System Design and Caching Round

What they ask

The technical interview is mainly system design, with side questions on databases, caching and optimization, and an emphasis on reliability and data consistency because the product moves real money.

Prompts in this area, for example:

• Design the live market data feed: prices for a few thousand symbols pushed to hundreds of thousands of app users during trading hours, with a latency budget of a couple of seconds.
• Design the portfolio and money-movement flow: deposits, withdrawals, buy orders, and a balance that must never go negative or double-spend.

Side questions reported: which cache to use and how to invalidate it when a price changes; when a read replica is enough and when it is not; which indexes you would add to a trades table; what ACID means for a transfer between two accounts; how you would find a slow endpoint.

What they look for

• Fan-out design for the feed (pub/sub, WebSockets or SSE, batching, backpressure) and a story for clients that disconnect.
• Transaction safety for money: idempotency keys, ledger-style append-only records, isolation level choices, reconciliation jobs.
• Practical caching: what is cached, TTL vs event-driven invalidation, stampede protection.
• Trade-off talk; they push on "what breaks first" and "what do you monitor".