Create a memcache session storage with optional storage backup

config.yml

framework:
    session:
        default_locale: %locale%
        auto_start: true
        lifetime: 7200
        storage_id: memcache_session_storage

parameters:
    pdo.db_options:
        db_table: session
        db_id_col: session_id
        db_data_col: session_value
        db_time_col: session_time
    memcache.options:
        host: localhost
        port: 11211

services:
    memcache_session_storage:
        class: JStout\MainBundle\Session\MemcacheSessionStorage
        arguments: [@memcache, %session.storage.options%, %memcache.options%, @pdo_session_storage]
    pdo_session_storage:
        class: Symfony\Component\HttpFoundation\SessionStorage\PdoSessionStorage
        arguments: [@pdo, %session.storage.options%, %pdo.db_options%]
    memcache:
        class: Memcache

MemcacheSessionStorage.php

<?php

namespace JStout\MainBundle\Session;

use Symfony\Component\HttpFoundation\SessionStorage\NativeSessionStorage;

/**
 * MemcacheSessionStorage.
 */
class MemcacheSessionStorage extends NativeSessionStorage
{
    private $memcache;
    private $storage;
    private $lifetime;

    /**
     * Constructor.
     *
     * @param \Memcache  $memcache A memcache instance
     * @param array $options   An associative array of session options
     * @param array $memcacheOptions An associative array of memcache options
     * @param Symfony\Component\HttpFoundation\SessionStorage\NativeSessionStorage $storage   An instance of NativeSessionStorage
     *
     * @see NativeSessionStorage::__construct()
     */
    public function __construct(\Memcache $memcache, array $options = array(), array $memcacheOptions = array(), NativeSessionStorage $storage = null)
    {
        $this->memcache = $memcache;
        $this->started  = false;

        if (!array_key_exists('host', $memcacheOptions)) {
            throw new \InvalidArgumentException('You must provide the "host" option for a Memcache.');
        }

        if (!array_key_exists('port', $memcacheOptions)) {
            throw new \InvalidArgumentException('You must provide the "port" option for a Memcache.');
        }

        if (array_key_exists('lifetime', $options)) {
            $this->lifetime = $options['lifetime'];
        } else {
            // get lifetime from php config
            $this->lifetime = intval(ini_get('session.gc_maxlifetime'));
        }
        
		@$this->memcache->connect($memcacheOptions['host'], $memcacheOptions['port']);

        // see if memcache server is available
        if (!$this->memcache->getServerStatus($memcacheOptions['host'])) {
            $this->memcache = null;
        }

        $this->storage = $storage;

        parent::__construct($options);
    }

    /**
     * Starts the session.
     *
     * @api
     */
    function start()
    {
        if (self::$sessionStarted) {
            return;
        }

        // if memcache server is down, use secondary session handler, otherwise throw exception
        if (null === $this->memcache) {
            if (null !== $this->storage && method_exists($this->storage, 'start')) {
                return $this->storage->start();
            } else {
                throw new \Exception('No session storages are available.');
            }
        }
        
        session_set_save_handler(
            array($this, 'sessionOpen'),
            array($this, 'sessionClose'),
            array($this, 'sessionRead'),
            array($this, 'sessionWrite'),
            array($this, 'sessionDestroy'),
            array($this, 'sessionGC')
        );

        parent::start();
    }

    /**
     * Opens a session.
     *
     * @param  string $path  (ignored)
     * @param  string $name  (ignored)
     *
     * @return Boolean true, if the session was opened, otherwise an exception is thrown
     */
    public function sessionOpen($path = null, $name = null)
    {
        return true;
    }

    /**
     * Closes a session.
     *
     * @return Boolean true, if the session was closed, otherwise false
     */
    public function sessionClose()
    {
        // do nothing
        return true;
    }

    /**
     * Destroys a session.
     *
     * @param  string $id  A session ID
     *
     * @return Boolean   true, if the session was destroyed, otherwise an exception is thrown
     *
     * @throws \RuntimeException If the session cannot be destroyed
     */
    public function sessionDestroy($id)
    {
        if (null !== $this->memcache) {
            $this->memcache->delete($id);
        }

        if (null !== $this->storage && method_exists($this->storage, 'sessionDestroy')) {
            $this->storage->sessionDestroy($id);
        }

        return true;
    }

    /**
     * Cleans up old sessions.
     *
     * @param  int $lifetime  The lifetime of a session
     *
     * @return Boolean true, if old sessions have been cleaned, otherwise an exception is thrown
     *
     * @throws \RuntimeException If any old sessions cannot be cleaned
     */
    public function sessionGC($lifetime)
    {
        if (null !== $this->storage && method_exists($this->storage, 'sessionGC')) {
            $this->storage->sessionGC($lifetime);
        }

        return true;
    }

    /**
     * Reads a session.
     *
     * @param  string $id  A session ID
     *
     * @return string      The session data if the session was read or created, otherwise an exception is thrown
     *
     * @throws \RuntimeException If the session cannot be read
     */
    public function sessionRead($id)
    {
        $data = '';
        
        if (null !== $this->memcache) {
		    $data = $this->memcache->get($id);
        }

		if ($data === false) {
            if (null !== $this->storage && method_exists($this->storage, 'sessionRead')) {
			    return $this->storage->sessionRead($id);
            } else {
                return array();
            }
		}

        if (null !== $this->memcache) {
            $this->memcache->set($id, $data, false, $this->lifetime);
        }

		return $data;
    }

    /**
     * Writes session data.
     *
     * @param  string $id    A session ID
     * @param  string $data  A serialized chunk of session data
     *
     * @return Boolean true, if the session was written, otherwise an exception is thrown
     *
     * @throws \RuntimeException If the session data cannot be written
     */
    public function sessionWrite($id, $data)
    {
        $result = false;

        if (null !== $this->memcache) {
            $result = $this->memcache->set($id, $data, false, $this->lifetime);
        }

        if (null !== $this->storage && method_exists($this->storage, 'sessionWrite')) {
            $this->storage->sessionWrite($id, $data);
        }

        return $result;
    }
}