I’ve started to work with ElasticSearch for a while now. I gotta say it’s a powerful open source for building distributed real-time search engines and analytics engines.
It also uses Shards and Replica in distributed machines to make your architecture reliable and scalable. It’s not just a simple full-text search engine even though it does that perfectly.
What I do with ElasticSearch is that it’s connected to my MongoDB replica set and it indexes tweets as they are streamed by Twitter API. I track tweets for few projects (health, news, UN GlobalPulse, etc.) and try to index some of my projects in ES on the fly.