mod_cache forgets to include cached headers when serving cached content under certain circumstances (2.2.x)
Under a very specific set of circumstances, apache proxy will not serve the correct headers for cached content. This was observed under 2.2.29 and seems to exist in the 2.2.31 release. The problem occurs when a client revalidates content that is both already cached on the client, and on the proxy. Revalidation is requested by the client via the If-Modified-Since header. If the upstream webserver responds to the revalidation with a 304 Not Modified that also contains headers indicating non-cacheable content, the proxy will serve its cached content to the client, but without the appropriate headers. This can result in errors on the client side as content encoding and content type information is lost. The specific circumstance is described in a comment in mod_cache: /* Hold the phone. Some servers might allow us to cache a 2xx, but * then make their 304 responses non cacheable. This leaves us in a * sticky position. If the 304 is in answer to our own conditional * request, we cannot send this 304 back to the client because the * client isn't expecting it. Instead, our only option is to respect * the answer to the question we asked (has it changed, answer was * no) and return the cached item to the client, and then respect * the uncacheable nature of this 304 by allowing the remove_url * filter to kick in and remove the cached entity. */ The code proceeds to push the cached content onto the bucket brigade but neglects to apply the matching cached headers. Originally I thought the solution would be to simply add the cached headers but in the 2.4.x branch, it seems that this case is handled by re-requesting the resource without the conditional. Should this code be backported into the the 2.2.x branch ?
Re: htcacheclean and expired cache entries
Thanks for the response. You're right, the -L option would be ideal. We probably won't be going to 2.4 for some time after release so I'll see if I can't backport htcacheclean to 2.2. Cheers 2011/12/2 Graham Leggett minf...@sharp.fm On 02 Dec 2011, at 8:52 PM, Rob Landrito wrote: I noticed that htcacheclean won't clean expired entries unless the specified size limit is reached. That's by design. Is there any specific reason for this ? I'm looking into freeing up some inodes on some of my servers where the total cache size hasn't reached my specified limit. Would it be worthwhile to add a switch to enable cleaning of expired entries even if the size limit isn't reached ? The short answer is that you're probably looking for the -L option in the (soon to be released) httpd v2.4 to limit to the given number of inodes, as described here: http://httpd.apache.org/docs/trunk/programs/htcacheclean.html#options You probably want to use both the -l and -L options together to keep files safely within limits on disk. To answer your question, creating content has a cost associated with it, you want to keep content around for as long as humanly possible. Unlike typical caching systems that have a dual state it's cached / it's deleted, HTTP caching offers a three state system: it's fresh / it's stale / it's deleted. Initially, when cached, content is typically fresh for a freshness lifetime, and when it gets old enough, the content goes into the second state, that of being stale. Using the conditional request mechanism, the backend server is given the opportunity to confirm whether the stale content is actually still fresh, and in many cases this is so. The stale content is then updated to be fresh again. Eventually, the cache becomes full, so htcacheclean kicks in and reduces the cache size until it fits inside the given limits, but no more. Apart from the obvious benefit of not having to regenerate content from scratch when it goes stale, having stale content available allows you to do some useful things, like serve stale content to a thundering herd while the stale content is being freshened, or returning stale content when the backend returned an error. None of this is possible if you strip the cache of stale data. Regards, Graham -- -- :wq!
htcacheclean and expired cache entries
Greetings, I noticed that htcacheclean won't clean expired entries unless the specified size limit is reached. Is there any specific reason for this ? I'm looking into freeing up some inodes on some of my servers where the total cache size hasn't reached my specified limit. Would it be worthwhile to add a switch to enable cleaning of expired entries even if the size limit isn't reached ?
