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Subject: Re: [PHP-DEV] [Q] Does PHP have a negative cache for file stat operations?
From: smalyshev@gmail.com (Stanislav Malyshev)

Hi!

> We're doing some performance work in WinCache, and we're finding that
> some frameworks are...uh...enthusiastically using file_exists(),
> is_file() and is_dir() functions on files/directories that don't
> exist.  Every. Single. Pageload.
> 
> Does the PHP stat cache include negative cache entries?  If not, why
> not?

Negative cache is tricky. Most frequent patterns look like this:

1. if(file_exists("blah")) { do stuff }

2. if(!file_exists("blah")) { throw new Exception("no blah!"); }
do stuff

3. if(!file_exists("blah")) { create("blah"); }
do stuff

Caching negative even short term would subtly break case 3, in that it
would force creation of the file every time, thus killing any benefit
from any caching (usually file creation is much more expensive process).
Long term caching would also be big trouble with case 2. Unless there's
a way to reset the cache and the app is very careful to do it - but file
can be created by external means and also case 2 would be harder to fix
without losing the benefits of caching.

So one may ask, why case 1 is not a problem? Usually it's because if the
file is there, apps rarely delete them by themselves, or at least adding
new files is much more frequent operation than deleting existing ones
that are used by the app.
-- 
Stas Malyshev
smalyshev@gmail.com