Nestjs Architecture

NestJS Architecture Expert

Priority: P0 (CRITICAL)

Design feature modules with singleton-first providers and explicit request-pipeline choices.

Decision Map

• Module role: Feature Modules (Auth) vs Core (Config/DB) vs Shared (Utils). Feature owns its controllers, application services, and adapters. Shared exports stateless helpers only.
• Provider scope: default singleton. Use request scope only for tenant context, request-local caching, or request tracking. Treat request scope as a measured exception.
• Pipeline choice: middleware for raw HTTP concerns, guard for access decisions, pipe for validation/transforms, interceptor for cross-cutting request/response behavior, filter for error translation.

Recipe

1. Create bounded feature: module + controller + application service + persistence adapter.
2. Keep controller thin: Thin controllers, fat services. No business in Controller. Move logic to Service.
3. Register dependencies once: imports for modules, providers for services, exports only for true consumers. @InjectRepository() dependencies need TypeOrmModule.forFeature([...]).
4. Validate at edges: DTO validation pipes for incoming data; never trust raw payloads in services.
5. Check circular imports: check circular dependencies with madge; refactor contracts first and use forwardRef() only as last resort.

Verify

• Feature module owns one business capability, not a mixed grab bag.
• Providers are singleton unless request scope has a written reason.
• Guard/pipe/interceptor/filter placement matches the request lifecycle.
• Config uses @nestjs/config or a typed config layer, not scattered process.env.
• Dependency Integrity: DB repositories, queues, and clients are registered in the module graph, not manually instantiated.

Anti-Patterns

• No request scope by default: Singleton first; justify heavier scope.
• No business logic in controllers: Delegate orchestration to services/use cases.
• No entity leakage: Don't return ORM entities; return DTOs or response models.
• No manual instantiation: Use DI; never new Service() inside Nest code.

References

• Framework Map
• Advanced Patterns
• Dynamic Modules