Mock system design interview at Stripe
A live 45-minute round on a question reported in Stripe interviews, run by an AI interviewer that asks one thing at a time, probes what you say, looks at your whiteboard, and weighs what that round is reported to weigh. It ends with a debrief: a score, what was strong, what was missing.
Design a Payment System
Accept card payments for an e-commerce platform, move money correctly exactly once, and be able to prove it.
You will not see the requirements, the numbers or the design before you start. That is the point.
What the Stripe round is reported to look like
45–60 minutes, typically money-adjacent: correctness, idempotency and reconciliation over raw scale.
What it weighs
- Exactly-once effects, idempotency keys and retries
- Ledger and state-machine design, including the unknown-outcome case
- API design and the developer experience of the thing you are building
- Reconciliation and what happens when an external system disagrees with you
The common mistake
Optimising for throughput when the question is about never losing or double-charging a payment.
From public interview guides and candidate reports, and Stripe’s own interview guidance where it publishes any. Not endorsed by Stripe. The interviewer is an AI running in that style; it is practice, not a prediction.
Start the round
The interviewer’s opening question is free. Answering it is part of Pro. See an example round and its debrief →
The 14 questions this round draws from
Each is reported in Stripe interviews. Pick one to sit, or read its reference design and walk through a strong answer first.
- Design a Payment SystemAccept card payments for an e-commerce platform, move money correctly exactly once, and be able to prove it.drawn
- Design a Distributed CacheA Redis-like cache as a service: hundreds of nodes, terabytes of memory, sub-millisecond gets, and a cluster that survives losing a machine mid-request.
- Design a Rate LimiterA distributed service that decides, in under a millisecond, whether a request from a client is allowed under its quota.
- Design TicketmasterSell 60 000 seats in ten minutes to a million people at once: hold a seat while someone pays, never sell it twice, and keep the site up when demand is a hundred times supply.
- Design a Distributed Job SchedulerRun millions of cron jobs and one-off tasks on time across a fleet, exactly once each, and keep going when a worker dies halfway through one.
- Design a Stock ExchangeMatch buy and sell orders in microseconds, in a strict and provable order, never lose a fill, and stay correct when the matching engine dies mid-trade.
- Design a Unique ID GeneratorHand out a million 64-bit ids a second across many hosts and regions, never twice, roughly in time order, with no central bottleneck and no trust in the wall clock.
- Design AWS Step FunctionsA managed workflow engine that runs 200 M state-machine executions a day, some lasting a year, applying every transition exactly once.
- Design an API GatewayOne front door for public and partner APIs: authenticate, limit and route 1 M requests a second to 300 services, adding under 10 ms.
- Design GitHubHost 400 M git repositories on replicated file servers, serve 100 k fetches a second, and run pull requests, forks and webhooks on top.
- Design Venmo80 M users paying friends instantly: 15 M transfers a day on a sharded double-entry ledger, funded and cashed out over slow bank rails.
- Design a Fraud Detection SystemAllow, block or challenge 10,000 card payments a second in under 100 ms, then learn from chargebacks that arrive months later.
- Design a Webhook Delivery SystemDeliver 3 billion signed webhooks a day to a million customer endpoints, retrying for three days, without one slow endpoint delaying anyone else.
- Design RobinhoodA retail brokerage: live prices to 3 M phones, 20 k orders a second at the open, every share and cent exact.
Other companies: Anthropic · Google · Meta · Amazon · OpenAI · Microsoft · Uber · DoorDash