Relational databases store the vast majority of web application persistent data. However, there are several alternative classifications of storage representations.
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 the column-family table class of NoSQL data stores builds on the key-value pair type. Each key-value pair is considered a row in the store while the column family is similar to a table in the relational database model.
A graph database represents and stores 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.
MongoDB for startups is a guide about using non-relational databases in green field environments.
Understand why NoSQL data stores are better for some use cases than relational databases. In general these benefits are only seen at large scale so they may not be applicable to your web application.
Integrate Redis into your project for a speed boost over slower persistent storage. Storing session data in memory is generally much faster than saving that data in a traditional relational database that uses persistent storage. Note that when memory is flushed the data goes away so anything that needs to be persistent must still be backed up to disk on a regular basis.
Evaluate other use cases such as storing transient logs in document-oriented data stores such as MongoDB.
