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

Qdrant ( quadrant ) is a vector similarity search engine. It provides a production-ready service with a convenient API to store, search, and manage points - vectors with an additional payload. Qdrant is tailored to extended filtering support. It makes it useful for all sorts of neural-network or semantic-based matching, faceted search, and other applications. With Qdrant, embeddings or neural network encoders can be turned into full-fledged applications for matching, searching, recommending, and much more.
 
The neural search uses semantic embeddings instead of keywords and works best with short texts. With Qdrant and a pre-trained neural network, you can build and deploy semantic neural search on your data in minutes.

Qdrant ( quadrant ) is a vector similarity search engine. It provides a production-ready service with a convenient API to store, search, and manage points - vectors with an additional payload. Qdrant is tailored to extended filtering support. It makes it useful for all sorts of neural-network or semantic-based matching, faceted search, and other applications. With Qdrant, embeddings or neural network encoders can be turned into full-fledged applications for matching, searching, recommending, and much more.

Qdrant - Neural Search Engine, Vector Similarity Search Engine with extended filtering support

Usergrid is an open-source Backend-as-a-Service (“BaaS” or “mBaaS”) composed of an integrated distributed NoSQL database, application layer and client tier with SDKs for developers looking to rapidly build web and/or mobile applications. It provides elementary services (user registration & management, data storage, file storage, queues) and retrieval features (full text search, geolocation search, joins) to power common app features.
 
It is a multi-tenant system designed for deployment to public cloud environments (such as Amazon Web Services, Rackspace, etc.) or to run on traditional server infrastructures so that anyone can run their own private BaaS deployment. For architects and back-end teams, it aims to provide a distributed, easily extendable, operationally predictable and highly scalable solution. For front-end developers, it aims to simplify the development process by enabling them to rapidly build and operate mobile and web applications without requiring backend expertise.

Usergrid is an open-source Backend-as-a-Service (“BaaS” or “mBaaS”) composed of an integrated distributed NoSQL database, application layer and client tier with SDKs for developers looking to rapidly build web and/or mobile applications. It provides elementary services (user registration & management, data storage, file storage, queues) and retrieval features (full text search, geolocation search, joins) to power common app features.

Usergrid - The BaaS Framework you run

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

Haystack is an end-to-end framework that enables you to build powerful and production-ready pipelines for different search use cases. Whether you want to perform Question Answering or semantic document search, you can use the State-of-the-Art NLP models in Haystack to provide unique search experiences and allow your users to query in natural language. Haystack is built in a modular fashion so that you can combine the best technology from other open-source projects like Huggingface's Transformers, Elasticsearch, or Milvus.
 
Haystack can perform semantic search and retrieve documents according to meaning, not keywords. It can ask questions in natural language and find granular answers in your documents. It can automate processes by automatically applying a list of questions to new documents and using the extracted answers.

Haystack is an end-to-end framework that enables you to build powerful and production-ready pipelines for different search use cases. Whether you want to perform Question Answering or semantic document search, you can use the State-of-the-Art NLP models in Haystack to provide unique search experiences and allow your users to query in natural language. Haystack is built in a modular fashion so that you can combine the best technology from other open-source projects like Huggingface's Transformers, Elasticsearch, or Milvus.

Haystack - Build a natural language interface for your data

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

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

Gigablast is one of the remaining four search engines in the United States that maintains its own searchable index of over a billion pages. It is scalable to thousands of servers. Has scaled to over 12 billion web pages on over 200 servers. It supports Distributed web crawler, Document conversion, Automated data corruption detection and repair,  Can cluster results from same site, Synonym search, Spell checker and lot more.  

Gigablast - Web and Enterprise search engine in C++

ucene.Net is a port of the Lucene search engine library, written in C# and targeted at .NET runtime users. The Lucene search library is based on an inverted index.

Lucene.Net - Lucene port in CSharp

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

BaasBox is an Open Source project that aims to provide a backend for mobile and web apps. It helps to manage the content, A document belongs to a Collection. It can create, read, update and delete a document, provided user have access to it. Additional to that it has support for File management, Backup and Restore, API access control, User and Profile management and lot more.

BaasBox - Backend for Mobile and Web Application

Open Distro for Elasticsearch is an Apache 2.0-licensed distribution of Elasticsearch enhanced with Enterprise Security, Alerting, SQL, Index Management, k-Nearest Neighbor Search, Performance Analyzer and more. 
 
It provides set of features to help you keep your data secure and stay compliant with regulations such as GDPR, HIPAA, PCI, and ISO. Whether you want to encrypt data in-transit, authenticate users against Active Directory, use Kerberos or JSON web tokens for single sign-on (SSO), or monitor and log any malicious access attempts, Open Distro for Elasticsearch has you covered.
 
It provides support to extract insights out of Elasticsearch using the familiar SQL query syntax. Use aggregations, group by, and where clauses to investigate your data. Read your data as JSON documents or CSV tables so you have the flexibility to use the format that works best for you

Open Distro for Elasticsearch is an Apache 2.0-licensed distribution of Elasticsearch enhanced with Enterprise Security, Alerting, SQL, Index Management, k-Nearest Neighbor Search, Performance Analyzer and more. 


Open Distro for Elasticsearch  - Elasticsearch enhanced with enterprise security, alerting, SQL, and more

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