CAllDatabase::DoConnect

public function DoConnect($connectionName = '')
{
	if ($this->connection && $this->connection->isConnected())
	{
		// the connection can reconnect outside
		$this->db_Conn = $this->connection->getResource();
		return true;
	}

	$application = MainApplication::getInstance();

	$found = false;

	// try to get a connection by its name
	$connection = $application->getConnection($connectionName ?: (string)$this->connectionName);

	if ($connection instanceof MainDBConnection)
	{
		// empty connection data, using the default connection
		if ((string)$this->DBHost === '')
		{
			$found = true;

			$this->DBHost = $connection->getHost();
			$this->DBName = $connection->getDatabase();
			$this->DBLogin = $connection->getLogin();
			$this->DBPassword = $connection->getPassword();
		}

		// or specific connection data
		if (!$found)
		{
			$found = (
				$this->DBHost == $connection->getHost()
				&& $this->DBName == $connection->getDatabase()
				&& $this->DBLogin == $connection->getLogin()
			);
		}
	}

	// connection not found, adding the new connection to the pool
	if (!$found)
	{
		if ((string)$connectionName === '')
		{
			$connectionName = "{$this->DBHost}.{$this->DBName}.{$this->DBLogin}";
		}

		$parameters = [
			'host' => $this->DBHost,
			'database' => $this->DBName,
			'login' => $this->DBLogin,
			'password' => $this->DBPassword,
		];

		$connection = $application->getConnectionPool()->cloneConnection(
			ConnectionPool::DEFAULT_CONNECTION_NAME,
			$connectionName,
			$parameters
		);

		if ($this->bNodeConnection && ($connection instanceof MainDBConnection))
		{
			$connection->setNodeId($this->node_id);
		}

		$found = true;
	}

	if ($found)
	{
		// real connection establishes here
		$this->db_Conn = $connection->getResource();

		$this->connection = $connection;
		$this->sqlTracker = null;
		$this->cntQuery = 0;
		$this->timeQuery = 0;
		$this->arQueryDebug = [];

		return true;
	}

	return false;
}