code: protected static String getWatermarkFromPreviousWorkUnits(SourceState state, String watermark) {
      if (state.getPreviousWorkUnitStates().isEmpty()) {
        return ComplianceConfigurationKeys.NO_PREVIOUS_WATERMARK;
      }
      return state.getPreviousWorkUnitStates().get(0)
          .getProp(watermark, ComplianceConfigurationKeys.NO_PREVIOUS_WATERMARK);
    }
docs: Fetches the value of a watermark given its key from the previous run.