95 lines
2.6 KiB
Markdown
95 lines
2.6 KiB
Markdown
---
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title: Understand Provider Scopes
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impact: CRITICAL
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impactDescription: Prevents data leaks and performance issues
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tags: dependency-injection, scopes, request-context
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---
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## Understand Provider Scopes
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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.
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**Incorrect (wrong scope usage):**
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```typescript
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// Request-scoped when not needed (performance hit)
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@Injectable({ scope: Scope.REQUEST })
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export class UsersService {
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// This creates a new instance for EVERY request
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// All dependencies also become request-scoped
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async findAll() {
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return this.userRepo.find();
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}
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}
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// Singleton with mutable request state
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@Injectable() // Default: singleton
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export class RequestContextService {
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private userId: string; // DANGER: Shared across all requests!
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setUser(userId: string) {
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this.userId = userId; // Overwrites for all concurrent requests
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}
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getUser() {
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return this.userId; // Returns wrong user!
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}
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}
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```
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**Correct (appropriate scope for each use case):**
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```typescript
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// Singleton for stateless services (default, most common)
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@Injectable()
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export class UsersService {
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constructor(private readonly userRepo: UserRepository) {}
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async findById(id: string): Promise<User> {
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return this.userRepo.findOne({ where: { id } });
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}
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}
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// Request-scoped ONLY when you need request context
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@Injectable({ scope: Scope.REQUEST })
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export class RequestContextService {
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private userId: string;
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setUser(userId: string) {
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this.userId = userId;
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}
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getUser(): string {
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return this.userId;
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}
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}
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// Better: Use NestJS built-in request context
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import { REQUEST } from '@nestjs/core';
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import { Request } from 'express';
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@Injectable({ scope: Scope.REQUEST })
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export class AuditService {
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constructor(@Inject(REQUEST) private request: Request) {}
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log(action: string) {
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console.log(`User ${this.request.user?.id} performed ${action}`);
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}
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}
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// Best: Use ClsModule for async context (no scope bubble-up)
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import { ClsService } from 'nestjs-cls';
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@Injectable() // Stays singleton!
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export class AuditService {
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constructor(private cls: ClsService) {}
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log(action: string) {
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const userId = this.cls.get('userId');
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console.log(`User ${userId} performed ${action}`);
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}
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}
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```
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Reference: [NestJS Injection Scopes](https://docs.nestjs.com/fundamentals/injection-scopes)
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