On Mon, 01 May 2006 22:46:44 +0200 Graham Leggett <[EMAIL PROTECTED]> wrote:
> Brian Akins wrote: > > >> That's two hits to find whether something is cached. > > > > You must have two hits if you support vary. > > You need only one - bring up the original cached entry with the key, and > then use cheap subkeys over a very limited data set to find both the > variants and the header/data. > > >> How are races prevented? > > > > shouldn't be any. something is in the cache or not. if one "piece" of > > an http "object" is not valid or in cache, the object is invalid. > > Although other variants may be valid/in cache. > > I can think of one race off the top of my head: > > - the browser says "send me this URL". > > - the cache has it cached, but it's stale, so it asks the backend > "If-None-Match". > > - the cache reaper comes along, says "oh, this is stale", and reaps the > cached body (which is independant, remember?). The data is no longer > cached even though the headers still exist. > > - The backend says "304 Not Modified". > > - the cache says "cool, will send my copy upstream. Oops, where has my > data gone?". Sorry, but this only happens in your imagination. It's pretty obvious that mod_cache_http will handle this. > The end user will probably experience this as "oh, the website had a > glitch, let me try again", so it won't be reported as a bug. No. > Ok, so you tried to lock the body before going to the backend, but > searching for and locking the body would have been an additional wasted > cache hit if the backend answered with its own body. Not to mention > having to write and debug code to do this. Locks are not necessary, perhaps you are imaginating something very different. If a data body disappears under mod_http_cache it is not a big deal! It will refuse to serve the request from the cache and a new version of the page will be cached. > Races need to be properly handled, and atomic cache operations will go a > long way to prevent them. I think we are discussing apples and oranges. First, we only want to *organize* the current cache code into a more layered solution. The current semantics won't change, yet! -- Davi Arnaut
