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copied!<p><strong>UPDATE (27 July 2012):</strong> <em>Link to Wikipedia article has been updated to reflect the version of the article that was current when this answer was posted. Please note that the <a href="http://en.wikipedia.org/wiki/Nosql" rel="noreferrer">current Wikipedia article</a> has been extensively revised!</em></p> <p>Well, according to an older version of a <a href="http://en.wikipedia.org/w/index.php?title=NoSQL&oldid=354774356" rel="noreferrer">Wikipedia article on NoSQL</a>:</p> <blockquote> <p>NoSQL is a movement promoting a loosely defined class of non-relational data stores that break with a long history of relational databases and ACID guarantees.</p> </blockquote> <p>and also:</p> <blockquote> <p>The name was an attempt to describe the emergence of a growing number of non-relational, distributed data stores that often did not attempt to provide ACID guarantees.</p> </blockquote> <p>and</p> <blockquote> <p>NoSQL systems often provide weak consistency guarantees such as eventual consistency and transactions restricted to single data items, even though one can impose full ACID guarantees by adding a supplementary middleware layer.</p> </blockquote> <p>So, in a nutshell, I'd say that one of the main benefits of a "NoSQL" data store is its distinct <em>lack</em> of <a href="http://en.wikipedia.org/wiki/ACID" rel="noreferrer">ACID</a> properties. Furthermore, IMHO, the more one tries to implement and enforce <a href="http://en.wikipedia.org/wiki/ACID" rel="noreferrer">ACID</a> properties, the further away from the "spirit" of a "NoSQL" data store you get, and the closer to a "true" <a href="http://en.wikipedia.org/wiki/RDBMS" rel="noreferrer">RDBMS</a> you get (relatively speaking, of course).</p> <p>However, all that said, "NoSQL" is a very vague term and is open to individual interpretations, and depends heavily upon just how much of a purist viewpoint you have. For example, most modern-day RDBMS systems don't actually adhere to <em>all</em> of <a href="http://en.wikipedia.org/wiki/Edgar_F._Codd" rel="noreferrer">Edgar F. Codd</a>'s <a href="http://en.wikipedia.org/wiki/Codd%27s_12_rules" rel="noreferrer">12 rules</a> of his <a href="http://en.wikipedia.org/wiki/Relational_model" rel="noreferrer">relation model</a>!</p> <p>Taking a pragmatic approach, it would appear that Apache's <a href="http://couchdb.apache.org/docs/intro.html" rel="noreferrer">CouchDB</a> comes closest to embodying both ACID-compliance whilst retaining loosely-coupled, non-relational "NoSQL" mentality.</p>

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