Sphinx is an open source full-text search server. Working with Sphinx is pretty much as with a database server . If you consider how to upgrade search engine in your application it can be an easiest way and number of relevance functions ensures you can tweak search quality as well.
This article is not about how bad MongoDB is, you can google it if you search for arguments. I heard the MongoDB had issues with failover, but I cannot confirm it from my experience. I heard bad opinions about this database engine, but it has good and bad sides. In my opinion, it is a good tool for coders.
What is Redis and what is Memcached? Why use Redis and when Memcached? Way we can call Redis a NoSQL database? Today I want to present my conclusions about which cache engine to use? Redis versus Memcached.