code: public long getReadyTimestamp(Constraints viewConstraints) {
      String normalizedConstraints = getNormalizedConstraints(viewConstraints);
  
      Path signalPath = new Path(signalDirectory, normalizedConstraints);
      // check if the signal exists
      try {
        try {
          FileStatus signalStatus = rootFileSystem.getFileStatus(signalPath);
          return signalStatus.getModificationTime();
        } catch (final FileNotFoundException ex) {
          // empty, will be thrown when the signal path doesn't exist
        }
        return -1;
      } catch (IOException e) {
        throw new DatasetIOException("Could not access signal path: " + signalPath, e);
      }
    }
docs: Check the last time the specified constraints have been signaled as ready.
  
  @param viewConstraints The constraints to check for a signal.
  
  @return the timestamp of the last time the constraints were signaled as ready.
  if the constraints have never been signaled, -1 will be returned.
  
  @throws DatasetException if the signals could not be accessed.