Abstracting the Database Using Hexagonal Architecture

In this lesson, we’ll apply what we’ve learned to our Wordz application and create a port suitable for fetching the words to present to a user.

Designing the repository interface

The first job in designing our port is to decide what it should be doing. For a database port, we need to think about the split between what we want our domain model to be responsible for and what we’ll push out to the database. The ports we use for a database are generally called repository interfaces.

The following three broad principles should guide us:

• Think about what the domain model needs. Why do we need this data? What will it be used for?

• Don’t simply echo an assumed database implementation, don’t think in terms of tables and foreign keys at this stage. That comes later when we decide how to implement the storage. Sometimes, database performance considerations mean we have to revisit the abstraction we create here. We would then trade off leaking some database implementation details here if it allowed the database to function better. We should defer such decisions as late as we can.

• Consider when we should leverage the database engine more. Perhaps we intend to use complex stored procedures in the database engine. Reflect this split of behavior in the repository interface. It may suggest a higher-level abstraction in the repository interface. ...