Product, Cart, Order classes in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the primary responsibility of the Order class

In a simple e-commerce system, what is the main responsibility of the Order class?

A Store details about a completed purchase including products, quantities, and payment status

B Calculate discounts and promotions for products in the cart

C Handle adding and removing products before checkout

D Manage the list of products a user wants to buy and their quantities

💡 Hint

Think about what happens after a user completes their purchase.

The Order class represents a finalized purchase. It stores the products bought, their quantities, and payment or shipping status. The Cart manages products before purchase.

❓ Architecture

intermediate

Choose the best class to calculate total price

Which class should be responsible for calculating the total price of all products including quantities and discounts?

A Product class, because it knows the price of each item

B Order class, because it represents the finalized purchase

C Cart class, because it holds all products and their quantities before purchase

D A separate PricingService class, to keep pricing logic outside data classes

💡 Hint

Consider where the products and quantities are managed before checkout.

The Cart class holds products and quantities before purchase, so it is best placed to calculate the total price including discounts. Product only knows single item price, Order is finalized, and PricingService is optional.

❓ scaling

advanced

Scaling the Cart for many users simultaneously

When many users add products to their carts at the same time, which design approach best ensures performance and data consistency?

A Store carts only in the database and query on every user action

B Use a global lock to allow only one cart update at a time

C Store all carts in a single shared database table with row-level locking

D Keep each user's cart in a separate in-memory cache instance with periodic database sync

💡 Hint

Think about fast access and avoiding bottlenecks for many users.

Using in-memory cache per user reduces database load and improves speed. Periodic sync ensures data persistence. Single table locking or global locks cause bottlenecks. Querying DB on every action is slow.

❓ tradeoff

advanced

Tradeoff between storing product details in Cart vs referencing Product IDs

What is a key tradeoff when the Cart stores full product details versus only product IDs with quantities?

A Storing full details increases data duplication but allows faster access without extra queries

B Storing only IDs causes data duplication but reduces database queries

C Storing full details reduces memory usage but requires more network calls

D Storing only IDs increases data duplication and slows down price calculations

💡 Hint

Consider data duplication and access speed.

Storing full product details in Cart duplicates data but avoids extra lookups, improving speed. Storing only IDs saves space but requires fetching product info separately.

❓ component

expert

Estimate capacity for Order processing system

Your e-commerce system expects 10,000 orders per minute at peak. Each order has on average 5 products. Estimate the minimum number of database writes per second needed to store orders and order items.

A Approximately 50,000 writes per second

B Approximately 3,000 writes per second

C Approximately 833 writes per second

D Approximately 10,000 writes per second

💡 Hint

Calculate total writes considering one write per order and one per product item.

10,000 orders/min ≈ 167 orders/sec. Each order: 1 write (order) + 5 writes (items) ≈ 1,000 writes/sec base. With overhead (logs, updates, etc.), approximately 3,000 writes/sec. Closest is C.