01
Level 1 of 5
Foundation
Where you learn to think in systems. Clean problems, clear tradeoffs, and the building blocks every harder level assumes you already know.
FOUNDATION
Unique ID Generator
Design a distributed ID generation service — globally unique, sortable, 10k IDs/sec. Covers UUID, Snowflake, ticket servers, and clock skew.
Snowflake UUID Ticket Server Clock Skew Fault Isolation
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FOUNDATION
URL Shortener
Design bit.ly — takes a long URL, returns a short code, redirects at scale. 100M DAU, 1B redirects/day.
Sharding Hot Keys Caching Pre-Generated Keys Cold Storage Fault Isolation
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