Design a Stock Order / Trade Management System

Problem Statement

Design a stock order and trade management system for a brokerage like Robinhood. Users should be able to place buy and sell orders, cancel open orders, see real-time status updates, and trust that orders are durable, auditable, and never executed twice.

Reported Robinhood interview signals for this question include:

• External exchange or broker APIs are provided, and you need to design how users place stock orders through them.
• The interviewer may emphasize reliability, durability, and explicit order state-machine behavior.
• Follow-ups often focus on database schema, API design, place/cancel edge cases, partial fills, and real-time order updates.
• Some loops connect this with Robinhood's fractional-share inventory problem: a user may trade 0.5 shares while the brokerage ultimately interacts with venues in whole-share quantities or through an internal inventory layer.

This is similar to the Coinbase crypto order-flow question because both are order-management systems with async venue callbacks, idempotency, timeouts, and reconciliation. The Robinhood version should lean harder into equity-brokerage constraints: market hours, buying-power and share reservations, cancel/fill races, fractional inventory, regulatory audit trails, and end-of-day reconciliation.