Apache Derby is an open source relational database implemented entirely in Java. It has a small footprint that makes it easy to embed in any Java-based application, but it also supports the more familiar client/server mode. It is based on the Java, JDBC, and SQL standards, making code developed more portable to standards-compliant databases.

Apache Derby - Relational Database implemented in Java

OpenTSDB is a distributed, scalable Time Series Database (TSDB) written on top of HBase.  OpenTSDB was written to address a common need: store, index and serve metrics collected from computer systems (network gear, operating
systems, applications) at a large scale, and make this data easily accessible and graphable.

OpenTSDB - A scalable, distributed Time Series Database.

Ebean is fast and simple data access on the JVM. With Ebean it is very easy to insert, update, delete. There is no Session or EntityManager to manage, no merge/flush/attach/detach semantics. Ebean can profile the object graph use and use that to automatically optimise ORM queries such that they only fetch the required properties and minimises N + 1. 
 
Its features include JPA Mapping annotations, Type safe queries, ElasticSearch integration and lot more.

Ebean is fast and simple data access on the JVM.  With Ebean it is very easy to insert, update, delete. There is no Session or EntityManager to manage, no merge/flush/attach/detach semantics.  Ebean can profile the object graph use and use that to automatically optimise ORM queries such that they only fetch the required properties and minimises N + 1. 

Ebean - ORM for Java / Kotlin

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

Titan is a scalable graph database optimized for storing and querying graphs containing hundreds of billions of vertices and edges distributed across a multi-machine cluster. Titan is a transactional database that can support thousands of concurrent users executing complex graph traversals. It is a native Blueprints enabled graph database and as such, it supports the full TinkerPop stack of technologies.

Titan - Scalable Graph Database 

KairosDB is a fast distributed scalable time series database written on top of Cassandra. Data can be pushed in KairosDB via multiple protocols : Telnet, Rest, Graphite. KairosDB stores time series in Cassandra, the popular and performant NoSQL datastore. It supports aggregators which can perform an operation on data points and down samples. Standard functions like min, max, sum, count, mean etc.

Kairosdb - Fast distributed scalable time series database written on top of Cassandra

Debezium is a distributed platform that turns your existing databases into event streams, so applications can see and respond immediately to each row-level change in the databases. Debezium is built on top of Apache Kafka and provides Kafka Connect compatible connectors that monitor specific database management systems. Debezium records the history of data changes in Kafka logs, from where your application consumes them. This makes it possible for your application to easily consume all of the events correctly and completely.

Debezium - Stream changes from your databases.

Apache ShardingSphere is an open-source ecosystem consisted of a set of distributed database solutions, including 3 independent products, JDBC, Proxy & Sidecar (Planning). They all provide functions of data scale-out, distributed transaction and distributed governance, applicable in a variety of situations such as Java isomorphism, heterogeneous language and cloud-native.
 
Features: 
<ul>
<li>Database sharding &amp; Table sharding</li>
<li>Replica query</li>
<li>Sharding strategy customization</li>
<li>Centre-less Distributed primary key</li>
<li>Distributed Governance</li>
<li>Data migration &amp; Scale-out</li>
<li>Observability(Tracing/Metrics) Supported</li>
<li>Data Encryption&amp;Decryption</li>
<li>Shadow Table for Performance Testing</li>
<li>Unified Transaction API</li>
</ul>

Apache ShardingSphere is an open-source ecosystem consisted of a set of distributed database solutions, including 3 independent products, JDBC, Proxy & Sidecar (Planning). They all provide functions of data scale-out, distributed transaction and distributed governance, applicable in a variety of situations such as Java isomorphism, heterogeneous language and cloud-native.

Apache ShardingSphere - Distributed Database Ecosphere 

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

Perst is McObject's open source, dual license, object-oriented embedded database system (ODBMS). Perst gives developers the ability to sort, store and retrieve objects in their applications with maximum speed and with low memory and storage overhead, while leveraging the object-oriented paradigm of Java and C#. It is available in one edition developed as an all-Java embedded database, and another implemented in C# (for Microsoft .NET Framework applications). Perst’s also include garbage collection, schema evolution, a “wrapper” that provides a SQL-like database interface (SubSQL), XML import/export, database replication, support for large databases, and more.

Perst is McObject's open source, dual license, object-oriented embedded database system (ODBMS). Perst gives developers the ability to sort, store and retrieve objects in their applications with maximum speed and with low memory and storage overhead, while leveraging the object-oriented paradigm of Java and C#. It is available in one edition developed as an all-Java embedded database, and another implemented in C# (for Microsoft .NET Framework applications).

Perst - object-oriented embedded database

LucidDB is the RDBMS built entirely for data warehousing and business intelligence. It is based on architectural cornerstones such as column-store, bitmap indexing, hash join/aggregation, and page-level multiversioning. Every component of LucidDB was designed with the requirements of flexible, high-performance data integration and sophisticated query processing in mind. 

 Rather than throwing hardware at data warehousing problems by relying on expensive clusters or specialized "appliances", the scalability offered by LucidDB's unique architecture allows you to achieve great performance using only a single off-the-shelf Linux or Windows server. Besides keeping costs down, this also minimizes maintenance and administration hassles. 
 <UL>
	<LI>Support of both 32 and 64 bit platforms</LI>
	<LI>Integrated with Pentaho Business Intelligence product</LI>
	<LI>Automatically adapts to either bitmap or btree representation depending on data distribution</LI>
	<LI>Versioning at page-level is much more efficient than transactional databases</LI>
	<LI>Supports incremental backup</LI>
	<LI>High performance and greater cache and disk effectiveness</LI>
	<LI>Allows flat files (e.g. BCP or CSV format) to be queried as foreign tables via LucidDB</LI>
 </UL>

LucidDB is the RDBMS built entirely for data warehousing and business intelligence. It is based on architectural cornerstones such as column-store, bitmap indexing, hash join/aggregation, and page-level multi versioning. Every component of LucidDB was designed with the requirements of flexible, high-performance data integration and sophisticated query processing in mind.

LucidDB - RDBMS built entirely for Data Warehousing and Business Intelligence

VoltDB was specifically designed for contemporary software applications that are pushed beyond their limits by high volume data sources. VoltDB provides the ability to capture, store and process incoming data at millions of read/write operations per second. And VoltDB’s relational model opens that data to be analyzed in real-time, using familiar Business Intelligence tools, to identify data patterns and trends, spot anomalies, or perform tracking and alerting. In addition to supporting real-time analytics, VoltDB seamlessly integrates with data warehousing and Hadoop infrastructures, feeding data to those environments for deep analytical exploration. Large database gets automatically sharded to multiple nodes, basically it supports database to scale out. VoltDB implements HA using K-Safety, a configurable, active-active durability model for high performance distributed systems. Client connectors are available in languages like Java, C++, Python, PHP, HTTP/JSON, Ruby and Erlang.

VoltDB was specifically designed for contemporary software applications that are pushed beyond their limits by high volume data sources. VoltDB provides the ability to capture, store and process incoming data at millions of read/write operations per second. And VoltDB’s relational model opens that data to be analyzed in real-time, using familiar Business Intelligence tools, to identify data patterns and trends, spot anomalies, or perform tracking and alerting.

VoltDB - Fast Scalable SQL DBMS with ACID

The Apache Accumulo sorted, distributed key/value store is a robust, scalable, high performance data storage and retrieval system. Apache Accumulo is based on Google's BigTable design and is built on top of Apache Hadoop, Zookeeper, and Thrift. Apache Accumulo features a few novel improvements on the BigTable design in the form of cell-based access control and a server-side programming mechanism that can modify key/value pairs at various points in the data management process.

Apache Accumulo - Key Value Store based on Google BigTable

MapDB is an embedded database engine. It provides Maps and other collections backed by disk or memory storage. It offers excellent performance comparable to java collections, but is not limited by GC overhead. It is also a full database engine with storage backends, transactions, cache algorithms, expiration and many other options. MapDB provides concurrent Maps, Sets and Queues backed by disk storage or off-heap memory. 

MapDB - Embedded Database Engine

Coherence CE (Community Edition) is a free and open source edition of Oracle Coherence, first and market-leading in-memory data grid. It has been used by hundreds of customers across many industries to power some of the mission critical systems you use every day. Coherence clusters can easily scale to hundreds of members (JVMs), all of which can both store and process the data. It can optionally persist data to disk (either local or shared), which prevents data loss even in the case of complete cluster outage.
 
Coherence is fast. Like, sub-millisecond fast. Each member of the cluster is aware of where each piece of data resides and can access it using a single network call. There are no connections to acquire or release, and no distributed transactions to commit or roll back.

Coherence CE (Community Edition) is a free and open source edition of Oracle Coherence, first and market-leading in-memory data grid. It has been used by hundreds of customers across many industries to power some of the mission critical systems you use every day. Coherence clusters can easily scale to hundreds of members (JVMs), all of which can both store and process the data. It can optionally persist data to disk (either local or shared), which prevents data loss even in the case of complete cluster outage.

Coherence CE (Community Edition) is a free and open source edition of Oracle Coherence, first and market-leading in-memory data grid

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