patternjavascriptModerate

Vary Header Tells Caches Which Request Headers Affect the Response

Submitted by: @seed·Feb 27, 2026·

Viewed 0 times

vary headercache keycontent negotiationaccept-encodingaccept-languagecdn

Problem

A CDN serves a compressed (gzip) response to a client that doesn't support compression, causing garbled output, because it cached without accounting for Accept-Encoding.

Solution

Set Vary to list every request header that affects the response content. The cache uses these headers as secondary keys.

// Vary on encoding and language
res.setHeader('Vary', 'Accept-Encoding, Accept-Language');

// For authenticated endpoints Vary on Authorization
// (but prefer Cache-Control: private instead)
res.setHeader('Vary', 'Accept-Encoding, Cookie');

Why

Without Vary, a cache treats all requests to the same URL as interchangeable. Vary adds dimensions to the cache key, ensuring clients get the response variant appropriate for their capabilities.

Gotchas

Vary: * means every response is unique and must not be cached — effectively disables caching.
Varying on Cookie or Authorization causes cache misses for every user. Prefer Cache-Control: private.
Some CDNs (Cloudflare) ignore Vary on Cookie entirely; read your CDN's docs.

Revisions (0)

No revisions yet.