The System Design Interview — RESHADED + a Traced Mock

A framework so you never freeze on "design X"

The system-design interview rewards a structured approach far more than a clever answer. RESHADED gives you a checklist and a time budget; the cardinal sin is drawing boxes before you've scoped the problem.

Traced mini-mock: "design a URL shortener"

1. R — Requirements: shorten a URL, redirect; ~100M new URLs/month, 100:1 read:write; low latency redirect; links don't expire. (Functional + non-functional, agreed first.)
2. E — Estimation (use the 5-step method): ~40 writes/s, ~4,000 reads/s; ~500 B/record × 5 yr ≈ ~3 TB. → read-heavy → cache + replicas. (Cross-link Capacity Estimation.)
3. S/A — Schema & API: POST /shorten {url} -> {short}; GET /{short} -> 301. Table (short_code PK, long_url, created_at).
4. H — High-level: client → LB → app → cache (hot codes) → DB; key generation = base62 of a counter / Snowflake id (cross-link Unique ID generation).
5. D — Deep dive: how to generate unique short codes without collisions; cache strategy for the hot read path; DB choice (KV store fits).
6. E — Evaluate: bottleneck = the redirect read path → CDN/cache; single counter = SPOF → ranges per host. Name these yourself.

The bar

"Why" beats "what." Scope first (R+E ~10 min), keep the high-level simple, then go deep on 1–2 areas the interviewer steers to, and volunteer the bottlenecks and trade-offs. Simplicity is a virtue — don't add a queue/shard you can't justify.

Pitfalls

• Designing before scoping — the #1 mistake; spend the first ~10 minutes on R + E.
• No numbers — estimation is what justifies caching/sharding decisions.
• No trade-offs / bottlenecks — sta+ candidates surface these unprompted.

Takeaways

• RESHADED: Requirements → Estimation → Schema/API → High-level → Deep dive → Evaluate.
• Scope + estimate first; keep high-level simple; go deep where steered; name bottlenecks.
• This is where your whole System Design + Estimation track pays off.

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Re-authored for this guide; RESHADED diagram hand-authored as SVG. Follows Educative's RESHADED and Hello Interview's time-boxed delivery. See also: the System Design subject, Capacity Estimation.

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