How to avoid a possible missing cache file in PHP? - php

Question

I have a simple caching system as
if (file_exists($cache)) {
echo file_get_contents($cache);
// if coming here when $cache is deleting, then nothing to display
}
else {
// PHP process
}
We regularly delete outdated cache files, e.g. deleting all caches after 1 hour. Although this process is very fast, but I am thinking that a cache file can be deleted right between the if statement and file_get_contents processes.
I mean when if statement checks the existence of cache file, it exists; but when file_get_contents tries to catch it, it is no longer there (deleted by simultaneous cache deleting process).
file_get_contents locks the file to avoid the undergoing delete process during the read process. But the file can be deleted when the if statement sends the PHP process to the first condition (before start of the file_get_contents).
Is there any approach to avoid this? Is the cache deleting system different?
NOTE: I did not face any practical problem, as it is not very probable to catch this event, but logically it is possible, and should happen on heavy loads.

Answer 1

Luckily file_get_contents return FALSE on error, so you could quick-bake it like:
if (FALSE !== ($buffer = file_get_contents())) {
echo $buffer;
return;
}
// PHP process
or similiar. It's a bit the quick and dirty way, considering you want to place the # operator to hide any warnings about non-existent files:
if (FALSE !== ($buffer = #file_get_contents())) {
The other alternative would be to lock, however that might prevent your cache-deletion to not delete the file if you have locked it.
Then left is to stall the cache your own. That means reading the file-creation time in PHP, check that it is < 5 minutes then for the file-deletion processing (5 minutes is exemplary) and then you would know that the file is already stale and for being replaced with fresh content. Re-create the file then. Otherwise read the file in, which probably is better then with readfile instead of file_get_contents and echo.

Answer 2

On failure, file_get_contents returns false, so what about this:
if (($output = file_get_contents($filename)) === false){
// Do the processing.
$output = 'Generated content';
// Save cache file
file_put_contents($filename, $output);
}
echo $output;
By the way, you may want to consider using fpassthru, which is more memory-efficient, especially for larger files. Using file_get_contents on large files (> 100 MB), will probably cause problems (depending on your configuration).
<?php
$fp = #fopen($filename, 'rb');
if ($fp === false){
// Generate output
} else {
fpassthru($fp);
}