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4.3.4. Using MySQL and memcached with PHP

PHP provides support for the Memcache functions through a PECL extension. To enable the PHP memcache extensions, you must build PHP using the --enable-memcache option to configure when building from source.

If you are installing on a RedHat based server, you can install the php-pecl-memcache RPM: 

root-shell> yum --install php-pecl-memcache

On Debian based distributions, use the php-memcache package.

You can set global runtime configuration options by specifying the values in the following table within your php.ini file.

Configuration option Default Description
memcache.allow_failover 1 Specifies whether another server in the list should be queried if the first server selected fails.
memcache.max_failover_attempts 20 Specifies the number of servers to try before returning a failure.
memcache.chunk_size 8192 Defines the size of network chunks used to exchange data with the memcached server.
memcache.default_port 11211 Defines the default port to use when communicating with the memcached servers.
memcache.hash_strategy standard Specifies which hash strategy to use. Set to consistent to allow servers to be added or removed from the pool without causing the keys to be remapped to other servers. When set to standard, an older (modula) strategy is used that potentially uses different servers for storage.
memcache.hash_function crc32 Specifies which function to use when mapping keys to servers. crc32 uses the standard CRC32 hash. fnv uses the FNV-1a hashing algorithm.

To create a connection to a memcached server, you need to create a new Memcache object and then specifying the connection options. For example: 

<?php
$cache = new Memcache;
$cache->connect('localhost',11121);
?>

This opens an immediate connection to the specified server.

To use multiple memcached servers, you need to add servers to the memcache object using addServer(): 

bool Memcache::addServer ( string $host [, int $port [, bool $persistent
                 [, int $weight [, int $timeout [, int $retry_interval
                 [, bool $status [, callback $failure_callback
                 ]]]]]]] )

The server management mechanism within the php-memcache module is a critical part of the interface as it controls the main interface to the memcached instances and how the different instances are selected through the hashing mechanism.

To create a simple connection to two memcached instances: 

<?php
$cache = new Memcache;
$cache->addServer('192.168.0.100',11211);
$cache->addServer('192.168.0.101',11211);
?>

In this scenario the instance connection is not explicitly opened, but only opened when you try to store or retrieve a value. You can enable persistent connections to memcached instances by setting the $persistent argument to true. This is the default setting, and will cause the connections to remain open.

To help control the distribution of keys to different instances, you should use the global memcache.hash_strategy setting. This sets the hashing mechanism used to select. You can also add an additional weight to each server, which effectively increases the number of times the instance entry appears in the instance list, therefore increasing the likelihood of the instance being chosen over other instances. To set the weight, set the value of the $weight argument to more than one.

The functions for setting and retrieving information are identical to the generic functional interface offered by memcached, as shown in this table.

PECL memcache Function Equivalent to
get() Generic get()
set() Generic set()
add() Generic add()
replace() Generic replace()
delete() Generic delete()
increment() Generic incr()
decrement() Generic decr()

A full example of the PECL memcache interface is provided below. The code loads film data from the Sakila database when the user provides a film name. The data stored into the memcached instance is recorded as a mysqli result row, and the API automatically serializes the information for you. 

<?php
$memc = new Memcache;
$memc->addServer('localhost','11211');
?>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 <title>Simple Memcache Lookup</title>
</head>
<body>
<form method="post">
  <p><b>Film</b>: <input type="text" size="20" name="film"></p>
<input type="submit">
</form>
<hr/>
<?php
  echo "Loading data...\n";
$value = $memc->get($_REQUEST['film']);
if ($value)
  {
    printf("<p>Film data for %s loaded from memcache</p>",$value['title']);
    foreach (array_keys($value) as $key)
      {
	printf("<p><b>%s</b>: %s</p>",$key, $value[$key]);
      }
  }
 else
   {
     $con = new mysqli('localhost','sakila','password','sakila') or
       die ("<h1>Database problem</h1>" . mysqli_connect_error());
     $result = $con->query(sprintf('select * from film where title ="%s"',$_REQUEST['film']));
     $row = $result->fetch_array(MYSQLI_ASSOC);
     $memc->set($row['title'],$row);
     printf("<p>Loaded %s from MySQL</p>",$row['title']);
   }
?>

With PHP, the connections to the memcached instances are kept open as long as the PHP and associated Apache instance remain running. When adding a removing servers from the list in a running instance (for example, when starting another script that mentions additional servers), the connections will be shared, but the script will only select among the instances explicitly configured within the script.

To ensure that changes to the server list within a script do not cause problems, make sure to use the consistent hashing mechanism.