MongoDB - BigData
MongoDB is a cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with optional schemas. Big Data refers to very large and often complex data sets, so massive in size that they’re beyond the capability of managing with traditional software tools. That's why non-relational databases such as MongoDB (which are inherently designed to handle unstructured inputs) are a great fit for Big Data, especially in the cloud.
CouchDB - BigData
PostgreSQL and PgPool-II
PostgreSQL is a powerful, open source object-relational Database system with a strong reputation for reliability, feature robustness, and performance. PgPool-II is a middleware that sits between PostgreSQL servers and a PostgreSQL database client that extends PostgreSQL with clustering capabilities and provide many features like: Connection Pooling, Load Balancing, Replication, Automated Fail Over and Recovery, etc.
MariaDB (MySQL) and Galera
MariaDB is a community-developed, commercially supported fork of the MySQL relational database management system (RDBMS), intended to remain free and open-source software under the GNU General Public License. MariaDB intended to maintain high compatibility with MySQL, ensuring a drop-in replacement capability with library binary parity and exact matching with MySQL APIs and commands. However, new features diverge more (ex: XtraDB replaces InnoDB). MariaDB server can be integrated with Galera Cluster that extends it with the feature of acting as a synchronous multi-master cluster.
Redis In-Memory DataStore
Redis is an open source in-memory data structure store, used as a database, cache and message broker that supports many types of data structures. It is considered being one of the fastest in-memory key-value database. Redis is a popular choice for caching, session management, gaming, leaderboards, real-time analytics, geospatial, ride-hailing, chat/messaging, media streaming, and pub/sub apps.