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

Vespa is an engine for low-latency computation over large data sets. It stores and indexes your data such that queries, selection and processing over the data can be performed at serving time. Vespa is serving platform for Yahoo.com, Yahoo News, Yahoo Sports, Yahoo Finance, Yahoo Gemini, Flickr.
 Queries can use both structured filters and unstructured text search to select data. All the matching data is then ranked according to a ranking function - typically machine learned - to implement such use cases as search relevance, recommendation, targeting and personalization.
 
Vespa is scalable. System sizes up to hundreds of nodes handling tens of billions of documents are not uncommon, and no harder to set up and modify than single node systems. Since all system components, as well as stored data is redundant and self-correcting, hardware failures are not operational emergencies and can be handled by re-adding capacity when convenient.
Its feature include:
<ul>
<li>Text search - Combine structured query and text search to select data</li>
<li>Advanced Ranking - Machine learned ranking</li>
<li>Aggregation</li>
<li>Elastic, Scalable</li>
<li>High Availability</li>
<li>Auto repair data corruption</li>
<li>Simple HTTP API interface&nbsp;</li>
<li>lot more...</li>
</ul>

Vespa is an engine for low-latency computation over large data sets. It stores and indexes your data such that queries, selection and processing over the data can be performed at serving time. Vespa is serving platform for Yahoo.com, Yahoo News, Yahoo Sports, Yahoo Finance, Yahoo Gemini, Flickr.

Vespa - Yahoo's big data serving engine

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

Lunr.js is a small, full-text search library for use in the browser. It indexes JSON documents and provides a simple search interface for retrieving documents that best match text queries. A bit like Solr, but much smaller and not as bright. Lunr enables you to provide a great search experience without the need for external, server-side, search services. Lunr has no external dependencies and works in your browser or on the server with node.js.
 
For web applications with all their data already sitting in the client, it makes sense to be able to search that data on the client too. A local search index will be quicker, there is no network overhead, and will remain available and usable even without a network connection.

Lunr.js is a small, full-text search library for use in the browser. It indexes JSON documents and provides a simple search interface for retrieving documents that best match text queries. A bit like Solr, but much smaller and not as bright. Lunr enables you to provide a great search experience without the need for external, server-side, search services. Lunr has no external dependencies and works in your browser or on the server with node.js.

LUNR.js - A bit like Solr, but much smaller and not as bright

Summa is a fast modular and scalable search engine written in Java. Summa is characterized by: 
 <ul><li>Integrated search. Summa can simultaneously access a number of different data and data sources and expose it in a unified interface.</li><li>Modular design. The Summa search system consists of a set of independent modules, which makes it simple and easy to maintain and upgrade.</li><li>Scalable. Summa supports a distributed architecture and can be scaled up or down to handle any amount of data.</li><li>Open standards. Summa is based upon modern web technologies and standards, and don’t include any proprietary codes or elements.</li><li>Failure tolerant. If a single source of data or service should fail, Summa will continue without that specific source.</li></ul>

Summa is a fast modular and scalable search engine written in Java. Sports a flexible workflow system and incremental indexing and facetting. Summa is primarily developed by the State and University Library of Denmark.

Summa

An open source .NET web crawler written in C# using SQL 2005/2008. Arachnode.net is a complete and comprehensive .NET web crawler for downloading, indexing and storing Internet content including e-mail addresses, files, hyperlinks, images, and Web pages. Its features include
 <UL>
	<LI>.NET architecture</LI>
	<LI>Configurable Rules and Actions</LI>
	<LI>Lucene.NET Integration</LI>
	<LI>SQL Server 2008 and full-text indexing</LI>
	<LI>.DOC/.PDF/.PPT/.XLS Indexing</LI>
	<LI>HTML to XML and XHTML</LI>
	<LI>Multi-threading and Throttling</LI>
	<LI>Respectful Crawling</LI>
	<LI>Analysis Services</LI>
	<LI>SQL Server 2008 and SSIS</LI>
	<LI>EXIF data extraction</LI>
 </UL>

An open source .NET web crawler written in C# using SQL 2005/2008. Arachnode.net is a complete and comprehensive .NET web crawler for downloading, indexing and storing Internet content including e-mail addresses, files, hyperlinks, images, and Web pages.

Arachnode.net

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.
 <UL>
	<LI>Multi-languages indexing</LI>
	<LI>The crawlers go through web sites and file systems to rapidly and easily build your index.</LI>
	<LI>Numerous document formats are supported, such as XML, HTML/XHTML, Adobe™ PDF, Microsoft™ Word™, PowerPoint™, OpenOffice™, etc</LI>
	<LI>Quick integration thanks to an XML interface via HTTP queries (XML over HTTP) and PHP classes</LI>
	<LI>The web interface is built around the power offered by the Zkoss framework. It runs with the main Ajax browsers. This RIA-type interface is as comfortable to use as that of a heavy client</LI>
 </UL>

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.

Open Search Server

Grub Next Generation is distributed web crawling system (clients/servers) which helps to build and maintain index of the Web. It is client-server architecture where client crawls the web and updates the server. The peer-to-peer grubclient software crawls during computer idle time.

Grub

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.

CLucene - Lucene C Port

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

Whoosh is a fast, featureful full-text indexing and searching library implemented in pure Python. Programmers can use it to easily add search functionality to their applications and websites. It has support of Fielded indexing, search, scoring, text analysis, storage, Pluggable scoring algorithm, Powerful query language and spell-checker.

Whoosh - Python Search Library

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

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