Dự án: refactor codebase legacy theo nguyên tắc SOLID

❌ Trước - God class, gắn chặt mọi thứ

class EnrollmentService {
  enroll(userId: string, courseId: string, card: Card): void {
    // (1) kiem tra hop le
    if (!userId || !courseId) throw new Error("Thieu thong tin");

    // (2) lay gia + thanh toan - gan chat Stripe & DB cu the
    const price = mysql.query("SELECT price FROM courses WHERE id=?", courseId);
    stripe.charge(card, price);

    // (3) luu ghi danh - gan chat MySQL
    mysql.query("INSERT INTO enrollments(user, course) VALUES(?,?)", userId, courseId);

    // (4) gui email - gan chat SMTP
    smtp.send(userId, "Ban da ghi danh thanh cong!");
  }
}

Đặc tả hành vi hiện tại (lưới an toàn)

it("ghi danh: charge dung gia roi luu + bao tin", () => {
  // thay the cac dich vu that bang spy de QUAN SAT hanh vi hien tai
  const calls = trackCalls(); // ghi lai charge/insert/send da xay ra
  new EnrollmentService().enroll("u1", "c1", card);
  expect(calls).toEqual(["charge", "insert", "send"]); // chot thu tu hien tai
});

Tách theo SRP

class EnrollmentValidator {
  validate(userId: string, courseId: string): void {
    if (!userId || !courseId) throw new Error("Thieu thong tin");
  }
}
class PaymentService    { pay(card: Card, amount: number): void { /* ... */ } }
class EnrollmentRepo    { save(userId: string, courseId: string): void { /* ... */ } }
class EnrollmentMailer  { confirm(userId: string): void { /* ... */ } }

Sau - nghiệp vụ chỉ phụ thuộc abstraction

interface PaymentGateway { charge(card: Card, amount: number): void; } // OCP: them cong = them lop
interface EnrollmentRepository { save(userId: string, courseId: string): void; } // DIP
interface Mailer { confirm(userId: string): void; } // ISP: interface nho
interface PriceLookup { of(courseId: string): number; } // tra cuu gia

class EnrollmentService {
  constructor(
    private validator: EnrollmentValidator,
    private payments: PaymentGateway,        // tiem abstraction
    private repo: EnrollmentRepository,
    private mailer: Mailer,
    private prices: PriceLookup,
  ) {}

  enroll(userId: string, courseId: string, card: Card): void {
    this.validator.validate(userId, courseId);
    this.payments.charge(card, this.prices.of(courseId));
    this.repo.save(userId, courseId);
    this.mailer.confirm(userId);
  }
}

// Composition root - chon implementation o MOT noi
const service = new EnrollmentService(
  new EnrollmentValidator(),
  new StripeGateway(),     // doi sang MoMoGateway -> chi sua o day
  new SqlEnrollmentRepo(),
  new SmtpMailer(),
  new SqlPriceLookup(),
);

enrollment.test.ts - tiêm fake, không chạm dịch vụ thật

class FakeGateway implements PaymentGateway {
  charged: number[] = [];
  charge(_card: Card, amount: number) { this.charged.push(amount); }
}

it("charge dung gia cua khoa hoc", () => {
  const gw = new FakeGateway();
  const service = new EnrollmentService(
    new EnrollmentValidator(), gw,
    new FakeRepo(), new FakeMailer(),
    { of: () => 499000 },           // PriceLookup gia lap
  );
  service.enroll("u1", "c1", card);
  expect(gw.charged).toEqual([499000]);
});