code: public function connect()
      {
          if ($this->isConnected() === false) {
              try {
                  if (isset($this->servers['all']) === true) {
                      $dsn = $this->providerDatabase->getDsn($this->servers['all']);
                      $this->servers['all']['link'] = new \PDO($dsn, $this->servers['all']['user'], $this->servers['all']['pass']);
                  } else {
                      $dsn = $this->providerDatabase->getDsn($this->servers['read']);
                      $this->servers['read']['link'] = new \PDO($dsn, $this->servers['read']['user'], $this->servers['read']['pass']);
                      $dsn = $this->providerDatabase->getDsn($this->servers['write']);
                      $this->servers['write']['link'] = new \PDO($dsn, $this->servers['write']['user'], $this->servers['write']['pass']);
                  }
              } catch (\PDOException $e) {
                  throw new ExternalLink('can\'t connect to database (' . $e->getMessage() . ')');
              }
          }
      }
docs: Connect to the database.
  
  @throws ExternalLink if database connection can't be established