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Design a frontend shopping cart and feature flags

DisneySoftware Engineer · Onsite
EasySoftware EngineerOnsiteSystem Design

Last updated

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.

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.