Short Link Service

Functional requirements

• Accept a long URL and return a short code that redirects to it
• Allow an optional custom code, rejected if already taken
• Redirect a short code to its target with an HTTP redirect
• Let the creator see how many times their link was followed
• Allow an optional expiry after which the code stops resolving

Non-functional requirements

• Redirects should complete well under 100 ms at p99
• A code, once issued, must never resolve to a different target
• Redirect availability matters far more than creation availability
• Codes should not be guessable in bulk

Scale assumptions

Worked estimates for this brief, not measurements of any real system. Disagree with them out loud — that is the exercise.

Code space

7 chars of [A-Za-z0-9] ≈ 3.5 trillion codes

Peak multiplier

~3x average, so ~6,000 redirects/second

Read:write ratio

20:1 — this is a read system with a write feature attached

Storage per year

~1.8 TB at ~500 bytes per link

New links per day

10 million (~100/second average)

Redirects per day

200 million (~2,000/second average)

Suggested API surface

• POST/v1/links

  Create a short link, optionally with a custom code

• GET/{code}

  Resolve and redirect; the hot path

• GET/v1/links/{code}/stats

  Follow count for the creator

• DELETE/v1/links/{code}

  Revoke a link you own

Decisions you should be able to defend

• How codes are generated
• When follow counts are updated
• Whether to cache at the edge
• Consistency of link creation