RE: Extension headers and caching

>IE has lots of issues with Vary

To be more specific, the WinINET networking stack does not cache outbound request headers, preventing IE and other applications from reusing resources that Vary without validation (except in narrow cases).  See for more information.

Revalidation ensures cache-correctness but entails a performance cost.


-----Original Message-----
From: [] On Behalf Of Julian Reschke
Sent: Monday, February 09, 2009 3:51 AM
To: Anne van Kesteren
Cc: HTTP Working Group
Subject: Re: Extension headers and caching

Anne van Kesteren wrote:
> Hi,
> If I perform a request to a server and then perform a subsequent request
> with an extension header set, should I get a cached copy or a fresh one
> for the subsequent request? Would it be different if the server had
> replied with a Vary header for the extension header?


if the response to the first request *was* cacheable, and the server
varies the response based on that Custom header, then it should have
included the name of that custom header in the Vary response header. If
it didn't, that would be a bug.

That being said, IE has lots of issues with Vary, so I wouldn't be
surprised if servers worked around that by not specifying Vary.

Related: <>

> It has been suggested that I should define how this case works in
> XMLHttpRequest, but I rather just defer to the HTTP for this.

In theory: yes, in practice it might be useful to call that out in the
XHR spec as well (if you finish before HTTPbis :-).

BR, Julian

Received on Tuesday, 10 February 2009 17:53:20 UTC