Prior to 2008, whatever your database question, the answer was always, "Oracle"—or sometimes, "MySQL" or, "SQL Server." The relational database management system (RDBMS) model—really a triumvirate of technologies combining Codd's relational data model, the ACID transaction model, and the SQL language—dominated database management systems completely.
However, around 2008–2009, a plethora of new database systems emerged, none of which closely followed the RDBMS model.
The straw that broke the back of relational dominance was the inability of RDBMS to satisfy the needs of the largest web companies—Google and Amazon.
At Google, the sheer volume of data involved in indexing the World Wide Web led the company to develop new approaches to massive data storage such as the Google File System (GFS) and MapReduce. These technologies led the open source community to develop Hadoop. Hadoop seriously challenged the enterprise data warehousing segment of the RDBMS market.
At Amazon, the need to maintain an “always-on” transactional system was at odds with the consistency model of the RDBMS. Amazon developed its own “eventually consistent” DBMS—Dynamo—which is now commercialized as DynamoDB. The Dynamo model inspired Cassandra and several other early NoSQL database systems.
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