Question

I’m new to HTTP caching and really need some help…

1. I saw many people set the "Cache-Control" header when sending XHR to server.
   What is the purpose of doing that? Does request header needs caching too?

2. I’ve set this header for a XHR:
   Cache-Control max-age=30, must-revalidate, but Firefox 3.6.13 seems do not re-validate the file when I refresh(even force refresh) after 30 seconds. No access-log in server either. Is this because I haven’t set expire header to the file in Apache?

   From Firebug log:

   Response Headers
   Date Sat, 22 Jan 2011 09:59:21 GMT
   Last-Modified Thu, 20 Jan 2011 09:05:11 GMT
   Etag “3e000000023ca7-20d-49a436e5b3413”

   Request Headers
   Keep-Alive 115
   Connection keep-alive
   Content-Type application/xml
   Cache-Control max-age=30, must-revalidate

   The file have been modified, but Firefox does not even do conditional get. Response Date and Last-Modified not update at all.

Answer 1

the server needs to set the re-validate directive, not the client.

the source controls if and how the content should be cacehd or not cached, the clients decides whether to honor it or not.

must-revalidate just means that a head request is sent and if the etag differes, the resources is refreshed. this is a very common an dsenseful practice for big files, because the e-tag generally changes once the content changes.

for small files however it may be your desire to not have any request at all, but a shorter time out. then must-revalidate is wrong.
the server load and the time it consumes of a head request to a 7kb image is not much less than serving the complete image.