S

Stripe

Very Hard
22Challenges
3War Rooms
~34hPrep Time
3/22Completed

Deeply technical. Expect payments, API design, and real production scenarios.

14% complete
Interview rounds:Take-HomeCoding ×2System DesignInfrastructureCultural

DSA4

Payment System Double-Billing (Array)Finance flagged that 3 customers were charged twice. The transaction log array has N+1 ite…
Junior~20 min
Ledger Reconciliation (Two Pointers)Finance is running a daily reconciliation job. We have a sorted array of transaction amoun…
Junior~20 min
Real-time API Analytics (Queue/Deque)Design a HitCounter class that tracks the number of API hits. It should support `hit(times…
Mid~25 min
Nested Payload Decoder (Stack)Given an encoded string, return its decoded string. The encoding rule is: `k[encoded_strin…
Mid~35 min

PR Review3

await Inside forEach — Silent Async FailureA junior dev refactored the order-fulfilment service to send confirmation emails after eac…
Junior~10 min
Missing Idempotency in Payment WebhooksThe customer support team is flooded with tickets. Users are complaining they are being do…
Mid~15 min
The "Double-Click" Race ConditionUsers with slow internet connections occasionally report being charged twice for the same …
Mid~12 min

War Room4

N+1 Query Kills Database After Feature LaunchA new "Customer Overview" dashboard feature launched 30 minutes ago. Database CPU went fro…
Mid~20 min
gRPC Deadline Propagation Causing Cascading TimeoutService A calls Service B (gRPC, 500ms deadline). Service B calls Service C (no deadline s…
Senior~25 min
Webhook Delivery Failure (Event Ordering)Your order management system processes Stripe webhooks. Orders are getting stuck in a "pen…
Mid~25 min
Race Condition in Distributed Payment ProcessingYour loyalty points redemption system has a bug: some customers have negative point balanc…
Senior~30 min

System Design3

Design a Payment Gateway (Stripe)Architect a B2B payment API. It must strictly guarantee 100% data consistency, no dropped …
Senior~60 min
Design a Rate LimiterDesign a rate limiter that enforces API quotas: 100 requests/minute per user, 1000 request…
Mid~40 min
Design a Banking Ledger (Double-Entry)Design an immutable double-entry accounting ledger for a fintech platform. Every financial…
Senior~55 min

Tech Debt Tribunal5

Refactor the God Class (AuthManager)AuthManager.java is a 4,000-line file with a massive if/else chain for Email, Google, Appl…
Mid~45 min
Mutable Default Arguments (Python Anti-Pattern)A Python function uses a mutable list as a default argument. State leaks between calls — a…
Junior~15 min
Distributed Transaction Without Saga PatternAn order service calls inventory, payment, and shipping APIs in sequence with no compensat…
Senior~45 min
No Input Validation on Form SubmissionA payment form accepts any string as card number, expiry, and amount. Empty strings and ne…
Junior~20 min
No Idempotency Keys on Payment APIA payment retry mechanism sends the same charge request without idempotency keys. Network …
Senior~30 min

War RoomStripe-specific3

Double-Charge on Payment IntentsA latency spike caused duplicate confirmations on payment intents. 0.1% of charges are dup…
P0+1600 XP
Webhook Fan-Out CollapseStripe webhook fan-out collapsed the DB connection pool across 6 regions. $2.4M in revenue…
P0+1500 XP
Idempotency Key CollisionA client library bug is generating colliding idempotency keys for different transactions. …
P1+1200 XP