PHP cache slam defense using a semaphore to prevent dogpile effect (aka clobbering updates, stampeding herd or Slashdot effect).

Problem: too many requests hit your website at the same time while it tries to regenerate same content slamming your database, eg. when cache expired.

Solution: first request generates new content while all the subsequent requests get (stale) content from cache until it's refreshed by the first request.

Read http://www.sobstel.org/blog/preventing-dogpile-effect/ for more details.

In composer.json file:

"require": {
  "sobstel/metaphore": "2.0.*"
}

or just composer require sobstel/metaphore

use Metaphore\Cache;
use Metaphore\Store\MemcachedStore;

// initialize $memcached object (new Memcached())

$cache = new Cache(new MemcachedStore($memcached));
$cache->cache('key', function() {
    // generate content
}, 30);

• __construct(ValueStoreInterface $valueStore, LockManager $lockManager = null)

• cache($key, callable $callable, [$ttl, [$onNoStaleCacheCallable]]) - returns result

• delete($key)

• getValue($key) - returns Value object

• setResult($key, $result, Ttl $ttl) - sets result (without anti-dogpile-effect mechanism)

• onNoStaleCache($callable)

• getValueStore()

• getLockManager()

Cache values and locks can be handled by different stores.

$valueStore = new Metaphore\MemcachedStore($memcached);

$lockStore = new Your\Custom\MySQLLockStore($connection);
$lockManager = new Metaphore\LockManager($lockStore);

$cache = new Metaphore\Cache($valueStore, $lockManager);

By default - if no 2nd argument passed to Cache constructor - value store is used as a lock store.

Sample use case might be to have custom MySQL GET_LOCK/RELEASE_LOCK for locks and still use in-built Memcached store for storing values.

You can pass simple integer value...

$cache->cache('key', callback, 30); // cache for 30 secs

.. or use more advanced Metaphore\TTl object, which gives you control over grace period and lock ttl.

// $ttl, $grace_ttl, $lock_ttl
$ttl = new Ttl(30, 60, 15);

$cache->cache('key', callback, $ttl);

• $ttl - regular cache time (in seconds)
• $grace_ttl - grace period, how long to allow to serve stale content while new one is being generated (in seconds), similar to HTTP's stale-while-revalidate, default is 60s
• $lock_ttl - lock time, how long to prevent other request(s) from generating same content, default is 5s

Ttl value is added to current timestamp (time() + $ttl).

In rare situations, when cache gets expired and there's no stale (generated earlier) content available, all requests will start generating new content.

You can add listener to catch this:

$cache->onNoStaleCache(function (NoStaleCacheEvent $event) {
    Logger::log(sprintf('no stale cache detected for key %s', $event->getKey()));
});

You can also affect value that is returned:

$cache->onNoStaleCache(function (NoStaleCacheEvent $event) {
    $event->setResult('new custom result');
});

Run all tests: phpunit.

If no memcached or/and redis installed: phpunit --exclude-group=notisolated or phpunit --exclude-group=memcached,redis.