The modern sense of , which dates from 2009, refers to databases that are not built on relational tables, unlike . Often, boast better design flexibility, horizontal scalability, and higher availability than traditional SQL databases, sometimes at the cost of weaker consistency.
NoSQL databases can take a number of forms. They can be cloud services or install on-premises. They can support one or more data models: key-value, document, column, graph, and sometimes even relational—which is one reason that NoSQL is sometimes parsed as “Not Only SQL.” They can also support a range of consistency models, from strong consistency to eventual consistency.
Key-value is the most basic of the four non-relational data models. Sometimes other database models are implemented on top of a key-value foundation layer.
Column databases have keys, values, and timestamps; the timestamp is used for determining the valid content. is a prominent example of a column database.
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