What is the real usage of apc_cas() function?

Question

I wonder what a real life example that I need to use the apc_cas function in PHP.

This change the cache variable with a new one but I also saw in the comments for apc_store: Note APC version 3.1.3 there is a bug (http://pecl.php.net/bugs/bug.php?id=16814) that will display a cache slam averted warning for all writes to a cache var that exists. I am guessing that using this function will not generate that warning — ka_lin, Aug 10 '15 at 11:03
Hope link will help you: http://www.cnblogs.com/argb/p/3604344.html — Vishal Bharti, Aug 10 '15 at 11:09

score 0 · Answer 1 · answered Aug 10 '15 at 11:15

Assures you that no one else updated the value in that key. Is for that reason that you have to pass the $old value. If the current value is different to your knowlegment of the key value (in this case $old) the key will not be updated and false is returned.

Implementation of http://en.wikipedia.org/wiki/Compare-and-swap commonly used in key stores databases.

score 0 · Accepted Answer · answered Aug 10 '15 at 11:18

A simple example is a counter. Assume we have a key counter and a function which increases the counter (note that this would be better suited for the apc_inc function but let's play along):

function incCounter(){
    //Get the current value increment and set
    $c = apc_fetch('counter');
    $c++;
    apc_store('counter',$c);
}

However the above has an issue. If two requests occur at the same time both will get the same value of $c and increment it meaning the counter will only be incremented by one.

Using apc_cas however let's you guarantee that the value you are updating is the old one and that it hasn't been changed in the meantime.

What is the real usage of apc_cas() function?

2 Answers2