One of the Northscale co-founders gave a very interesting interview about making databases scale horizontally, like Google does, so the database becomes elastic.
With common databases like MySQL handling more traffic meant scaling vertically, building a bigger more powerful server, but thanks to Memcached cheap servers, like Cloud servers, can be used in front of the database to grow horizontally for a lot less money. Memcached also allows MySQL, and similar databases, to handle far larger traffic than they were designed to handle.
One concept in this video is the idea of storing some less important data that doesn’t have to be saved per say in the database, but needs to be stored in something like the database, can be stored in the less costly Memcached memory, which is RAM. In WordPress, temporary data can be stored in the Object Cache, and if it is temporary then it is best to use the Transient API. If Memcached is doing the Object Caching for WordPress, then data from the Transient API gets stored in Memcached.
See the video below.