/**
* Since our CAS is deferred, the CAS token we got from our original
* get() will likely not be valid by the time we want to store it to
* the real cache. Imagine this scenario:
* * a value is fetched from (real) cache
* * an new value key is CAS'ed (into temp cache - real CAS is deferred)
* * this key's value is fetched again (this time from temp cache)
* * and a new value is CAS'ed again (into temp cache...).
*
* In this scenario, when we finally want to replay the write actions
* onto the real cache, the first 3 actions would likely work fine.
* The last (second CAS) however would not, since it never got a real
* updated $token from the real cache.
*
* To work around this problem, all get() calls will return a unique
* CAS token and store the value-at-that-time associated with that
* token. All we have to do when we want to write the data to real cache
* is, right before was CAS for real, get the value & (real) cas token
* from storage & compare that value to the one we had stored. If that
* checks out, we can safely resume the CAS with the real token we just
* received.
*
* {@inheritdoc}
*/
public function cas($token, $key, $value, $expire = 0)
{
$originalValue = isset($this->tokens[$token]) ? $this->tokens[$token] : null;
// value is no longer the same as what we used for token
if (serialize($this->get($key)) !== $originalValue) {
return false;
}
// "CAS" value to local cache/memory
$success = $this->local->set($key, $value, $expire);
if ($success === false) {
return false;
}
// only schedule the CAS to be performed on real cache if it was OK on
// local cache
$this->defer->cas($originalValue, $key, $value, $expire);
return true;
}
