NoSQL and when to use it

In 2007 Amazon engineers published the design of Dynamo, the store behind services such as the shopping cart. Their requirement was blunt: a customer must always be able to add an item, even while servers fail or the network splits. A cart that briefly shows an older version is acceptable; a cart that refuses a write is lost revenue.

Google's Bigtable paper had appeared a year earlier. Together they showed a different set of trade-offs from Codd's: give up joins and some consistency, and you can spread data across thousands of cheap machines. The open-source systems that followed are grouped under the loose label NoSQL.