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 but also about invoking external services.
The classical approach is to use a Relational Database Management System (RDMS). In such case we strongly recommend to follow our JPA Guide. In case you are using Oracle you should also consider the Oracle guide.
In case of specific demands and requirements you may want to choose for a Not only SQL database (NoSQL). There are different categories of such products so you should first be aware what fits your requirements best:
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key/value DB
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document DB
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graph DB
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wide-column DB
As there are many such products and the market is evolving very fast, we do not yet give clear recommendations here. If you are doing a devon project and consider NoSQL please contact us for further details.