function getFreeConnection() {
    pool = getConnectionsPool();
    try {
        pool.getConnection(function cb(err, connection) {

            if (err) {
                console.error(`error getting connection from pool; ${err}`);
                throw  err;
            }

            if (!connection) {
                console.error(`error getting connection from pool`);
                return null;
            }
        });
    } catch (e) {
        console.error(e);
    }
}

function getConnectionsPool() {
    pool = pool ?
        pool :
        mysql.createPool({
            host: process.env.MYSQL_HOST,
            user: process.env.MYSQL_USER,
            password: process.env.MYSQL_PASSWORD,
            database: process.env.MYSQL_DB
        });

    if (!pool) {
        console.error(`error getting connections pool`);
        return null;
    }
    return pool;
}