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Relational databases store the vast majority of web application persistent data. However, there are several alternative classifications of storage representations.
- Key-value pair
- Document-oriented
- Column-family table
- Graph
These persistent data storage representations are commonly used to augment, rather than completely replace, relational databases.
A document-oriented database provides a semi-structured representation for nested data.
Key-value pair data stores are based on hash map data structures.
A graph database represent and store data in three aspects: nodes, edges, and properties.
A node is an entity, such as a person or business.
An edge is the relationship between two entities. For example, an edge could represent that a node for a person entity is an employee of a business entity.
A property represents information about nodes. For example, an entity representing a person could have a property of "female" or "male".
Neo4j is one of the most widely used graph databases and runs on the Java Virtual Machine stack.
MongoHQ provides MongoDB as a service. It's easy to set up with either a standard LAMP stack or on Heroku.
NoSQL Weekly is a free curated email newsletter that aggregates articles, tutorials, and videos about non-relational data stores.
NoSQL comparison is a large list of popular, BigTable-based, special purpose, and other datastores with attributes and the best use cases for each one.