Software Architect's Handbook

Data tier

The data tier provides functionality to access and manage data. The data tier contains a data store for persistent storage, such as an RDBMS. It provides services and data for the business tier.

There are variations of n-tier architectures that go beyond just three tiers. For example, in some systems, there is a data access or persistence layer in addition to a data or database layer. The persistence layer contains components for data access, such as an object-relational mapping (ORM) tool, and the database layer contains the actual data store, such as an RDBMS. One reason to separate these into two distinct layers is if you wanted the ability to switch out your data access or database technology for a different one.