Memcached, Locking and Race Conditions

Question

We are trying to update memcached objects when we write to the database to avoid having to read them from database after inserts/updates.

For our forum post object we ha

Answer 1

We encountered this in our system. We modified get so

• If the value is unset, it sets it with a flag ('g') and [8] second TTL, and returns false so the calling function generates it.
• If the value is not flagged (!== 'g') then unserialize and return it.
• If the value is flagged (==='g') then wait 1 second and try again until it's not flagged. It will eventually be set by the other process, or expired by the TTL.

Our database load dropped by a factor of 100 when we implemented this.

function get($key) {
  $value=$m->get($key);
  if ($value===false) $m->set($key, 'g', $ttl=8);
  else while ($value==='g') {
    sleep(1);
    $value=$m->get($key);
  }
  return $value;
}