Design a scalable stock trading platform that can handle heavy traffic without system downtime

Start by clarifying the exact contract for Stock Trading Platform: input representation, mutation rules, duplicate behavior, and what should happen on invalid or empty input. The core idea is scoping financial-system correctness before infrastructure. Work a tiny example aloud and name the invariant after each step, because that is the proof that the approach stays correct as the input grows.

A dependable implementation plan is to clarify order and latency requirements; use idempotent durable commands; partition by instrument; sequence matching per partition; derive read models from immutable events. Acknowledged orders need auditability and replay, not only cache availability. Keep the code aligned with that invariant instead of adding special cases that silently change the contract. Test the smallest input, a normal case, duplicates or ties where relevant, an adversarial boundary, and a case that exercises the failure path. The expected complexity is matching updates are commonly O(log n); capacity depends on workload. Explain both time and auxiliary memory, including recursion or helper structures; do not hide sorting, copying, or repeated scans. If the title omits critical constraints, say what you need to know before committing to a more specialized solution.