5 Designing good abstractions

Good abstractions allow us to add new functionality to a system without constantly changing existing code. For example, think of a bookstore with a range of discounts like “Buy three books, get one free,” “45% off during Christmas,” and “Buy five e-books, get one printed copy free.” The marketing team proposes new discounts regularly, so the development team needs an easy way to add them to the code. A well-designed software system will have abstractions in place so developers can add new discounts with minimal effort.

It’s difficult to define what abstractions are in one sentence, so I’ll use a few:

Abstractions work well with extension points. An extension point enables developers to extend or modify the system’s functionality. In the bookstore, an extension point would allow developers to plug in or unplug whatever discounts should be applied for a given basket.

5.1 Design abstractions and extension points

5.1.1 Identifying the need for an abstraction

5.1.2 Designing an extension point

5.1.3 Attributes of good abstractions

5.1.4 Learn from your abstractions

5.1.5 Learn about abstractions

5.1.6 Abstractions and coupling

5.1.7 Example: Giving badges to employees

5.2 Generalize important business rules

5.2.1 Separate the concrete data from the generalized business rule

5.2.2 Example: Generalizing the badge rules

5.3 Prefer simple abstractions