Design a Real-Time Recommendation System for E-Commerce

You are designing a real-time recommendation system for a large e-commerce platform with the following scale:

• 100 million monthly active users
• 50 million products in the catalog
• 500 million user-item interaction events per day
• Target latency: under 100ms for personalized recommendations on the homepage
• The system must serve recommendations across homepage, product detail page, and email campaigns

Requirements

1. Design the end-to-end architecture from user event ingestion to serving recommendations.
2. Address the cold-start problem for new users and new products.
3. Explain how you would handle the explore-exploit tradeoff.
4. Describe how you would measure success and run online experiments.
5. Discuss how the system handles seasonality (e.g., holiday shopping spikes).