Onion Architecture

Onion Architecture places an independent domain model at the center, followed by domain services and application services. Infrastructure stays at the edge and implements interfaces owned by inner layers.

src/app/
├── domain_model/
├── domain_services/
├── application/
└── infrastructure/

Complete Configuration

deply:
  paths: ["."]
  exclude_files:
    - '.*/\.venv/.*'
    - '.*/tests?/.*'
  layers:
    - name: domain_model
      collectors:
        - type: directory
          directories: ["src/app/domain_model"]
    - name: domain_services
      collectors:
        - type: directory
          directories: ["src/app/domain_services"]
    - name: application
      collectors:
        - type: directory
          directories: ["src/app/application"]
    - name: infrastructure
      collectors:
        - type: directory
          directories: ["src/app/infrastructure"]
  ruleset:
    domain_model:
      disallow_layer_dependencies: [domain_services, application, infrastructure]
      disallow_external_imports: [django, fastapi, flask, sqlalchemy, requests]
    domain_services:
      disallow_layer_dependencies: [application, infrastructure]
    application:
      disallow_layer_dependencies: [infrastructure]

Do not create domain services only to mirror the diagram. Keep behavior on domain objects until a real operation spans multiple domain concepts.

Deply can enforce inward source dependencies, but it cannot verify runtime dependency inversion or the quality of domain-service boundaries.

Validate

deply validate --config=deply.yaml
deply analyze --parallel --config=deply.yaml

Reference: Jeffrey Palermo’s Onion Architecture series.