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

Module-boundary checklist

• Separate Feature Modules (Auth) from Core (Config/DB) and Shared (Utils); keep controllers thin and do not return ORM entities directly.

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• Dependency Integrity
• Don't return ORM entities
• Feature Modules (Auth) vs Core (Config/DB) vs Shared (Utils)