Apache Solr is open source software which can be used as a full-text Enterprise search platform.
It was derived from the Apache Lucene, a Java library that provides high performance full-text search engine that is written in Java programming language.
Apache Solr supports Full-text search engine capabilities, Indexing, Rich document processing, and clustering, faceted search. Solr can run as standalone full-text server and cloud server.
According to Apache, Lucene Search library is in the top 15 open source software projects and in top 5 from Apache projects which installed by 4000 companies.
Many applications are using the Apache Solr for Auto suggestions, spatial search, full-test log search, Range or category browsing using facets. Also enterprises which are using the FAST or Google Search Appliance (GSA) as a commercial search engine showing the interest in Apache Solr or Lucene implementations as this is open source and free to access the source for modifications as per the requirements.