Design real-time stock price viewer

Design a system to let clients view real-time stock prices only (no historical queries). Describe ingestion from market data feeds, symbol normalization, deduplication, and aggregation; how you would disseminate updates to clients (e.g., pub/sub, WebSockets vs. long polling); API design and throttling; caching and fan-out; latency and consistency goals; backpressure and delivery semantics; horizontal scaling to millions of concurrent users; fault tolerance, failover, and multi-region deployment; and monitoring, alerting, and testing strategies.