code: public static void notifyAboutCloudSize(InetAddress ip, int port, InetAddress leaderIp, int leaderPort, int size) {
      if (ARGS.notify_local != null && !ARGS.notify_local.trim().isEmpty()) {
        final File notifyFile = new File(ARGS.notify_local);
        final File parentDir = notifyFile.getParentFile();
        if (parentDir != null && !parentDir.isDirectory()) {
          if (!parentDir.mkdirs()) {
            Log.err("Cannot make parent dir for notify file.");
            H2O.exit(-1);
          }
        }
        try(BufferedWriter output = new BufferedWriter(new FileWriter(notifyFile))) {
          output.write(SELF_ADDRESS.getHostAddress());
          output.write(':');
          output.write(Integer.toString(API_PORT));
          output.flush();
        } catch ( IOException e ) {
          e.printStackTrace();
        }
      }
      if (embeddedH2OConfig == null) { return; }
      embeddedH2OConfig.notifyAboutCloudSize(ip, port, leaderIp, leaderPort, size);
    }
docs: Tell the embedding software that this H2O instance belongs to
  a cloud of a certain size.
  This may be non-blocking.
  
  @param ip IP address this H2O can be reached at.
  @param port Port this H2O can be reached at (for REST API and browser).
  @param size Number of H2O instances in the cloud.