RE: [PHP-DEV] [Q] Does PHP have a negative cache for file stat operations?

From: Stanislav Malyshev [mailto:smalyshev@gmail.com] > > 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). Yes, and that was my biggest worry about implementing a negative cache. I'd have to hook all the ways a file could be created in order to correctly flush entries in the negative cache. And, in the case of a site that doesn't pathologically try to hit non-existent files, the overhead of a negative cache is wasted cycles. > 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. Yes, it's the problem of detecting & hooking all the ways a file could be created, and then flushing negative entries. And, as you've pointed out, since there's potentially other, non-hookable ways to create files, I'd have to provide API surface to flush entries out of the cache. Thank you for the thoughtful response! --E.