Key Concepts of Chapter 24
1. What Are Partial Boundaries?
- Definition: Partial boundaries are less rigid separations between components or layers in a system.
- Instead of fully isolating layers or modules, partial boundaries allow some controlled leakage of details to reduce complexity or overhead.
- They are useful in situations where enforcing strict boundaries would lead to diminishing returns.
2. Why Use Partial Boundaries?
- Pragmatism: Strict boundaries can sometimes introduce unnecessary complexity, especially for smaller or simpler systems.
- Performance: Fully abstracting boundaries may result in performance overhead that isn’t justified in certain contexts.
- Team Constraints: Smaller teams may lack the resources to implement and maintain a fully decoupled architecture.
- Development Speed: Partial boundaries can speed up development by simplifying interactions between layers.
3. Examples of Partial Boundaries
Database Access:
- A fully abstracted repository pattern might be overkill for simple CRUD applications.
- Instead of abstracting all database queries, you might allow some queries to reside closer to the application logic.
Partial Boundary Example:
public class UserService {
private Database database;
public User getUserById(int userId) {
return database.query("SELECT * FROM users WHERE id = ?", userId);
}
}
UI Layer:
- For simpler applications, some UI logic may interact directly with use cases or even entities, bypassing strict boundaries.
- Example: A view might directly access a data model to render content.
External Systems:
- Instead of fully abstracting an external API behind interfaces, you might call the API directly within the use case for simplicity.
4. Risks of Partial Boundaries
- Coupling: Partial boundaries can lead to tighter coupling between components, making future changes harder.
- Reduced Testability: Direct dependencies between layers might complicate testing.
- Technical Debt: Over-reliance on partial boundaries can create long-term maintenance challenges.
5. When to Use Partial Boundaries
- Simple Systems: When the application has limited complexity and well-defined requirements.
- Prototyping: During early stages of development when speed is more important than architectural purity.
- Low-Risk Areas: For parts of the system that are unlikely to change frequently or impact core business rules.
6. Evolving from Partial to Full Boundaries
- Martin emphasizes that partial boundaries should not become permanent unless justified.
- As the system grows or requirements change, you may need to refactor partial boundaries into fully abstracted ones.
- Incremental Refactoring:
- Replace direct database calls with repository interfaces.
- Move business logic out of the UI layer into use cases.
Example: E-Commerce System
Partial Boundary (Pragmatic):
public class ProductService {
private Database database;
public List<Product> getProductsByCategory(String category) {
return database.query("SELECT * FROM products WHERE category = ?", category);
}
}
Full Boundary (Abstracted):
public interface ProductRepository {
List<Product> findByCategory(String category);
}
public class ProductService {
private ProductRepository productRepository;
public ProductService(ProductRepository productRepository) {
this.productRepository = productRepository;
}
public List<Product> getProductsByCategory(String category) {
return productRepository.findByCategory(category);
}
}
Key Takeaways
- Balance Purity and Practicality:
- Use partial boundaries when full boundaries introduce unnecessary complexity or overhead.
- Understand the Risks:
- Be aware of the trade-offs, such as reduced testability and tighter coupling.
- Evolve Over Time:
- Start with partial boundaries and refactor to full boundaries as the system grows or requirements change.
- Keep Business Rules Independent:
- Even with partial boundaries, core business rules should remain isolated from external systems and implementation details.