Lucene is most popular and java based searchengine library. It offers near real time search. Its features include Ranked search, many powerful query types: phrase queries, wildcard queries, proximity queries, range queries and more, fielded searching (e.g., title, author, contents), date-range searching, sorting by any field , multiple-index searching with merged results , allows simultaneous update and searching.
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Solr is the popular, blazing fast open source enterprise search platform from the Apache Lucene project. Its major features include powerful full-text search, hit highlighting, faceted search, dynamic clustering, database integration, and rich document (e.g., Word, PDF) handling. Solr is highly scalable, providing distributed search and index replication, and it powers the search and navigation features of many of the world's largest internet sites.
CLucene is a port of the very popular Java Lucene text search engine API. CLucene aims to be a good alternative to Java Lucene when performance really matters or if you want to stick to good old C++. CLucene is faster than Lucene as it is written in C++, meaning it is being compiled into machine code, has no background GC operations, and requires no any extra setup procedures.
Open Search Server is both a modern crawler and search engine and a suite of high-powered full text search algorithms. Built using the best open source technologies like lucene, zkoss, tomcat, poi, tagsoup. Open Search Server is a stable, high-performance piece of software.
MG4J (Managing Gigabytes for Java) is a free full-text search engine for large document collections written in Java. MG4J is a highly customisable, high-performance, full-fledged search engine providing state-of-the-art features (such as BM25/BM25F scoring) and new research algorithms. The main points of MG4J are Powerful indexing, Multi-index interval semantics, Virtual fields, Clustering and lot more.
The Lemur Project develops search engines, browser toolbars, text analysis tools, and data resources that support research and development of information retrieval and text mining software. The project is best known for its Indri search engine, Lemur Toolbar, and ClueWeb09 dataset.
Xapian is an Open Source Search Engine Library. It is written in C++, with bindings to allow use from Perl, Python, PHP, Java, Tcl, C# and Ruby. Xapian is a highly adaptable toolkit which allows developers to easily add advanced indexing and search facilities to their own applications. It supports the Probabilistic Information Retrieval model and also supports a rich set of boolean query operators.
Katta is a scalable, failure tolerant, distributed, data storage for real time access. Katta serves large, replicated, indices as shards to serve high loads and very large data sets. These indices can be of different type. Currently implementations are available for Lucene and Hadoop mapfiles.
Carrot2 is an Open Source Search Results Clustering Engine. It could cluster the search results from various sources and generates small collection of documents. Carrot2 offers ready-to-use components for fetching search results from various sources including YahooAPI, GoogleAPI, Bing API, eTools Meta Search, Lucene, SOLR, Google Desktop and more.
IndexTank search engine powers search in Reddit, Social bookmarking site. IndexTank is acquired by LinkedIn and released the project as open source. It includes features like Variables boosts, Facets, Faceted search, Snippeting, Custom scoring functions, Suggest, and Autocomplete.