NoSQL DEFINITION: Next Generation Databases mostly addressing some of the points: being non-relational, distributed, open-source and horizontally scalable. The original intention has been modern web-scale databases. The movement began early 2009 and is growing rapidly. Often more characteristics apply such as: schema-free, easy replication support, simple API, eventually consistent / BASE (not ACID), a huge amount of data and more.
So the misleading term “nosql” (the community now translates it mostly with “not only sql”) should be seen as an alias to something like the definition above. A NoSQL database provides a simple, lightweight mechanism for storage and retrieval of data that provides higher scalability and availability than traditional relational databases. The NoSQL data stores use looser consistency models to achieve horizontal scaling and higher availability. Many authors refer to them as “Not only SQL”; because the data stores generally do not use a ‘SQL’ approach for storing data, i.e. in the form of tables.
NoSQL database systems are often highly optimized for retrieval and appending operations and often offer little functionality beyond record storage (e.g. key–value stores). The reduced run-time flexibility compared to full SQL systems is compensated by marked gains in scalability and performance for certain data models.
In short, NoSQL database management systems are useful when working with a huge quantity of data when the data’s nature does not require a relational model. The data can be structured, but NoSQL is used when what really matters is the ability to store and retrieve great quantities of data, not the relationships between the elements. Usage examples might be to store millions of key–value pairs in one or a few associative arrays or to store millions of data records. This organization is particularly useful for statistical or real-time analyses of growing lists of elements (such as Twitter posts or the Internet server logs from a large group of users).
NoSQL does not use SQL as its query language. NoSQL database systems arose alongside major Internet companies, such as Google, Amazon, and Facebook, which had challenges in dealing with huge quantities of data that conventional RDBMS solutions could not cope with. Note that both the Facebook and Twitter infrastructures are largely based on MySQL databases. NoSQL database systems are developed to manage large volumes of data that do not necessarily follow a fixed schema. Data is partitioned among different machines (for performance reasons and size limitations) so that traditional JOIN operations cannot be used.
NoSQL cannot necessarily give full ACID guarantees. Usually only eventual consistency is guaranteed or transactions limited to single data items. This means that given a sufficiently long period of time over which no changes are sent, all updates can be expected to propagate eventually through the system.
NoSQL has a distributed, fault-tolerant architecture. Several NoSQL systems employ a distributed architecture, with the data held in a redundant manner on several servers.
In this way, the system can easily scale out by adding more servers, and failure of a server can be tolerated. This type of database typically scales horizontally and is used for managing large amounts of data, when the performance and real-time nature is more important than consistency (as in indexing a large number of documents, serving pages on high-traffic web sites, and delivering streaming media).
“NoSQL is a whole new way of thinking about a database. Though NoSQL is not a relational database, the reality is that a relational database model may not be the best solution for all situations.”