# Design a shopping cart service: system design interview question

> Persistent carts across devices: concurrent edits, guest cart merges, and changes in product availability.

Source: https://sysround.com/questions/design-shopping-cart-service
Updated: 2026-11-05


Difficulty: medium. Levels: Mid-level, Senior, Staff+.

## How the interviewer opens it

"Design a persistent shopping cart for an online store. Shoppers can shop as guests or sign in from different devices."

## What to clarify first

- Scale and traffic
- Cart size and lifetime
- Guest and account identity
- Cart edits
- Merge rules
- Product changes
- Latency
- Correctness and durability
- Deployment and dependencies
- Scope limits

## Scope of a strong answer

Core requirements:

- Keep guest carts and account carts across visits
- Apply concurrent cart edits without silent data loss
- Merge a guest cart into an account cart once
- Show product prices and availability without inventory reservations

Usually out of scope:

- Checkout and payment
- Inventory reservations
- Coupons and tax calculation
- Implementation of the identity and catalog services

## What interviewers probe

### Concurrent cart edits

Two devices can act on the same old cart. The service must distinguish additions from replacements to avoid silent data loss.

### Lost responses and repeated requests

An addition can commit even when the client receives no response. A later retry must not repeat that addition.

### Guest carts at sign-in

A merge changes two carts while devices can still edit both. The service must apply quantity caps without repeated contributions or partial results.

### Product changes

A durable cart does not guarantee a price or stock. The catalog can fail while the cart remains available.

## Common mistakes

- The design stores the only cart copy in the browser or a cache.
- The server replaces the whole cart without a version check.
- A retry adds an item twice or repeats a guest merge.
- The merge overwrites the account cart, loses an edit, or applies the wrong quantity cap.
- The cart reserves stock or promises a price that checkout cannot honor.
- The server changes millions of carts when one product becomes unavailable.

## The bar by level

- **Mid-level:** The candidate defines guest and account ownership, a durable data model, and clear cart APIs. The candidate prevents lost edits and applies the stated merge limits.
- **Senior:** The candidate makes edits and merges atomic and safe to retry. The candidate resolves merge races, defines conflict responses, and handles catalog failures.
- **Staff+:** The candidate traces failures across guest edits, account edits, and retries. The candidate protects account access after a merge. The candidate justifies region failover limits, conflict boundaries, and catalog costs with numbers.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=shopping-cart


---
SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.