Object-Oriented Design — Senior¶

Use domain events, aggregates, value objects, policies, and services only where they preserve real domain distinctions. Aggregate boundaries protect invariants and transaction scope; they should not mirror database tables or UI screens.

Design thread-safe objects with ownership, immutability, confinement, locks, or actors. Document atomic operations and never expose partially valid state.

Extension through polymorphism is valuable when variations share a stable contract. If consumers need type checks or subclasses reject base behavior, the hierarchy is lying. Favor replaceable collaborations and explicit data transformations when object identity adds no value.

Test yourself¶

1. Which invariant defines an aggregate boundary?
2. How do you prevent partially valid concurrent state?
3. What signals a false inheritance hierarchy?
4. When is functional data transformation simpler than objects?

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