Oracle 5.0 Reference Manual Download

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Mysqlnd query result cache plugin (

mysqlnd_qc

)

2668

is_select( CREATE TABLE test(id INT)): do not cache

is_select( INSERT INTO test(id) VALUES (1), (2), (3)): do not cache

is_select( SELECT id FROM test WHERE id = 1): cache

is_select( SELECT * FROM test): cache

Cache put: 2

Cache hit: 1

The examples callback tests if a statement string matches a pattern. If this is the case, it either returns

TRUE

to cache the statement using the global default TTL or an alternative TTL.

To minimize application changes the callback can put into and registered in an auto prepend file.

20.7.7.4.6. Slam defense

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 20.312. Enabling the slam defense mechanism

mysqlnd_qc.slam_defense=1

mysqlnd_qc.slam_defense_ttl=1

The slam defense mechanism is enabled with the PHP configuration directive

mysqlnd_qc.slam_defense

. The extended life time of a cache entry is set with

mysqlnd_qc.slam_defense_ttl

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 place.

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 accordingly.

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