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From Brian Pane <bri...@apache.org>
Subject [PATCH] mod_cache: support caching of streamed responses
Date Mon, 02 Sep 2002 08:45:13 GMT
This patch allows mod_cache to cache a response that isn't
all contained in the first brigade sent through cache_in_filter().
(This scenario happens, for example, with reverse-proxy requests,
CGIs, and various app server connectors.)

Can someone familiar with the caching code please scrutinize
the patch and send comments?  There's one known problem with
the code (details below), but it's otherwise ready for review.

The way it works is:

  * If the first call to cache_in_filter() includes an EOS,
    the operation is unchanged: put the response in the cache (
    assuming it meets all the other cacheability criteria) and
    pass the brigade to the next filter.

  * If the brigade passed to cache_in_filter() doesn't have an
    EOS, though, the filter now makes a copy of the brigade and
    passes the original on to the next filter.  In subsequent
    calls, cache_in_filter() continues buffering up copies of
    the buckets while streaming the response on to the next
    filter in the chain.  Once it finally sees an EOS, it
    concatenates the setaside buckets into a single brigade
    for storage in the cache.  It then streams the last bit
    of the output to the next filter.

  * If the response contains any bucket with length==-1 (an
    unread pipe bucket, for example), the filter doesn't attempt
    to cache it.  (It's probably possible to add support for
    this in the future.)

The one thing that's missing is a check to avoid setting aside
too much data.  If the total size of the setaside buckets would
exceed the maximum object size for the cache, cache_in_filter()
should immediately discard the setaside buckets and give up on
caching the response.  What's the right way to implement this
check?  It looks like the max object size is a property of each
specific cache implementation (mod_mem_cache, mod_disk_cache).
The first solution that comes to mind is to make each cache
implementation provide a callback function that says whether
it's willing to cache an object of size X.  Is there a cleaner
solution?

-Brian


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