Vary with Care

re: Vary with Care

EricLaw [ex-MSFT] — Thu, 27 Dec 2012 21:54:50 GMT

In particular, it should be noted that, in virtually all cases I can think of, it would be preferable for the client to retrieve a locally cached albeit decompressed response in preference to a remote retrieval of a compressed version of the resource.

re: Vary with Care

EricLaw [ex-MSFT] — Thu, 27 Dec 2012 21:48:14 GMT

@Mark: No one at Microsoft updates IE6-9's functionality, if that's what you're asking; only security patches are issued. I'm afraid I don't understand your claim of "intermediary cache thrash"-- it would be interesting to have a fuller explanation of the problem you think would arise. It would also be interesting for you to be more explicit about where, exactly in RFC2616 requires "Vary" as real-world practice suggests that it's insufficiently plain.

re: Vary with Care

Mark Nottingham — Fri, 26 Oct 2012 05:28:14 GMT

Just a note - your "best practice":

Only send a Vary: Accept-Encoding header when you have compressed the content (e.g. Content-Encoding: gzip).

"""

Will make pretty much every intermediary cache thrash, and violates HTTP. Anybody left at MSFT who can fix this? ;)

re: Vary with Care

Steve — Tue, 28 Feb 2012 18:19:16 GMT

Eric, Is it fair to presume that in most cases... a web server that is serving up content with Expires and Cache-Control headers to explicitly cache static resources (CSS/JS/GIF/JPG/PNG) and not cache HTML pages should not need to send a Vary header... ever?

e.g. if the server passes the correct headers for caching... can we ditch the Vary header completely?

EricLaw: @Steve: The idea behind Vary is that the same URL may yield a different resource based on some factor in the headers. For instance, if I had a "Buy" PNG file, I could use VARY: Accept-Language and return a localized PNG file for a localized page. In practice, of course, sites will typically use a different URL for this purpose, because support for Vary is sporadic and causes cache bloat problems for shared caches.

re: Vary with Care

Vincent Voyer — Wed, 24 Nov 2010 15:39:29 GMT

Just found out that the second server is sending Vary: host while in HTTP 1.0, I guess this is why IE6 can still send if modified since.. :)

https://gist.github.com/713829 for headers

re: Vary with Care

EricLaw [ex-MSFT] — Wed, 24 Nov 2010 13:24:57 GMT

@Vincent: I suspect you're seeing this behavior because perhaps your response is compressed. As noted, IE6 entirely ignores Vary if compression is used. That's the only explanation I see. (I-M-S and If-None-Match will be sent for an expired cached response, or in IE7+ if a response requires validation due to Vary).

re: Vary with Care

Vincent Voyer — Wed, 24 Nov 2010 09:46:38 GMT

I have request being cached even if vary host is set.

IE6 will send "if modified since" even if vary host is set, is this because I also have cache control max age public set ? Thanks

re: Vary with Care

EricLaw [MSFT] — Tue, 03 Aug 2010 01:14:58 GMT

@Dan: Yes, Cache-Control: private tells a (shared) proxy not to cache the resource.

Just because the client requests GZIP'd content doesn't mean that the server must give it back (for instance, many sites send the first response without compression and spin up a background thread to compress the content and cache it on the server for later use). Additionally, just because IE supports compressed content doesn't mean that it will always ask for it-- users can disable compression, or edge proxies may strip the client request's Accept-Encoding header.

re: Vary with Care

Dan Zollman — Sun, 01 Aug 2010 04:48:04 GMT

I've got a question as someone who's still learning how HTTP caching works. Let's say a resource is set up as follows:

- Normally, send "Vary: Accept-Encoding, User-agent"

- If client is IE8 or earlier, and if content is not compressed, don't send the vary header, but send "Cache-Control: private"

Wouldn't that prevent proxies from storing the uncompressed version?

And, a separate question: if we're talking about a gzipped page, and if IE supports gzip anyway, why would uncompressed content be sent to IE (with the Vary header) in the first place?

re: Vary with Care

EricLaw [ex-MSFT] — Wed, 02 Jun 2010 21:18:07 GMT

I've updated this post with updated analysis of IE6 behavior after code inspection. A future update to this post will cover IE8 and IE9 behavior.