In its default configuration, Petclinic uses an in-memory database (HSQLDB) which gets populated at startup with data. A similar setup is provided for MySql in case a persistent database configuration is needed. Note that whenever the database type is changed, the data-access.properties file needs to be updated and the mysql-connector-java artifact from the pom.xml needs to be uncommented.The Spring Petclinic master branch in the main spring-projects GitHub org is the "canonical" implementation, currently based on Spring Boot and Thymeleaf. There are quite a few forks in a special GitHub org spring-petclinic. If you have a special interest in a different technology stack that could be used to implement the Pet Clinic then please join the community there.