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Problem

Build a URL shortening service like bit.ly. Users submit a long URL and receive a short alias (e.g. bit.ly/xK9a). When someone visits the short URL they are redirected to the original. The service must handle millions of redirects per day with very low latency.

Functional Requirements

• Shorten a given long URL to a unique short code
• Redirect short code to the original URL (< 50 ms p99)
• Allow custom aliases (optional)
• Track click analytics per short URL
• Expire short URLs after a configurable TTL

Non-Functional

• 100M new URLs created per day
• 10B redirects per day (read-heavy: ~100:1 read/write ratio)
• 99.99% availability — redirection must never be down
• Horizontally scalable — no single points of failure