Carlo Strozzi used the term NoSQL in 1998 to name his lightweight, open-source relational database that did not expose the standard SQL interface. Strozzi suggests that, as the current NoSQL movement"departs from the relational model altogether; it should therefore have been called more appropriately 'NoREL'.
Eric Evans (then a Rackspace employee) reintroduced the term NoSQL in early 2009 when Johan Oskarsson of Last.fm wanted to organize an event to discuss open-source distributed databases. The name attempted to label the emergence of a growing number of non-relational, distributed data stores that often did not attempt to provide atomicity, consistency, isolation and durability guarantees that are key attributes of classic relational database systems.
In 2011, work began on UnQL (Unstructured Query Language), a specification for a query language for NoSQL databases. Like XQuery it is designed to query collections (versus tables) of documents (versus rows) with loosely defined fields (versus columns). UnQL is claimed to be a superset of SQL within which SQL is a very constrained type of UnQL for which the queries always return the same fields (same number, names and types). However, UnQL does not cover the data definition language (DDL) SQL statements like CREATE TABLE or CREATE INDEX.
A NoSQL database provides a mechanism for storage and retrieval of data that use looser consistency models than traditional relational databases in order to achieve horizontal scaling and higher availability. Some authors refer to them as "Not only SQL" to emphasize that some NoSQL systems do allow SQL-like query language to be used.
NoSQL database systems are often highly optimized for retrieval and appending operations and often 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 (especially big 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 analysis of growing lists of elements (such as Twitter posts or the Internet server logs from a large group of users).
Other usages of this technology are related with the flexibility of the data model; a lot of applications might gain from this unstructured data model: tools like CRM, ERP, BPM, etc., could use this flexibility to store their data without performing changes on tables or creating generic columns in a database. These databases are also good to create prototypes or fast applications, because this flexibility provides a tool to develop new features very easily.
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. Although most of the NoSQL systems have transactions over single documents, other NoSQL systems such as eXtreme Scale, FoundationDB, OrientDB, and djondb state that they are able to execute transactions over multiple documents, similar to what RDBMS systems support over multiple rows.
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).
Often, NoSQL databases are categorized according to the way they store the data and fall under categories such as:
- Key-value stores
- BigTable implementations
- Document store databases
- Graph databases
With the rise of the real-time web, there was a need to provide information out of large volumes of data which more or less followed similar horizontal structures. As such, NoSQL databases are often highly optimized for retrieve and append 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 large gains in scalability and performance for certain data models.
Key–value stores allow the application to store its data in a schema-less way. The data could be stored in a datatype of a programming language or an object. Because of this, there is no need for a fixed data model.
The central concept of a document store is the notion of a "document". While each document-oriented database implementation differs on the details of this definition, in general, they all assume that documents encapsulate and encode data (or information) in some standard formats or encodings. Encodings in use include XML, YAML, and JSON as well as binary forms like BSON, PDF and Microsoft Office documents (MS Word, Excel, and so on).
Different implementations offer different ways of organizing and/or grouping documents:
- Non-visible Metadata
- Directory hierarchies
Compared to relational databases, for example, collections could be considered as tables as well as documents could be considered as records. But they are different: every record in a table has the same sequence of fields, while documents in a collection may have fields that are completely different.
Documents are addressed in the database via a unique key that represents that document. One of the other defining characteristics of a document-oriented database is that, beyond the simple key-document (or key–value) lookup that you can use to retrieve a document, the database will offer an API or query language that will allow retrieval of documents based on their contents. Some NoSQL document stores offer an alternative way to retrieve information using MapReduce techniques, in CouchDB the usage of MapReduce is mandatory if you want to retrieve documents based on the contents, this is called "Views" and it's an indexed collection with the results of the MapReduce algorithms.
This kind of database is designed for data whose relations are well represented as a graph (elements interconnected with an undetermined number of relations between them). The kind of data could be social relations, public transport links, road maps or network topologies, for example.