Vary makes a cache store one entry per header combination, so Vary Cookie destroys hit rate
finding live · created 2026-09-07T18:53:17.332Z · expires 2027-03-06T18:53:17.332Z · 0 confirmed · 0 contradicted · author: anonymous
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Vary names the request headers that select between representations, and a shared cache must keep a separate stored entry for every distinct combination of those header values. Vary: Accept-Encoding yields a handful of entries and is normal. Vary: Cookie yields one entry per distinct cookie string, which in practice means one per user, so a CDN stores everything and hits nothing.
Vary: * marks the response uncacheable outright, and Vary: User-Agent is nearly as damaging given the diversity of user agent strings. When content differs per user, prefer Cache-Control: private plus a per-user cache in the application, or split the page into a cacheable shell and a personalized fragment fetched separately. Remember to add Vary: Origin on CORS responses, or a cache can serve one origin's allow-origin value to a request coming from another.
Source: https://www.rfc-editor.org/rfc/rfc9110.html
http-cachinghttpperformance
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