# ADR-0001: Modular Architecture

## Status

Accepted

## Context

The Meldestelle system was initially developed as a monolithic application. As the system grew in complexity and size, several challenges emerged:

1. The codebase became difficult to maintain and understand
2. Development teams had to coordinate closely, slowing down development
3. Scaling the entire application was necessary even when only specific parts needed more resources
4. Technology choices were constrained by the monolithic architecture

The team needed to decide whether to continue with the monolithic approach or migrate to a more modular architecture.

## Decision

We decided to migrate from a monolithic structure to a modular architecture, organizing the system into the following modules:

- **core**: Shared core components
- **masterdata**: Master data management
- **members**: Member management
- **horses**: Horse registration
- **events**: Event management
- **infrastructure**: Shared infrastructure components
- **client**: Client applications

Each domain module (masterdata, members, horses, events) follows a clean architecture approach with separate API, application, domain, infrastructure, and service layers.

## Consequences

### Positive

- **Improved maintainability**: Smaller, focused modules are easier to understand and maintain
- **Independent development**: Teams can work on different modules with minimal coordination
- **Selective scaling**: Individual modules can be scaled based on their specific requirements
- **Technology flexibility**: Different modules can use different technologies as appropriate
- **Clear boundaries**: Domain boundaries are explicitly defined, improving the conceptual integrity of the system

### Negative

- **Increased complexity**: The overall system architecture is more complex
- **Deployment overhead**: More components to deploy and manage
- **Performance considerations**: Inter-module communication adds latency
- **Migration effort**: Significant effort required to migrate from the monolithic structure

### Neutral

- **Team organization**: Teams need to be reorganized around modules rather than features
- **Documentation needs**: More comprehensive documentation is required to understand the system as a whole

## Alternatives Considered

### Enhanced Monolith

We considered improving the internal structure of the monolith with better module boundaries but keeping it as a single deployable unit. This would have been simpler to deploy but wouldn't have addressed the scaling and technology flexibility issues.

### Microservices

We considered a more fine-grained microservices approach with many smaller services. This would have provided maximum flexibility but introduced excessive complexity and operational overhead for our current needs.

## References

- [Migration notes in README.md](../../../../README.md#aktuelle-migrationshinweise)
- [Modular Monoliths by Simon Brown](https://simonbrown.je/blog/modularity-and-microservices/)
- [Clean Architecture by Robert C. Martin](https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html)
