Vertical Slice Architecture

Quick Reference: The 5 Rules

1. One feature = one directory containing handler, request/response types, validation, and tests
2. One entry point per feature — a setup/registration function that receives the router and dependencies. Name varies by convention (Setup, RegisterRoute, Map); the role is the invariant, not the name.
3. Minimize coupling between slices, maximize coupling within a slice
4. No premature abstractions — no shared repository/service layers until genuine duplication emerges across multiple slices
5. Test each feature primarily through its entry point, verifying outcomes (DB state, API calls, response). Platform/adapter tests are also encouraged.

Project Structure

{project}/
  features/              # or internal/features/ (Go), Features/ (.NET)
    {domain}/            # orders/, users/, kvs/
      {operation}/       # create/, list/, delete/
        handler          # Single entry point + orchestration
        request/response # DTOs
        validator        # Input validation (optional)
        test             # Co-located integration test
        internal/        # Feature-private helpers (optional)
  platform/              # or Infrastructure/ — shared cross-cutting concerns
    middleware/           # Auth, error handling, idempotency
    database/            # Connection pooling, circuit breakers
    observability/       # Metrics, tracing, structured logging
    opqueue/             # Operation queues, outbox patterns (if needed)
  main                   # Composition root wires features + infrastructure

Workflow: Adding a New Feature

1. Create directory: features/{domain}/{operation}/
2. Define the handler with a single exported setup function
3. Define request/response types (inline for simple cases)
4. Add validation logic
5. Register in the composition root (main)
6. Write integration test that calls the setup function, sends request, verifies outcomes

Workflow: Adding Cross-Cutting Concerns

Place in platform/ (not inside a feature). Examples:

• Auth middleware, error handling, request logging
• Database connection pooling, circuit breakers
• Idempotency middleware, operation queues, event notifications
• Observability (metrics, tracing, structured logging)

Workflow: Extracting Shared Logic

Only extract when genuine duplication emerges across multiple slices (use judgment — the "3+ slices" heuristic is guidance, not a hard rule):

• Duplicate business rule: extract to a domain entity/value object
• Duplicate data access pattern: extract to a shared repository (only for that specific pattern)
• Duplicate HTTP helper: extract to platform/httpx/

Key Decisions by Language

Detect the project's language/framework and consult the appropriate reference:

• Patterns per language: See [references/patterns-by-language.md](references/patterns-by-language.md) for Go, .NET, Java, TypeScript, Python
• Testing per language: See [references/testing.md](references/testing.md) for testcontainers, mock verification, integration test patterns
• Core principles: See [references/principles.md](references/principles.md) for detailed rules, anti-patterns, and shared domain model guidance

Single Entry Point Contract

Every feature exposes one primary setup/registration function. Internal types stay private. The entry point name is conventional — the invariant is: one public function per feature that wires the slice to the framework.

• Go — convention Setup or RegisterRoute; signature func Setup(r gin.IRoutes, repo Repository); DI via explicit params.
• .NET — convention Map (static); signature static void Map(IEndpointRouteBuilder app); DI container resolves deps in handler.
• Java/Kotlin — @RestController class discovered by component scan; Spring DI (constructor injection).
• TypeScript — convention setup; signature function setup(router: Router, db: Database): void; DI via explicit params.
• Python — convention setup; signature def setup(router: APIRouter, db: Database) -> None; DI via explicit params or Depends().

Exceptions: Versioned APIs may have SetupV1/SetupV2 wrappers sharing internal handler wiring. Frameworks with auto-discovery (Spring, NestJS) use the controller/module class itself as the entry point.

Testing

See [references/testing.md](references/testing.md) for full testing strategy per language, including feature integration tests, platform/adapter tests, mock verification patterns, and test naming conventions.