Both ElasticSearch and Solr are services, and both are built from Apache Lucene, a set of Java libraries providing indexed search, with facets and many other fe

Both ElasticSearch and Solr are services, and both are built from Apache Lucene, a set of Java libraries providing indexed search, with facets and many other features. Xapian is a C++ library with similar functionality, although it doesn't seem to have the same level of popularity as Lucene.