Medium

High-level design problems at SDE-2 level. Focus: distributed systems, caching, replication, load balancing.

Problems

Problem	File	Key concepts
Twitter / News Feed	twitter-news-feed.md	Timeline, fan-out, Redis; see also twitter.md
Instagram	instagram.md	Image storage, CDN, follower/feed
YouTube	youtube.md	Video transcoding, CDN, recommendations
WhatsApp	whatsapp.md	Chat, WebSockets, message queue
Notification Service	notification-service.md	Push, pub-sub, fan-out, queues

Use the HLD template and trade-offs cheat sheet when practicing.