How i can use multiple connection? (Master-Slave)

How i can use Master-Slave?? I added 2 databases in services.php:

$di->setShared('db', function () {
    $config = $this->getConfig();
    $class = 'Phalcon\Db\Adapter\Pdo\\' . $config->database->adapter;
    $params = [
        'host' => $config->database1->host,
        'username' => $config->database1->username,
        'password' => $config->database1->password,
        'dbname' => $config->database1->dbname,
        'charset' => $config->database1->charset
    ];
    $connection = new $class($params);
    return $connection;
});

$di->setShared('dbSlave', function () {
    $config = $this->getConfig();
    $class = 'Phalcon\Db\Adapter\Pdo\\' . $config->database->adapter;
    $params = [
        'host' => $config->database2->host,
        'username' => $config->database2->username,
        'password' => $config->database2->password,
        'dbname' => $config->database2->dbname,
        'charset' => $config->database2->charset
    ];
    $connection = new $class($params);
    return $connection;
});

in model add next code:

    public function initialize()
    {
        $this->setReadConnectionService('dbSlave');

        $this->setWriteConnectionService('db');
    }

why all my queries was executed only for 'db' connection?? and what i did wrong?? if possible, please give me a real working example, not a quote from the documents..thanks