private function newClusterConnection($database, $mode) {
  $master = PhabricatorDatabaseRef::getMasterDatabaseRef();

  if ($master && !$master->isSevered()) {
    $connection = $master->newApplicationConnection($database);
    if ($master->isReachable($connection)) {
      return $connection;
    } else {
      if ($mode == 'w') {
        $this->raiseImpossibleWrite($database);
      }
      PhabricatorEnv::setReadOnly(
        true,
        PhabricatorEnv::READONLY_UNREACHABLE);
    }
  }

  $replica = PhabricatorDatabaseRef::getReplicaDatabaseRef();
  if (!$replica) {
    throw new Exception(
      pht('No valid databases are configured!'));
  }

  ...