System Design: Real‑Time Stock Prices Viewer (No History)

You are designing a real‑time system that lets clients view current stock prices only. No historical queries are supported.

Assume:

• Real‑time view of last trade price and top‑of‑book (best bid/ask) per symbol.
• Multiple upstream market feeds (e.g., SIP and/or direct exchange feeds).
• Millions of concurrent end users with watchlists.

Describe and justify a design that covers:

1. Ingestion and Processing
   • Ingesting market data feeds (parsing, sequencing, network considerations).
   • Symbol normalization and reference data management.
   • Deduplication and out‑of‑order handling.
   • Aggregation (per‑venue books, cross‑venue top‑of‑book, last trade).
2. Client Dissemination
   • Transport and pub/sub choices (e.g., WebSockets vs. SSE vs. long polling).
   • Fan‑out strategy and edge distribution.
   • Caching current values for fast snapshots.
3. API and Controls
   • API design for initial snapshot and live updates.
   • Throttling and quotas (per connection, per symbol, message rates).
4. Performance and Reliability Goals
   • Latency and consistency targets.
   • Backpressure policy and delivery semantics to clients.
   • Horizontal scaling to millions of concurrent users.
5. Resilience and Operations
   • Fault tolerance, failover, and multi‑region active/active deployment.
   • Monitoring, alerting, and on‑call runbooks.
   • Testing strategies (including load tests and simulators).

Keep the scope to real‑time only (no historical query capabilities). State reasonable assumptions where needed and explain trade‑offs.