Why should you care?

Online retail needs fast product APIs, stock consistency, and payment integration patterns.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

// meanverse-shop/
//   api/src/routes/orders.routes.ts
//   web/src/app/features/shop/cart.service.ts

router.post('/orders', auth, validateCreateOrder, async (req, res, next) => {
  try {
    const order = await orderService.checkout(req.user.id, req.body);
    res.status(201).json(order);
  } catch (e) { next(e); }
});

@Injectable({ providedIn: 'root' })
export class CartService {
  items = signal<CartItem[]>([]);
  total = computed(() => this.items().reduce((s, i) => s + i.priceCents * i.qty, 0));
  private http = inject(HttpClient);
  checkout() { return this.http.post<Order>('/api/shop/orders', { items: this.items() }); }
}

What happened?

• checkout service runs transaction: decrement stock, create order, call payment.
• CartService uses signals for reactive total; checkout POST sends cart snapshot.

Practice next

1. Model Product with embedded variants; Order with line items.
2. build checkout Mongo transaction.
3. Angular product list with cart signal state.
4. Add coupon code validation middleware.
5. Implement order status Socket.IO push to customer.

Remember

E-commerce = catalog + cart + checkout transaction. Signals drive cart UI reactively. Payment tokens only — PCI scope minimal.

Flash sale

10k concurrent checkout attempts on limited SKU.

Outcome: Transaction + inventory lock prevents oversell; queue smooths payment API.