System design (HLD), interview edition

API: POST /shorten → short code; GET /{code} → redirect.
Code: base62 of an auto-increment ID or hash: 7 chars gives 62⁷ ≈ 3.5 trillion URLs.
Storage: key-value store (code → long URL); read-heavy, so cache hot codes in Redis.
Redirect: 301 (permanent, browser caches) vs 302 (temporary, lets you count clicks).

Crisp answers on scaling, caching, databases at scale, queues, reliability: plus outline answers for the classic “design X” questions.

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