Design a frontend shopping cart and feature flags

Q: Design a frontend shopping cart and feature flags

This question evaluates frontend system-design skills including client-side state management, synchronization and conflict-resolution strategies, caching and offline resilience, performance for large carts, observability, and feature-flag/remote-config design for targeted rollouts and experiments.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a easy difficulty System Design question, commonly asked during Onsite rounds at Disney.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Disney during technical interviews.

Question

Scenario

You are designing frontend-focused systems for a large e-commerce web app.

Part A — Shopping cart (frontend focus)

Design the shopping cart experience across web sessions and devices.

Cover:

Requirements: anonymous vs. logged-in carts, multi-tab behavior, offline/poor network, real-time price/stock changes.
Data model (cart item, quantity, variants, promotions).
Client state management and caching strategy.
Sync strategy with backend (optimistic UI, conflict resolution, idempotency).
Performance (rendering large carts, minimizing API calls).
Observability (metrics, logging) and failure handling.

Part B — Feature flag system (frontend focus)

Design a feature flag / remote config system used to gate UI features.

Cover:

Targeting (user/segment/percentage rollout), environments, and kill switch.
Delivery to clients (SDK, bootstrap, caching, polling vs. push).
Consistency guarantees (what users see across refreshes/devices).
Safety (flag dependencies, defaults, failure modes).
Experimentation/A-B testing hooks and metrics.

Assume you can propose APIs and backend components as needed, but focus on frontend architecture and end-to-end behavior.

Design a frontend shopping cart and feature flags

Quick Overview

Scenario

Part A — Shopping cart (frontend focus)

Part B — Feature flag system (frontend focus)

Solution

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