WordPress has alot of filters that allow you to take over/modify just about anything including internal functions. We overrode get_posts() to use ElasticSearch

WordPress has alot of filters that allow you to take over/modify just about anything including internal functions. We overrode get_posts() to use ElasticSearch instead of MySQL and added lots of caching to alot of things with Memcache. I built a plugin which cached all of the HTML output of every WordPress page and all of the widgets in Memcache. On top of all the server side caching, there was Akamai in front caching every page at the network level.