title: “URL Shortener Additional”

linkTitle: “URL Shortener Additional”

Back-of-envelope (40-50s)

• Keyspace: need ≥1B codes -> base62^5 ≈ 916M (<1B), base62^6 ≈ 56.8B -> choose 6-char minimum, expandable to 7-8.
• Traffic: say 100M DAU, ~5 resolves/day ⇒ 500M/day ≈ 5.8k QPS avg; peak ×10 ≈ 58k QPS read. Writes are much smaller (≪1k QPS).
• Storage: 1B rows × ~1 KB/row (URL, code, meta) ≈ ~1 TB raw (before replication).

(These numbers justify caches, partitioning, and avoiding hot databases on read path.)

“Definition of Done” checklist for each FR

• Actor, input, output, HTTP codes defined
• Validation rules explicit (allowed schemes, length bounds)
• Uniqueness & collision behavior explicit
• Expiration semantics & statuses explicit
• Case sensitivity and charset documented
• Idempotency/dedup policy defined (and parameterizable)
• Security/abuse basics noted (rate limit, reserved aliases)
• Acceptance tests enumerated (unit + e2e)

(?) how we can come up and reason this design? What problems does it solve?

URL Shortener “design skeleton” to draw

1. Clients -> CDN/Edge
2. Edge cache (POP/regional Redis or provider KV)
3. Service (stateless)
4. NoSQL KV (hash-sharded by code)
5. Async bus (cache invalidation, replication, expiry jobs)
6. Admin/ops (blocklists, takedown, reserved aliases)

Label latencies: Edge hit ≈ few ms; regional cache ≈ 5-20 ms; DB ≈ 2-10 ms (in-region) + network.

Prepare a Degradation Plan (for the CV path)#

Demonstrates operational maturity anchored to value.

• If DB hot: serve cached redirects and negative-cache 404/410; trip circuit breakers to protect p95.
• If cache cold: return short TTL 503 with human-friendly fallback (minimize blast radius).
• If expiration job lags: tolerate brief stale resolves; correctness catches up (availability over strict freshness).

(?) deep dives # If they push on one dimension

• Bigger scale: increase TTLs on very hot codes, push mapping into provider edge KV (Cloudflare KV/Workers, Fastly KV), or preload hot sets.
• Stronger consistency on create: conditional writes/unique index + per-alias token; still async propagate to caches.

Stateless Service: The redirection service should be stateless so you can scale it horizontally behind a load balancer.

Edge Cases

• Case sensitivity: codes are case‑insensitive (normalize to lower)
• Very long URLs: set a server‑side max length (e.g., ≤ 2,048 chars) with clear error
• Negative caching: treat 404/410 as cacheable for a short TTL (ops consideration; influences availability but not contract)