ghostfolio/.agents/skills/nestjs-best-practices/rules/di-scope-awareness.md

title	impact	impactDescription	tags
Understand Provider Scopes	CRITICAL	Prevents data leaks and performance issues	dependency-injection, scopes, request-context

Understand Provider Scopes

NestJS has three provider scopes: DEFAULT (singleton), REQUEST (per-request instance), and TRANSIENT (new instance for each injection). Most providers should be singletons. Request-scoped providers have performance implications as they bubble up through the dependency tree. Understanding scopes prevents memory leaks and incorrect data sharing.

Incorrect (wrong scope usage):

// Request-scoped when not needed (performance hit)
@Injectable({ scope: Scope.REQUEST })
export class UsersService {
  // This creates a new instance for EVERY request
  // All dependencies also become request-scoped
  async findAll() {
    return this.userRepo.find();
  }
}

// Singleton with mutable request state
@Injectable() // Default: singleton
export class RequestContextService {
  private userId: string; // DANGER: Shared across all requests!

  setUser(userId: string) {
    this.userId = userId; // Overwrites for all concurrent requests
  }

  getUser() {
    return this.userId; // Returns wrong user!
  }
}

Correct (appropriate scope for each use case):

// Singleton for stateless services (default, most common)
@Injectable()
export class UsersService {
  constructor(private readonly userRepo: UserRepository) {}

  async findById(id: string): Promise<User> {
    return this.userRepo.findOne({ where: { id } });
  }
}

// Request-scoped ONLY when you need request context
@Injectable({ scope: Scope.REQUEST })
export class RequestContextService {
  private userId: string;

  setUser(userId: string) {
    this.userId = userId;
  }

  getUser(): string {
    return this.userId;
  }
}

// Better: Use NestJS built-in request context
import { REQUEST } from '@nestjs/core';
import { Request } from 'express';

@Injectable({ scope: Scope.REQUEST })
export class AuditService {
  constructor(@Inject(REQUEST) private request: Request) {}

  log(action: string) {
    console.log(`User ${this.request.user?.id} performed ${action}`);
  }
}

// Best: Use ClsModule for async context (no scope bubble-up)
import { ClsService } from 'nestjs-cls';

@Injectable() // Stays singleton!
export class AuditService {
  constructor(private cls: ClsService) {}

  log(action: string) {
    const userId = this.cls.get('userId');
    console.log(`User ${userId} performed ${action}`);
  }
}

Reference: NestJS Injection Scopes