php-memcached.md

PHP Memcached

PHP Sessions don't work on clusters, memcached provides a solution. The procedure is as follows:

create a new Memcached object
call memcached::addServer
retrieve the value with memcached::get
if the value was retrieved successfully
  carry on
else
  store the value with memcached::set

• When the page is first loaded, the value will not be found in the cache (obviously), so we need to store it.
• When the page is loaded subsequently the value should be found, the program can proceed normally.
• There is a timeout, after which time the value will be removed. Many memcached members have a default parameter to set your own timeout. This is a hint only.

From Wikipedia:

"Memcached's APIs provide a very large hash table distributed across multiple machines. When the table is full, subsequent inserts cause older data to be purged in least recently used (LRU) order. Applications using Memcached typically layer requests and additions into RAM before falling back on a slower backing store, such as a database".

A little experiment

The first time round the value is not found, so we store it. Subsequently it is retrieved successfully. We can reset by adding ?reset=1 to the uri.

// reset
if (isset($_GET['reset'])) {
  echo "Deleting cached value";
  $cache->delete('testValue');
  echo "Result is " . $cache->getResultMessage();
  exit();
}

// Try to retrieve the cached value
$cachedValue = $cache->get('testValue');

if ($cachedValue) {                                            // we got the value
  echo "Value successfully retrieved: $cachedValue ";
} else {                                                      // value not found, so store it
  echo "failed to retreive testvalue, so setting testvalue";
  $cache->set('testValue', 12345);
}

echo "Result was " . $cache->getResultMessage();

Profiler results

constructor  0.5
addServer    0.5
set         21.5
get         57.7