System Design Interview Playbook

Design a high-scale system capable of monitoring real-time stock price movements and triggering user-defined alerts (e.g., 'Notify me if AAPL goes above $180'). The system must handle 10 million daily active users, 100 million configured alerts, and a stream of 100,000 price updates per second. Focus on minimizing the latency between a price crossing a threshold and the notification delivery, while ensuring the system remains resilient to massive bursts in market activity.

Design a global calendar system similar to Google Calendar. The system must support creating, updating, and deleting events, including complex recurring schedules (e.g., 'every second Tuesday'). It must handle invitations and RSVPs for millions of concurrent users. Key challenges include efficient time-range queries across different timezones, handling recurring event exceptions, and ensuring high availability for viewing schedules while maintaining transactional consistency for event edits. Provide a detailed storage schema and explain how you would scale the system for 100 million daily active users.

Design a comprehensive geospatial system similar to Google Maps that supports global map rendering, high-concurrency point-of-interest (POI) search, and real-time navigation. The system must handle 100M+ daily active users, provide sub-second routing responses, and incorporate real-time traffic data to adjust ETAs. Address specific challenges regarding geospatial indexing, massive data ingestion for location pings, and efficient distribution of map assets at scale.

Design a globally distributed URL shortening service capable of handling 1 million redirection requests per second and 10,000 shortening requests per second. The system must ensure unique short-code generation without collisions, provide sub-50ms latency for redirects, and scale to store over 1 trillion mappings over 5 years. Address how you would handle hot keys, distributed ID generation, and data consistency across multiple geographic regions.

Design a distributed web crawler capable of traversing and indexing the global web. The system must manage 10 billion+ documents monthly, ensuring strict adherence to politeness protocols (robots.txt), efficient URL deduplication, and high fault tolerance against spider traps and varied server behaviors. Discuss how you would handle URL prioritization and the trade-offs between storage costs and crawl freshness.

Design a high-concurrency railway booking system similar to IRCTC capable of handling 10 million daily users and extreme traffic surges (e.g., millions of concurrent users during a 1-hour window). The system must support train searches, real-time availability checks, and atomic booking transactions with strong consistency. Address how you would prevent overbooking, handle massive spikes in traffic, and ensure low-latency search results under load.

Design a globally distributed payment system similar to Stripe. The system must support high-volume transaction processing, ensure strict idempotency to prevent double-charging, and maintain an immutable ledger for financial auditing. Focus on the end-to-end lifecycle of a payment, including interaction with external Payment Service Providers (PSPs), handling asynchronous webhooks, and ensuring system-wide consistency and PCI-DSS compliance constraints.

Design a globally scalable real-time messaging application similar to WhatsApp or Slack. The system must support millions of concurrent WebSocket connections, ensure message persistence with high write throughput, and provide real-time presence tracking (online/offline status). Detail how you would handle message delivery for users who are currently offline, ensure message ordering, and scale the session management layer to handle millions of active users while maintaining low latency.

Design a real-time chat application capable of supporting 10 million daily active users and 1 million concurrent connections. The system must support instant 1-on-1 messaging, group chats with up to 500 members, and real-time user presence (online/offline status). Ensure the design addresses message persistence, low-latency delivery, and synchronization across multiple devices, while discussing the trade-offs between consistency and availability in a distributed environment.

Design a high-performance, distributed rate-limiting system capable of handling millions of requests per second across a global API infrastructure. The system must support multiple limiting strategies (e.g., sliding window), ensure sub-millisecond overhead, and maintain high availability even during partial network partitions or cache failures. Address how you would handle race conditions in a distributed environment and discuss the trade-offs between accuracy and system latency.

Design a high-scale real-time messaging platform like Slack that supports millions of concurrent users. The system must handle private and group channels (up to 100k members), provide instant message delivery with strict ordering within channels, and track user presence (online/offline). Discuss how you would handle massive fan-out for popular channels, ensure high availability despite stateful connections, and optimize storage for billions of daily messages.

Design a global music streaming service similar to Spotify. The system must support a library of 100 million tracks and 500 million users. Key challenges include low-latency audio delivery (minimal buffering), efficient search across a massive catalog, and high-throughput tracking of playback events for royalty and analytics purposes. Address how you would handle global content distribution, metadata management, and the trade-offs between consistency and availability in user-generated content like playlists.