we build a web site or an application and want to add search to it, and then it hits us: getting search working is hard. We want our search solution to be fast, we want a painless setup and a completely free search schema, we want to be able to index data simply using JSON over HTTP, we want our search server to be always available, we want to be able to start with one machine and scale to hundreds, we want real-time search, we want simple multi-tenancy, and we want a solution that is built for the cloud.
"Carrot2 is an Open Source Search Results Clustering Engine. It can automatically organize small collections of documents (search results but not only) into thematic categories."