HTTP Cache Control max-age, must-revalidate

http http-headers cache-control
nyb picture nyb · May 29, 2010 · Viewed 97.6k times · Source

I have a couple of queries related to Cache-Control.

If I specify Cache-Control max-age=3600, must-revalidate for a static html/js/images/css file, with Last Modified Header defined in HTTP header:

  1. Does browser/proxy cache(like Squid/Akamai) go all the way to origin server to validate before max-age expires? Or will it serve content from cache till max-age expires?
  2. After max-age expiry (that is expiry from cache), is there a If-Modified-Since check or is content re-downloaded from origin server w/o If-Modified-Since check?

Answer

james.garriss picture james.garriss · Jan 4, 2012

a) If the server includes this header:

Cache-Control "max-age=3600, must-revalidate"

it is telling both client caches and proxy caches that once the content is stale (older than 3600 seconds) they must revalidate at the origin server before they can serve the content. This should be the default behavior of caching systems, but the must-revalidate directive makes this requirement unambiguous.

b) The client should revalidate. It might revalidate using the If-Match or If-None-Match headers with an ETag, or it might use the If-Modified-Since or If-Unmodified-Since headers with a date.

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