The world used to get by on just a smattering of databases. You know, trusty relational workhorses like Oracle, Microsoft SQL Server, Ingres, and IBM DB2. Soon enough, however, open source crashed the party with MySQL and PostgreSQL. A bit later, NoSQL databases hit the market, with the likes of MongoDB, Redis, and Apache Cassandra growing in popularity. In terms of numbers, by January 2013 DB-Engines, which ranks database popularity, listed 109 databases. Today? DB-Engines catalogs 356 databases, more than triple the number just seven years ago.