System Design Problems
22 canonical problems Staff candidates are asked across companies. Each comes with realistic scale numbers, requirements, key architectural decisions with rationale, common pitfalls, and the follow-up questions interviewers actually ask.
Social & Feeds
ML Systems
Platform
Rate Limiter at Scale
Enforce per-principal and global request limits across a distributed fleet with minimal latency overhead.
Webhook Delivery with Retries
At-least-once event delivery to customer endpoints with retries, isolation, and observability.
A/B Testing & Experimentation Infrastructure
Run thousands of overlapping experiments with statistically valid analysis and safe rollouts.
Real-time
Real-time Messaging Delivery
Reliably deliver chat messages in order, online and offline, with read receipts and presence.
Notification Fan-out at Billion Scale
Deliver targeted notifications to billions of devices with priority and per-user policy.
Real-time Collaborative Document Editor
Multiple users editing the same document with low latency and consistent convergence.
Storage
Financial
Idempotent Payment Processing API
Charge customers exactly once even when networks, clients, and servers retry aggressively.
Double-entry Ledger at Scale
Authoritative financial ledger that never loses, double-counts, or reorders entries.
Multi-tenant SaaS Billing System
Subscription, usage, and proration billing across many tenants without lost or duplicate charges.
Marketplace
Real-time Marketplace Dispatch & Matching
Match riders and drivers (or orders and couriers) in real time with geo-aware routing.
Real-time Surge / Dynamic Pricing
Compute geo-zone pricing multipliers in seconds based on local supply-demand imbalance.
Booking System Without Double-booking
Reservation system that never double-books inventory under concurrent demand.