At Robinhood, we have an order entry system that accepts both buy and sell orders from our clients. These orders must be validated first before they are forwarded to a third party exchange.
For this question, we will only focus on limit order instead of market orders. Because limit orders do not receive immediate executions from the stock market, there can be an arbitrary wait. This means the orders can stay open forever.
Our system must tolerate concurrent requests and server instance failures without data corruption or 'double spending' issues.