Design an e-commerce platform (Amazon) order management system.

Product catalog: MongoDB (variable attributes per product type) Inventory: Redis for real-time count + PostgreSQL for ACID stock management Orders: PostgreSQL for ACID transactions (payment + order + inventory must be atomic) Order history: MongoDB or Cassandra for fast retrieval by userId Cart: Redis Hash per user with TTL; persist to DB on checkout Sessions: Redis with HttpSession (Spring Sessio