A badly designed cache can do more harm than good.
In the worst case a cache can increase database server load instead
of minimizing it. An overload situation can occur if a highly
shared cache entry expires (cache stampeding).
Cache entries are shared and reused to a different
degree depending on the storage used. The default storage handler
stores cache entries in process memory. Thus, a cache entry can be
reused for the life-span of a process. Other PHP processes cannot
access it. If Memcache is used, a cache entry can be shared among
multiple PHP processes and even among multiple machines, depending
on the set up being used.
If a highly shared cache entry stored, for example,
in Memcache expires, many clients gets a cache miss. Many client
requests can no longer be served from the cache but try to run the
underlying query on the database server. Until the cache entry is
refreshed, more and more clients contact the database server. In
the worst case, a total lost of service is the result.
The overload can be avoided using a storage handler
which limits the reuse of cache entries to few clients. Then, at
the average, its likely that only a limited number of clients will
try to refresh a cache entry concurrently.
Additionally, the built-in slam defense mechanism
can and should be used. If slam defense is activated an expired
cache entry is given an extended life time. The first client
getting a cache miss for the expired cache entry tries to refresh
the cache entry within the extended life time. All other clients
requesting the cache entry are temporarily served from the cache
although the original TTL of the cache entry has expired.
The other clients will not experience a cache miss before the
extended life time is over.
Example #1 Enabling the slam defense
The function mysqlnd_qc_get_core_stats() returns an array
of statistics. The statistics slam_stale_refresh and
slam_stale_hit are incremented if slam defense takes
It is not possible to give a one-fits-all
recommendation on the slam defense configuration. Users are advised
to monitor and test their setup and derive settings