Persisting PHP sessions into mongodb (allows NLB without affinity)

example.php

<?php
// all default values
MongoSessionPersistence::create()
    ->connect()
    ->register();

// connect to concrete server/db
MongoSessionPersistence::create()
    ->connect('mongodb://mongo_server:27017', 'databaseName')
    ->useLifeTime(60*60) // lifetime=1 hour
    ->register();


// use existing database connection
MongoSessionPersistence::create()
    ->useCollection($db->phpSessionStoreCollection)
    ->register();


?>

mongo-session-persistance.php

<?php
class MongoSessionPersistence implements SessionHandlerInterface
{

    private $collection=null;
    private $lifeTime;

    public function register()
    {
        if(is_null($this->collection))
            $this->connect();
        $result = session_set_save_handler($this);
        return $result;
    }

    public static function create(){
        return new MongoSessionPersistence();
    }

    public function useLifeTime($lifetime){
        $this->lifeTime = $lifetime;
        return $this;
    }

    public function useCollection(MongoCollection $collection){
        $this->collection = $collection;
        $this->ensureIndex();
        return $this;
    }

    public function connect($server = 'mongodb://localhost:21017',
                            $database = 'test',
                            $collection = 'php_session')
    {
        return $this->useCollection((new MongoClient($server))
            ->selectDB($database)->selectCollection($collection));
    }


    function __construct()
    {
        $this->lifeTime = intVal(get_cfg_var('session.gc_maxlifetime'));
    }


    /**
     * PHP >= 5.4.0<br/>
     * Close the session
     * @link http://php.net/manual/en/sessionhandlerinterface.close.php
     * @return bool <p>
     * The return value (usually TRUE on success, FALSE on failure).
     * Note this value is returned internally to PHP for processing.
     * </p>
     */
    public function close()
    {
        // do nothing
        return true;
    }

    /**
     * PHP >= 5.4.0<br/>
     * Destroy a session
     * @link http://php.net/manual/en/sessionhandlerinterface.destroy.php
     * @param int $session_id The session ID being destroyed.
     * @return bool <p>
     * The return value (usually TRUE on success, FALSE on failure).
     * Note this value is returned internally to PHP for processing.
     * </p>
     */
    public function destroy($session_id)
    {
        $this->collection->remove(['_id' => $session_id]);
        return true;
    }

    /**
     * PHP >= 5.4.0<br/>
     * Cleanup old sessions
     * @link http://php.net/manual/en/sessionhandlerinterface.gc.php
     * @param int $maxlifetime <p>
     * Sessions that have not updated for
     * the last maxlifetime seconds will be removed.
     * </p>
     * @return bool <p>
     * The return value (usually TRUE on success, FALSE on failure).
     * Note this value is returned internally to PHP for processing.
     * </p>
     */
    public function gc($maxlifetime)
    {
        // do nothing, actual GC performed by MongoDB ttl index
        return true;
    }

    /**
     * PHP >= 5.4.0<br/>
     * Initialize session
     * @link http://php.net/manual/en/sessionhandlerinterface.open.php
     * @param string $save_path The path where to store/retrieve the session.
     * @param string $session_id The session id.
     * @return bool <p>
     * The return value (usually TRUE on success, FALSE on failure).
     * Note this value is returned internally to PHP for processing.
     * </p>
     */
    public function open($save_path, $session_id)
    {
        // do nothing
        return true;
    }

    /**
     * PHP >= 5.4.0<br/>
     * Read session data
     * @link http://php.net/manual/en/sessionhandlerinterface.read.php
     * @param string $session_id The session id to read data for.
     * @return string <p>
     * Returns an encoded string of the read data.
     * If nothing was read, it must return an empty string.
     * Note this value is returned internally to PHP for processing.
     * </p>
     */
    public function read($session_id)
    {
        $obj = $this->collection->findOne(['_id' => $session_id]);
        if ($obj) {
            $data = $obj['data'];
            return $data->bin;
        }
        return '';
    }

    /**
     * PHP >= 5.4.0<br/>
     * Write session data
     * @link http://php.net/manual/en/sessionhandlerinterface.write.php
     * @param string $session_id The session id.
     * @param string $session_data <p>
     * The encoded session data. This data is the
     * result of the PHP internally encoding
     * the $_SESSION superglobal to a serialized
     * string and passing it as this parameter.
     * Please note sessions use an alternative serialization method.
     * </p>
     * @return bool <p>
     * The return value (usually TRUE on success, FALSE on failure).
     * Note this value is returned internally to PHP for processing.
     * </p>
     */
    public function write($session_id, $session_data)
    {
        // obtain proper time from mongodb server
        $ts = $this->collection->db->command(['serverStatus' => 1
            , 'globalLock' => 0
            , 'metrics' => 0
            , 'repl' => 0
            , 'locks' => 0])['localTime'];

        $ttl = new MongoDate($ts->sec + $this->lifeTime, $ts->usec);

        $this->collection->save(['_id' => $session_id,
            'data' => new MongoBinData($session_data),
            'ttl' => $ttl]);
        return true;
    }

    public function create_sid()
    {
        $id = new MongoId();
        return "{$id}";
    }

    private function ensureIndex()
    {
        $this->collection->ensureIndex(['ttl' => 1], ['expireAfterSeconds' => 3600]); // +1 hour for sessions
    }
}
?>