# ADR-0002: Domain-Driven Design

## Status

Accepted

## Context

As the Meldestelle system evolved to handle complex business rules for equestrian event management, we faced challenges in:

1. Maintaining a clear separation between business logic and technical concerns
2. Ensuring that the system accurately reflects the domain expert's understanding of the problem space
3. Creating a shared language between technical and non-technical stakeholders
4. Organizing code in a way that reflects the business domains

We needed an architectural approach that would address these challenges and provide a solid foundation for the modular architecture described in [ADR-0001](0001-modular-architecture.md).

## Decision

We decided to adopt Domain-Driven Design (DDD) principles for organizing our codebase and designing our system. This includes:

1. **Ubiquitous Language**: Developing a common language shared by domain experts and developers
2. **Bounded Contexts**: Defining explicit boundaries between different domain areas (masterdata, members, horses, events)
3. **Layered Architecture**: Organizing each domain module into layers:
   - Domain Layer: Contains domain models, entities, value objects, and domain services
   - Application Layer: Contains application services, use cases, and command/query handlers
   - Infrastructure Layer: Contains technical implementations of repositories, messaging, etc.
   - API Layer: Defines the interfaces for interacting with the domain
4. **Aggregates**: Identifying aggregate roots that maintain consistency boundaries
5. **Repositories**: Using the repository pattern to abstract data access
6. **Domain Events**: Using events to communicate between bounded contexts

## Consequences

### Positive

- **Business-technology alignment**: The code structure directly reflects the business domains
- **Improved communication**: Ubiquitous language facilitates communication between technical and non-technical stakeholders
- **Maintainability**: Clear separation of concerns makes the code easier to maintain
- **Testability**: Domain logic can be tested independently of infrastructure concerns
- **Flexibility**: Changes in one bounded context have minimal impact on others

### Negative

- **Learning curve**: DDD concepts require time to learn and apply correctly
- **Initial development overhead**: More upfront design and discussion is needed
- **Potential overengineering**: Risk of applying complex DDD patterns where simpler solutions would suffice

### Neutral

- **Team organization**: Teams need domain knowledge as well as technical skills
- **Documentation needs**: Domain models and bounded contexts need to be well-documented

## Alternatives Considered

### Transaction Script Pattern

We considered using a simpler transaction script pattern where business logic is organized around procedures rather than domain objects. This would have been simpler to implement initially but would have become harder to maintain as the business logic grew more complex.

### Anemic Domain Model

We considered using an anemic domain model where domain objects are simple data containers and business logic is in separate service classes. This would have been more familiar to developers coming from a CRUD-based background but would not have provided the benefits of encapsulation and rich domain modeling.

## References

- [Domain-Driven Design by Eric Evans](https://domainlanguage.com/ddd/)
- [Implementing Domain-Driven Design by Vaughn Vernon](https://vaughnvernon.co/?page_id=168)
- [Clean Architecture by Robert C. Martin](https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html)
