Voldemort is a distributed key-value data store used at LinkedIn for high-scalability storage problems where simple functional partitioning is not sufficient.
It is named after the very popular fictional Harry Potter villain Lord Voldemort. Voldermort contains in-memory caching with storage system hence a separate caching tier is not needed. It supports horizontal scalability for reads and writes. It is a more of fault tolerant hash table.
Horizontal scalability and High availability for O/R mapper such as hibernate and active-record
Support for distribution across data centers that are far apart by pluggable data placement strategies
Automatic data replication over large number of servers
Versioned data items to maintain and maximize data integrity
Cassandra is an open source distributed database management system and an Apache Software Foundation project having Apache License (version 2.0).
It is designed to handle enormous amounts of data spread out across many commodity servers in traditional environment or in Cloud environment while providing a highly available service with no single point of failure. It is a NoSQL solution that was developed by Facebook and now used by companies that have large, active data sets such as eBay, Twitter, Reddit, Cisco, OpenX, Digg etc.
FlockDB is an open source, fault-tolerant, and distributed graph database licensed under the Apache license for managing data at webscale. Twitter used it to build user database and manage relationships. It can be efficiently used in high throughput and low latency environments. FlockDB was created by Twitter for relationship related analytics. FlockDB is a database that stores graph data which is optimized for very large adjacency lists, and quick reads and writes but not optimized for graph traversal operations.
In FlockDB, graphs are stored as sets of edges between nodes which are identified by 64 bit integers. Each edge between nodes is also marked with a 64 bit position. Edge can be used for sorting. For social graphs, integer node IDs will be user IDs while in a graph containing favorite tweets, the destination will be a tweet ID.
It is an open source property graph database. It is implemented in Java. It is stores data structured in graphs. Graph based model makes it highly agile and fast. It is massively scalable, up to several billion nodes and highly available when it is distributed across multiple nodes. It can be easily embedded by including the Neo4j library jars in your build.
In high availability mode, it has single master and zero or more slaves. It’s high availability feature can handle write requests on all machines so there is no need to redirect those to the master particularly. A slave can handle writes by synchronizing with the master to maintain consistency. All updates propagate from the master to other slaves in due course so a write from one slave may not be immediately visible on all other slaves.
MongoDB is an open source, scalable, high-performance, and document-oriented database optimized for highly transient data and written in the C++ programming language. It provides RESTful API. Free Cloud based monitoring service is provided for monitoring MongoDB deployments. It supports search by range queries, fields, and regular expressions. Master slave replication is supported where master can perform read and write operations while slaves can read or take backup.MongoDB supports horizontal scaling with the use of sharding. It can be effectively used as a efficient file storage which is capable of taking benefits of load balancing and data replication.