. Without an object cache, the transients are stored in the database itself. This is the default. You could verify this yourself by looking at the (wp_)options table in the database. Fortunately, we have options to store these transients in the memory using one of the backends mentioned above. Among these, Redis has become stable and is production-ready. There are two WP plugins available that use Redis as WP Object Cache. First one is Eric Mann’s plugin. The second plugin is by Till Krüss who forked the former and made it better.