Postgresql database

PostgreSQL is a powerful, open source object-relational database system with over 30 years of active development that has earned it a strong reputation for reliability, feature robustness, and performance.

PostgreSQL is a common choice for Clojure WebApps that require a persistent store for data, especially where that data is relational in nature. PostgreSQL is also a great choice for JSON and other formats.

Heroku Postgres

Heroku provides a Posgresql service (free 10,000 rows limit per database) which provisions PostgreSQL databases on demand, so is a simple way to start development.