Core Principles

1. The domain is the center of the universe. Business logic has zero knowledge of databases, HTTP frameworks, message queues, or any external technology.
2. Dependencies point inward. Outer layers depend on inner layers — never the reverse.
3. Communication through ports. The application core defines interfaces (ports) that describe what it needs. External systems provide implementations (adapters) for those interfaces.
4. Swappable infrastructure. Because all infrastructure is behind interfaces, you can replace MySQL with Postgres, Redis with Memcached, or REST with gRPC without touching business logic.

                ┌─────────────────────────────────────┐
                │          Primary Adapters            │
                │   (gRPC handlers, HTTP controllers,  │
                │    CLI, event consumers)             │
                └───────────────┬─────────────────────┘
                                │ calls
                ┌───────────────▼─────────────────────┐
                │        Application Layer             │
                │   (Commands, Queries, Use Cases)     │
                │                                      │
                │   Uses ports (interfaces) to talk     │
                │   to the outside world               │
                └───────────────┬─────────────────────┘
                                │ depends on
                ┌───────────────▼─────────────────────┐
                │          Domain Layer                │
                │   (Entities, Value Objects,           │
                │    Business Rules)                   │
                └───────────────┬─────────────────────┘
                                │ defined by
                ┌───────────────▼─────────────────────┐
                │            Ports                     │
                │   (Repository interfaces,            │
                │    Service interfaces)               │
                └───────────────┬─────────────────────┘
                                │ implemented by
                ┌───────────────▼─────────────────────┐
                │        Secondary Adapters            │
                │   (MySQL, Redis, S3, GraphQL         │
                │    clients, external APIs)           │
                └─────────────────────────────────────┘