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81 lines
2.0 KiB
Markdown
81 lines
2.0 KiB
Markdown
---
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title: Avoid Circular Dependencies
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impact: CRITICAL
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impactDescription: "#1 cause of runtime crashes"
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tags: architecture, modules, dependencies
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---
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## Avoid Circular Dependencies
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Circular dependencies occur when Module A imports Module B, and Module B imports Module A (directly or transitively). NestJS can sometimes resolve these through forward references, but they indicate architectural problems and should be avoided. This is the #1 cause of runtime crashes in NestJS applications.
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**Incorrect (circular module imports):**
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```typescript
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// users.module.ts
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@Module({
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imports: [OrdersModule], // Orders needs Users, Users needs Orders = circular
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providers: [UsersService],
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exports: [UsersService],
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})
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export class UsersModule {}
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// orders.module.ts
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@Module({
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imports: [UsersModule], // Circular dependency!
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providers: [OrdersService],
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exports: [OrdersService],
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})
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export class OrdersModule {}
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```
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**Correct (extract shared logic or use events):**
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```typescript
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// Option 1: Extract shared logic to a third module
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// shared.module.ts
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@Module({
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providers: [SharedService],
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exports: [SharedService],
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})
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export class SharedModule {}
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// users.module.ts
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@Module({
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imports: [SharedModule],
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providers: [UsersService],
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})
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export class UsersModule {}
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// orders.module.ts
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@Module({
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imports: [SharedModule],
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providers: [OrdersService],
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})
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export class OrdersModule {}
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// Option 2: Use events for decoupled communication
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// users.service.ts
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@Injectable()
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export class UsersService {
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constructor(private eventEmitter: EventEmitter2) {}
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async createUser(data: CreateUserDto) {
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const user = await this.userRepo.save(data);
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this.eventEmitter.emit('user.created', user);
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return user;
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}
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}
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// orders.service.ts
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@Injectable()
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export class OrdersService {
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@OnEvent('user.created')
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handleUserCreated(user: User) {
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// React to user creation without direct dependency
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}
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}
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```
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Reference: [NestJS Circular Dependency](https://docs.nestjs.com/fundamentals/circular-dependency)
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