Design a stock trading platform

Design a stock trading platform. Users place market and limit orders; the platform matches buy and sell orders, updates each user's cash and positions exactly once per trade, and streams prices to users in real time.

Requirements

• Functional: place, modify and cancel market and limit orders; check buying power or holdings before accepting an order; match orders per symbol by price and time priority; settle trades into balances and positions; stream quotes and trades; show order status and history.
• Non-functional: very low latency in matching; strict ordering of orders per symbol; every trade applied to balances exactly once; no loss of accepted orders on failure; peaks at market open and close; a full audit trail.

Clarifying questions

• Do we run our own matching engine, or route orders to exchanges?
• How many symbols, users and orders per second at peak?
• Which order types beyond market and limit (stop, time in force)?
• Are there margin accounts or only cash accounts?
• How fast must price updates reach users?

What a strong answer covers

• An order flow: gateway, risk checks that reserve buying power, a sequencer that gives each order a position in a per-symbol log, and the matching engine.
• The matching engine: an in-memory order book per symbol (price levels with queues), single-threaded per symbol for determinism, partitioned across machines by symbol.
• Durability and recovery by replaying the sequenced log; hot standby engines.
• Settlement: trades as events applied idempotently to a ledger of cash and positions; releasing reservations on cancel or partial fill.
• Market data fan-out to many clients, with throttling and snapshots plus deltas.
• Handling the open and close peaks, audit logs, and reconciliation.

Follow-ups

• The matching engine for a busy symbol crashes mid-trade. How do you recover without losing or duplicating trades?
• Add stop orders that trigger on the last trade price.
• A user submits thousands of orders a second. How do you protect the system?
• Halt trading in one symbol on regulatory request.