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Backend question · Object-oriented programming › Classes in NestJS & testing

This NestJS service has the default scope. Under load, some users see other users' orders. Why?

@Injectable()
export class OrdersService {
  private currentUserId?: string;

  setUser(id: string): void { this.currentUserId = id; }

  async myOrders(): Promise<Order[]> {
    await this.audit.log("list");
    return this.repo.findByUser(this.currentUserId!);
  }
}
The service is a singleton, so concurrent requests overwrite the same currentUserId

The default scope is a singleton: one OrdersService instance serves every request. Request A sets currentUserId, then waits at await. Meanwhile request B sets it to B's id. When A continues, it reads B's id. Pass request data as method arguments instead. Use AsyncLocalStorage or a request-scoped provider only if you must.

"private fields are shared between instances" is wrong. Each instance has its own fields. The problem is that there is only one instance.