🛒 System Design: Dynamic eCommerce UIs with BFF Pattern (Senior Frontend Interview Guide)
Category: system-design
Difficulty: hard
Target Level: Senior Frontend Engineer / Staff Engineer Duration: 45-60 minutes Interview Focus: Backend for Frontend (BFF), Config-Driven UI, Feature Flags, Dynamic Forms, A/B Testing Interview Importance: 🔴 Critical — eCommerce companies heavily test on this topic because dynamic UIs drive conversion rates, enable rapid experimentation, and power festival/event-based campaigns that generate massive revenue spikes. Interview Approach & What Interviewers Look For When asked to design a dynamic eCommerce UI system that adapts based on festivals, user segments, or A/B tests, interviewers are evaluating: BFF Pattern Understanding: Do you know when and why to use Backend for Frontend as an orchestration layer? Config-Driven Architecture: Can you design UIs that change without code deployments? Dynamic Forms: Can you build form systems that adapt based on user context, product types, or business rules? Feature Flags & A/B Testing: How do you enable rapid experimentation without breaking production? Performance: Can you keep the UI fast despite fetching configurations, user data, and personalization? Scalability: Can your system handle Black Friday/Diwali traffic spikes (10-100x normal load)? Pro Tip: Real eCommerce sites (Amazon, Flipkart, Shopify) switch themes, layouts, and forms instantly during festivals. Your design must support zero-downtime updates and instant rollbacks. 1️⃣ What are Dynamic eCommerce UIs? Dynamic eCommerce UIs are interfaces that automatically adapt their appearance, layout, content, and behavior based on: Temporal Events: Festivals (Diwali, Christmas, Black Friday), flash sales, seasonal campaigns User Context: Location, language, purchase history, device type, loyalty tier Business Rules: Inventory levels, regional regulations, payment methods, shipping options Experimentation: A/B tests, feature rollouts, personalization algorithms Visual Example [code example] Real-World Analogy: Think of your favorite streaming app's homepage. Netflix shows...