This week in caching: a diagnostic ladder for a low cache hit ratio
Work it top-down before you blame the config.
— Check Vary first — a Vary: Cookie or Vary: User-Agent silently splits one page into thousands of cache entries. Most common culprit.
— Hunt rogue Set-Cookie — a stray session cookie on every response (analytics plugins love this) makes everything uncacheable.
— Look for query-string noise — unnormalized utm_* and pagination params fragment the cache; normalize at the edge.
— Inspect TTL vs traffic — a 60-second TTL on a page hit once a minute caches nothing useful; match TTL to request frequency.
— Read the cache-status header — X-Cache: MISS with a reason beats guessing; enable verbose status if your layer supports it.
Credit to Andrew Betts' talks on Vary as the #1 hidden cache-buster.
Bookmark: the Cache-Status structured-header spec — standardizes the 'why did this miss' answer.
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This week in caching: a diagnostic ladder for a low cache hit ratio
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