Table of Contents
The data-access layer is responsible for all outgoing connections to access and process data. This is mainly about accessing data from a persistent data-store. External system could also be accessed from the data-access layer if they match this definition, e.g. a mongo-db via rest services.
You need to make your choice for a database. Options are documented here.
The classical approach is to use a Relational Database Management System (RDMS). In such a case, we strongly recommend to follow our JPA Guide. Some NoSQL databases are supported by spring-data so you can consider the repository guide.