Design a Stock Trading System (Zerodha)

Design a stock-trading platform like Zerodha, end to end, with both HLD and LLD. Functional requirements: user registration, buy/sell stock, view portfolio, and fetching live stock data from the exchange. Non-functional: consistency, scalability, availability, data recovery. Provide classes, an ER diagram / DB design, and address follow-ups: data recovery if a regional cluster goes down, scalable notifications when a stock fluctuates, limitations from stock-exchange API availability, and circuit breaking when an exchange is down.