Adding Rules and Behavior
Domain-Driven Design emphasizes the importance of building a rich domain model that accurately captures business rules and behaviors. Protean provides a comprehensive set of mechanisms to define and enforce these rules in your domain model.
Core Concepts in Domain Behavior
Validations
Validations ensure that data meets basic requirements before it can be processed. In Protean, validations can be applied at multiple levels:
- Field-level restrictions - Define type constraints, required fields, uniqueness
- Built-in validators - Leverage pre-defined validators for common validation patterns
- Custom validators - Create domain-specific validation logic
Learn more about validations →
Invariants
Invariants are business rules that must always hold true within your domain model. They preserve the consistency and integrity of your domain objects:
- Always valid - Invariants are conditions that must hold true at all times
- Domain-driven - Invariants stem from business rules and policies
- Immediate validation - Triggered automatically during initialization and state changes
Aggregate Mutation
Aggregates encapsulate the state and behavior of your domain. Mutating their state is how you implement business operations:
- State change methods - Well-defined methods for modifying aggregate state
- Invariant enforcement - All state changes are validated against defined invariants
- Explicit behavior - Business operations are expressed as meaningful methods
Learn more about aggregate mutation →
Raising Events
Domain events record significant state changes and enable communication between different parts of your system:
- Delta events - Generated when aggregates mutate to record state changes
- Entity events - Any entity in an aggregate cluster can raise events
- Event dispatching - Events are automatically dispatched or can be manually published
Learn more about raising events →
Domain Services
Domain services encapsulate business logic that doesn't naturally fit within any single aggregate:
- Stateless operations - Pure functions that operate on multiple aggregates
- Complex workflows - Coordinate operations that span multiple aggregates
- Business rules - Enforce constraints that involve multiple objects
Learn more about domain services →
Best Practices
When implementing domain behavior in Protean:
- Keep aggregates focused - Aggregates should encapsulate only their own state and behavior
- Make business rules explicit - Use invariants to clearly express domain constraints
- Use domain events for cross-aggregate communication - Avoid direct dependencies between aggregates
- Consider domain services for complex operations - When logic spans multiple aggregates
- Validate early and often - Apply appropriate validation at all levels