Design a Stock Price Watcher

Read this as three connected jobs before drawing anything. Prices flow in from a market data feed and have to reach every screen watching that symbol, fast. Watchlists and alert rules flow the other way, from the member into storage, at a trickle. And a background loop watches every incoming price against the stored rules and fires a notification the instant a threshold is crossed. The first path is read-heavy and latency-sensitive. The third is where the correctness lives. Naming those three paths in the first two minutes is how you keep this from turning into a vague "design a trading app."

The scale is smaller than it first sounds, and saying so early earns credit. The US market has on the order of ten thousand tradable symbols, and watchers cluster hard on a few hundred popular names. The latest price for every symbol is a few hundred kilobytes total, so the whole live-price state fits in memory many times over. What is genuinely large is the fan-out on reads during market hours, and the number of alert rules to check on every tick. That is where the work is. Those two are the design. Everything else is plumbing.

The SoFi angle

This is a SoFi system. SoFi Invest is a real member-facing product, so a price watcher with alerts is the kind of thing the team operates. The interviewer will treat correctness as a trust question. A duplicate alert that pushes the same "TSLA crossed $250" twice looks broken, and a missed alert on a threshold a member set on purpose is worse. That maps straight onto the value SoFi leads its rubric with, putting members' interests first. The round is reported as a dedicated forty-five to sixty minute session, weighted to the senior and final loops. The sibling billing prompt states the house register out loud: solve it as simply as possible, do not introduce too much extraneous architecture. Bring the same restraint here. The signal is in reading the access pattern and getting the notify-on-crossing path exactly right, not in bolting on infrastructure the numbers do not ask for.