Product, Cart, Order classes in LLD - System Design Exercise

Design: E-commerce Cart and Order Management

Design focuses on the core classes and their interactions for product catalog, cart management, and order processing. Payment, shipping, and user authentication are out of scope.

Functional Requirements

FR1: Allow users to browse products with details like name, price, and stock quantity

FR2: Users can add products to a shopping cart with specified quantities

FR3: Users can update or remove items in the cart

FR4: Users can place orders from their cart

FR5: Orders should capture product details, quantities, prices, and total amount

FR6: Support multiple users with separate carts and orders

Non-Functional Requirements

NFR1: Handle up to 10,000 concurrent users

NFR2: Ensure cart updates and order placements have p99 latency under 200ms

NFR3: Maintain data consistency between cart and order

NFR4: Availability target of 99.9% uptime

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Product catalog management

Cart service to manage user carts

Order service to process and store orders

Database or in-memory storage for persistence

APIs for user interactions

Design Patterns

Domain-Driven Design for modeling Product, Cart, and Order entities

Event-driven updates for stock changes

Repository pattern for data access abstraction

Transactional consistency for order placement

Reference Architecture

  +-------------+       +------------+       +------------+
  |   Product   |<----->|    Cart    |<----->|   Order    |
  +-------------+       +------------+       +------------+
        ^                     ^                    ^
        |                     |                    |
  Product Catalog       User Session          Order History
  (Database)            (In-Memory/DB)        (Database)

Components

Product

Class with attributes

Represents items available for purchase with details like id, name, price, and stock quantity

Cart

Class with collection of CartItems

Holds products selected by a user with quantities before order placement

Order

Class capturing finalized purchase details

Records purchased products, quantities, prices, total amount, and order status

Request Flow

1. User browses products from Product catalog

2. User adds product with quantity to Cart

3. Cart updates item quantities or removes items as requested

4. User places order from Cart

5. Order is created with snapshot of Cart items and prices

6. Stock quantity in Product catalog is updated accordingly

7. Order status is tracked until completion

Database Schema

Entities: - Product(id PK, name, price, stock_quantity) - Cart(id PK, user_id FK) - CartItem(id PK, cart_id FK, product_id FK, quantity) - Order(id PK, user_id FK, total_amount, status, created_at) - OrderItem(id PK, order_id FK, product_id FK, quantity, price_at_purchase) Relationships: - One Product can be in many CartItems and OrderItems - One Cart has many CartItems - One Order has many OrderItems - User has one Cart and many Orders

Scaling Discussion

Bottlenecks

Database contention on Product stock updates during high order volume

Cart data consistency when multiple devices update simultaneously

Latency in order placement under heavy load

Storage growth for order history over time

Solutions

Use optimistic locking or versioning on Product stock to prevent overselling

Implement distributed locks or atomic operations for Cart updates

Introduce caching layers and asynchronous processing for order creation

Archive old orders and use partitioned databases for scalability

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing classes and their interactions, 10 minutes discussing scaling and trade-offs, and 5 minutes summarizing.

Explain clear separation of concerns between Product, Cart, and Order

Discuss data consistency challenges and solutions

Highlight how the design supports multiple users and concurrent updates

Mention scalability considerations and how to handle growth

Show understanding of real-world e-commerce workflows