Elasticsearch is a distributed, RESTful search and analytics engine capable of solving a growing number of use cases. As the heart of the Elastic Stack, it centrally stores your data so you can discover the expected and uncover the unexpected. 
 
 Its feature include: 
 <ul>
	<li>Distributed and Highly Available Search Engine.
	<ul>
		<li>Each index is fully sharded with a configurable number of shards.</li>
		<li>Each shard can have one or more replicas.</li>
		<li>Read / Search operations performed on either one of the replica shard.</li>
	</ul></li>
	<li>Multi Tenant with Multi Types.
	<ul>
		<li>Support for more than one index.</li>
		<li>Support for more than one type per index.</li>
		<li>Index level configuration (number of shards, index storage, &#8230;).</li>
	</ul></li>
	<li>Various set of APIs
	<ul>
		<li>HTTP RESTful API</li>
		<li>Native Java API.</li>
		<li>All APIs perform automatic node operation rerouting.</li>
	</ul></li>
	<li>Document oriented
	<ul>
		<li>No need for upfront schema definition.</li>
		<li>Schema can be defined per type for customization of the indexing process.</li>
	</ul></li>
	<li>Reliable, Asynchronous Write Behind for long term persistency.</li>
	<li>(Near) Real Time Search.</li>
	<li>Built on top of Lucene
	<ul>
		<li>Each shard is a fully functional Lucene index</li>
		<li>All the power of Lucene easily exposed through simple configuration / plugins.</li>
	</ul></li>
	<li>Per operation consistency
	<ul>
		<li>Single document level operations are atomic, consistent, isolated and durable.</li>
	</ul></li>
</ul>

Elasticsearch is a distributed, RESTful search and analytics engine capable of solving a growing number of use cases. As the heart of the Elastic Stack, it centrally stores your data so you can discover the expected and uncover the unexpected. 

ElasticSearch - Distributed, RESTful search and analytics engine

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. Solr is written in Java and runs as a standalone full-text search server within a servlet container such as Tomcat. Solr uses the Lucene Java search library at its core for full-text indexing and search, and has REST-like HTTP/XML and JSON APIs that make it easy to use from virtually any programming language. Solr's powerful external configuration allows it to be tailored to almost any type of application without Java coding, and it has an extensive plugin architecture when more advanced customization is required. Its feature set include Rich Document Parsing and Indexing (PDF, Word, HTML, etc) using Apache Tika, An Administration Interface, Monitorable Logging, Fast Incremental Updates and Index Replication, Highly Scalable Distributed search with sharded index across multiple hosts, HTTP interface with configurable response formats (XML/XSLT, JSON, Python, Ruby, PHP, Velocity) Solrj is embedded solr. It provides Java based API and it takes care of constructing, parsing, sending and receiving HTTP request.

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. 

Solr - Blazing-fast, open source enterprise search platform

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. 
 Homepage: <A HREF="http://indextank.com/" target="_blank">http://indextank.com/</A>

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.

IndexTank - Search Engine powers Reddit

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. 

Apache Lucene is a high-performance, full-featured text search engine library written entirely in Java. It is a technology suitable for nearly any application that requires full-text search, especially cross-platform.

Lucene - A high-performance, full-featured text search engine library

OpenSearch is a community-driven, open source search and analytics suite derived from Apache 2.0 licensed Elasticsearch 7.10.2 & Kibana 7.10.2. It consists of a search engine daemon, OpenSearch, and a visualization and user interface, OpenSearch Dashboards. OpenSearch enables people to easily ingest, secure, search, aggregate, view, and analyze data. These capabilities are popular for use cases such as application search, log analytics, and more.
 
Its features include: 
<ul><li>Log analytics</li><li> Real-time application monitoring</li><li> Clickstream analytics</li><li>Use SQL or a piped processing language to query your data</li><li>Automate index operations</li><li>Monitor and optimize your cluster</li><li>Run search requests in the background</li><li>KNN- Find “nearest neighbors” in your vector data</li><li>Authentication and access control for your cluster</li><li>Anomaly Detection</li></ul>

OpenSearch is a community-driven, open source search and analytics suite derived from Apache 2.0 licensed Elasticsearch 7.10.2 & Kibana 7.10.2. It consists of a search engine daemon, OpenSearch, and a visualization and user interface, OpenSearch Dashboards. OpenSearch enables people to easily ingest, secure, search, aggregate, view, and analyze data. These capabilities are popular for use cases such as application search, log analytics, and more.

OpenSearch - Open source distributed and RESTful search engine

Constellio Open Source Enterprise Search is based on Apache Solr and using Google Search Appliances connectors architecture, it allows, with a single click, to find all relevant content in your organization (Web, email, ECM, CRM etc.). 

 It supports Federated search, Using the single interface data in the organization could be discovered or searched. It provides multi-lingual search. Support for more than 15 languages are available. It has support of automatic document classification, adding tags or keywords to the documents. Additional to that this product has all kind of feature a Enterprise Search engine should have. It supports reporting on indexing and search, Sort results, Synonyms search, Thesaurus support, Auto complete, Facet etc. Constellio can index SharePoint sites, Documentum, FileNet, LiveLink. 

 Google's Enterprise Connector Manager provides support to retrieve data from different sources like database, file system, CRM, Email etc. Data from database could be retrieved using <A HREF="http://code.google.com/p/google-enterprise-connector-database/" target="_blank">Database connector</A>. This product most efficiently using this and retrieves the data from different sources. It uses Apache Solr to index the content. 

 Constellio's secure web interface will allow you to search your email and access attached files. It also provides API support in Ruby, PHP, Java, Python, JSon, C#, ColdFusion. Google's Enterprise Connector used in Google Search Appliance is constantly adding more connectors to retrieve data from different sources. Since Constellio uses this more Enterprise search support will be provided.

Constellio Open Source Enterprise Search is based on Apache Solr and using Google Search Appliances connectors architecture, it allows, with a single click, to find all relevant content in your organization (Web, email, ECM, CRM etc.).

Constellio - Enterprise Search engine

Crate is an open source, highly scalable, shared-nothing distributed SQL database. Crate offers the scalability and performance of a modern No-SQL database with the power of Standard SQL. Crate’s distributed SQL query engine lets you use the same syntax that already exists in your applications or integrations, and have queries seamlessly executed across the crate cluster, including any aggregations, if needed.

Crate - The fast, scalable, easy to use SQL database with native full text search

H2 database is very fast, open source database engine. 
 It supports : 
 <UL>
	<LI>It supports SQL and JDBC standards</LI>
	<LI>Encrypted database</LI> 
	<LI>Security and Authentication</LI>
	<LI>Multiple index types (b-tree, tree, hash)</LI> 
	<LI>Contains a recovery tool that can dump the contents of the database</LI> 
	<LI>Clustering</LI>
	<LI>Full text search</LI>
	<LI>Temporary Tables</LI> 
	<LI>User-Defined Datatypes</LI>
 </UL>

H2 database is very fast, open source database engine. It supports SQL and JDBC standards. It can run in Embedded and Server mode and it has clustering support.  


H2 Database - Java based Database Engine

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. Release 5.0 has several source and binary incompatibilities, and introduces quasi-succinct indices.

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. 

MG4J - Managing Gigabytes for Java

Terrier is a highly flexible, efficient, and effective open source search engine, readily deployable on large-scale collections of documents. Terrier implements state-of-the-art indexing and retrieval functionalities, and provides an ideal platform for the rapid development and evaluation of large-scale retrieval applications. Terrier can index large corpora of documents, and provides multiple indexing strategies, such as multi-pass, single-pass and large-scale MapReduce indexing.

Terrier - Information Retrieval Platform

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. 

Lemur - Search Engine

Norconex HTTP Collector is a full-featured web crawler (or spider) that can manipulate and store collected data into a repositoriy of your choice (e.g. a search engine). It very flexible, powerful, easy to extend, and portable.

Norconex HTTP Collector - Enterprise Web Crawler

Nutch is open source web-search software. It builds on Lucene Java, adding web-specifics, such as a crawler, a link-graph database, parsers for HTML and other document formats, etc. Its main feature include
 <UL>
	<LI>Fetching, parsing and indexation in parallel and distributed</LI>
	<LI>Plugin support</LI>
	<LI>Ontology</LI>
	<LI>Clustering</LI>
	<LI>Distributed filesystem (via Hadoop)</LI>
	<LI>Link-graph database</LI>
	<LI>NTLM authentication</LI>
	<LI>MapReduce</LI>
	<LI>Many formats: plain text, HTML, XML, ZIP, OpenDocument (OpenOffice.org), Microsoft Office (Word, Excel, Powerpoint), PDF, JavaScript, RSS, RTF, MP3 (ID3 tags)</LI>
 </UL>

Nutch is open source web-search software. It builds on Lucene Java, adding web-specifics, such as a crawler, a link-graph database, parsers for HTML and other document formats, etc.

Nutch - Highly extensible, highly scalable Web crawler

Puggle is an open-source desktop search engine written exclusively in Java. It provides full text and metadata search over files, folders, music, photos, web pages and more that are stored locally on your computer. Moreover, a portable edition of Puggle is available that you can carry around with you on a portable device, like a USB flash drive or an external hard drive. Puggle will index all the data contained in and allow you to search over them on any computer.

Puggle

Sensei is a distributed data system that was built to support many product initiatives at LinkedIn, including the real-time faceted search in <A HREF="http://www.linkedin.com/signal/" target="_blank">LinkedIn Signal</A> and the news feed and tabs on the Homepage. Sensei is both a search engine and a database. It is designed to query and navigate through documents that consist of unstructured text and well-formed and structured metadata. 
 It supports Fast realtime updates, Complex query language and REST/JSON api, Hadoop integration and lot more.

Sensei is a distributed data system that was built to support many product initiatives at LinkedIn, including the real-time faceted search in LinkedIn Signal and the news feed and tabs on the Homepage. Sensei is both a search engine and a database. It is designed to query and navigate through documents that consist of unstructured text and  well-formed and structured metadata. Sensei is both a search engine and a database.

SenseiDB - Distributed, Realtime, Semi-Structured Database from LinkedIn

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