$db   = new swoole_mysql;
$serv = new swoole_http_server("127.0.0.1", 9501);

$serv->on('WorkerStart', function(swoole_server $server) {
    global $db;
    $db->connect($db->server, function($db, $r) {
        if ($r === false) {
            var_dump($db->connect_errno, $db->connect_error);
            die;
        }
    });
});
$serv->on('Request', function($request, $response) {
        global $db;
        $db->query($sql, function(swoole_mysql $db, $r) {
        ........
        });
}

After a while, the mysql connection will be closed. How can I never close it?