Design an electronic trading platform that supports equities trading with limit and market orders. Specify how you would: ( 1) maintain per-symbol order books and implement a price–time priority matching engine; ( 2) ingest and normalize market data from multiple venues and publish snapshots plus incremental updates with bounded skew; ( 3) expose authenticated APIs for order entry (REST/gRPC) and a low-latency streaming protocol for market data; ( 4) perform pre-trade risk checks (credit/position/price bands), ensure idempotent submissions, and provide exactly-once execution reports; ( 5) achieve P99 end-to-end acknowledgement under 5 ms within one region at 200k orders/second peak, and scale horizontally for bursts; ( 6) persist state with an append-only log or event sourcing, support deterministic replay/backtesting, and meet strict auditability; ( 7) handle partial fills, cancels, mass cancels, auctions, market halts, and symbol pauses; ( 8) design partitioning/sharding of order books, concurrency control inside the matcher, recovery and disaster recovery (RPO ≈ 0, RTO < 1 minute); ( 9) address fault tolerance, time synchronization and clock skew (e.g., PTP), fairness across partitions, backpressure, and flow control; ( 10) define monitoring, SLOs, capacity planning, and strategies to test latency and correctness under failures.