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The internet's phone book. A globally distributed, read-dominated, cache-everywhere hierarchy that has to answer in single-digit milliseconds and never go down.
Design the Domain Name System: the service that translates human-readable names (example.com) into IP addresses, and that powers traffic routing, failover, and load distribution for nearly every internet service.
This is a Hard topic because DNS is the canonical "globally distributed read system" - it teaches caching hierarchies, TTL trade-offs, anycast routing, and availability engineering under adversarial load. Interviewers use it to see whether you understand layered caching and eventual consistency at planetary scale.
Asking these before diving into a solution is the difference between a "hire" and a "no signal" rating. Pick the questions whose answers would change your design.
Capacity estimation · architecture with all 6 components explained · 4 deep dives · trade-off analysis · 8 common follow-up questions
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Edge cache hierarchies, cache key design, invalidation, origin shield, and edge compute - the system every other system relies on without thinking about it.
L4 vs L7, consistent hashing, health checks, connection draining, and the difference between a fleet that survives partial failures and one that cascades into outage.
The canonical bounded system design problem. Read-heavy, hot-key prone, and a great vehicle for hashing, caching, and capacity estimation.
Reading is the floor. The interview signal is in walking through this live with someone probing follow-ups. Use the AI mock interview to practice talking through requirements, architecture, and trade-offs out loud.
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