DO NOT REPLY [Bug 35211] New: - RFE: Better support for Vary Headers in mod_cache

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http://issues.apache.org/bugzilla/show_bug.cgi?id=35211

           Summary: RFE: Better support for Vary Headers in mod_cache
           Product: Apache httpd-2.0
           Version: 2.1-HEAD
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P4
         Component: mod_cache
        AssignedTo: bugs@httpd.apache.org
        ReportedBy: chip@force-elite.com
                CC: chip@force-elite.com


Currently, mod_cache is RFC compliant in its handling of Vary headers, but it
behaves in a less than optimal manner.

If a reply contains the vary header, it is noted.  If the varied headers match
in both the stored version and the client request, the cache is used. If the
varied headers are missing or do not match, the cached copy is not used.  When
the cached copy is not used, a new version is fetched from the backend, and
replaces the previously cached copy, that contained different values for the
Varied headers.  This effectively kills the cache hit rate when using the Vary
header, since anytime a header does vary the cached copy is destroyed.

The enhancement would be storing one copy of the page, for every Varied header
value/combination.

One possible solution is to include the Varied headers in the Cache Hash.  This
would give each combination of varied headers, values and URLs their own unique
hash, and would enable the current mod_disk_cache and mod_mem_cache to operate
with little or no changes.  The problem is that we do not know if a URL should
be varied, until after we have already determined the Cache Hash.  If this route
is taken, some method to change the cache hash at a later time might be needed.

Another possible solution, that would require changes to both mod_mem_cache,
mod_disk_cache, and htcacheclean, would be to make some knid of 'index' node. 
This node would contain a list of all known combinations for each varied header,
and the location of the cached version of each.

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