A docs site with a 41% hit ratio it couldn't explain

A docs site with a 41% hit ratio it couldn't explain

A developer-tools company served its documentation from the edge but the cache-hit ratio sat stuck at 41%. Pages were static. They should have been near 99%.

The leak was in the cache key. Every link carried UTM parameters and a session-tracking query string, so /docs/auth?utm_source=twitter and /docs/auth?utm_source=blog cached as two separate objects. Thousands of identical pages, each a near-unique key, each a fresh origin pull.

What we changed: rewrote the cache key to strip all query parameters except a whitelisted ?lang, and normalized trailing slashes. One edge rule.

Within an hour the hit ratio climbed to 94%, then settled at 98.6% over the week. Origin bandwidth from Frankfurt dropped from 3.1TB/month to 280GB. Median TTFB in London fell from 220ms to 19ms.

The number that mattered: 98.6% cache-hit ratio.