Apache Solr

Apache Solr is a standalone enterprise search server with a REST-like API. Solr has a large developer community, extensive plugins, and is one of the most widely adopted open source search products with support from Drupal hosting platforms, Acquia and Pantheon.

Search is a key component of a rich content and user experience. Search platforms allow for building rich faceted search experiences that allow users to drill down and find the content they seek. Apache Solr integrates at a deeper level than other search servers, working hand-in-hand with Drupal's content access controls to allow site editors and administrators to search for content that isn't published or publicly available. Examples of Solr-enabled faceted searches include Netflix, Zappos, and The Guardian.