Indeed.So either you create a sript/program that goes directly in the cache and clears the document (which would mean you can recalculate the hash that mod_cache uses for a given URL), or else you find another clever way.
Let's say that for Tomcat, the expensive part of the operation is to re-generate the PDF response. But since this seems to be some kind of database-based application, I imagine that : - when your application on Tomcat gets a request for an item, it goes off into the database to retrieve the information, then formats this into a PDF and sends this as a response. That's expensive. - but the information in the database must have some kind of timestamp, since you can determine when the response would need to be regenerated and when not. - so what about your application checking first if the information is really "new", and if not returning some other kind of response to the caller (in this case the caller would be Apache/mod_cache).
What I mean is a bit complicated to explain, but roughly :- when mod_cache examines a request, it can find that the document is not in the cache, and then the request goes through to Tomcat. The response from Tomcat is then stored in the cache, by a filter set up by mod_cache for this request and for this purpose. I imagine that in this case, the request as it goes to Tomcat is "unconditional", meaning that it tells Tomcat "give me this document, no matter what". - on the other hand, when mod_cache finds the document already in the cache, it must still, under certain circumstances, let a request go through to Tomcat, to verify that the cached document is still valid.
I am not quite clear myself on which combination of circumstances and request and response headers control this behaviour, but I have a feeling somehow that it must be possible to play with this, to avoid the Tomcat application having to unnecessarily generate those expensive PDF answers. It would still get calls, and would still need to interrogate the database, but in many cases it could answer with something like a "Not modified" status instead of a new PDF.
Any real guru available here ? Karim Zaki wrote:
Seems I'm back at square 1. The idea of using max-age=0 did not work, which, on second thought, makes sense. CacheIgnoreCacheControl should ignore the "cache-control" header regardless of its value. The virtual host solution did not work either. It created new cache entries for the vhost. -----Original Message----- From: André Warnier [mailto:aw@xxxxxxxxxx] Sent: Monday, December 01, 2008 6:20 PM To: users@xxxxxxxxxxxxxxxx Subject: Re: Clearing cache selectively on Apache + mod_cache Eric Covener wrote:On Mon, Dec 1, 2008 at 11:09 AM, André Warnier <aw@xxxxxxxxxx> wrote:Which leads me to believe that, as such, the scheme proposed with the separate VirtualHost will not work as simply as expected.Yep, it looks like you'd need to jump through hoops to have a vhost that was able to nuke these requests (using the same hostname on a separate interface).Just intellectual curiosity : other than going through the source code of mod_cache, is there a description somewhere of the algorithm used by mod_cache to calculate this hashcode ? --------------------------------------------------------------------- The official User-To-User support forum of the Apache HTTP Server Project. See <URL:http://httpd.apache.org/userslist.html> for more info. To unsubscribe, e-mail: users-unsubscribe@xxxxxxxxxxxxxxxx " from the digest: users-digest-unsubscribe@xxxxxxxxxxxxxxxx For additional commands, e-mail: users-help@xxxxxxxxxxxxxxxx --------------------------------------------------------------------- The official User-To-User support forum of the Apache HTTP Server Project. See <URL:http://httpd.apache.org/userslist.html> for more info. To unsubscribe, e-mail: users-unsubscribe@xxxxxxxxxxxxxxxx " from the digest: users-digest-unsubscribe@xxxxxxxxxxxxxxxx For additional commands, e-mail: users-help@xxxxxxxxxxxxxxxx
--------------------------------------------------------------------- The official User-To-User support forum of the Apache HTTP Server Project. See <URL:http://httpd.apache.org/userslist.html> for more info. To unsubscribe, e-mail: users-unsubscribe@xxxxxxxxxxxxxxxx " from the digest: users-digest-unsubscribe@xxxxxxxxxxxxxxxx For additional commands, e-mail: users-help@xxxxxxxxxxxxxxxx
|