code: public void setLWMScn(String source, long scn) {
          WaterMarkEntry e = sourceWaterMarkMap.get(source);
          if (e == null) {
              e = new WaterMarkEntry(source);
              sourceWaterMarkMap.put(source, e);
          }
          e.setLWMScn(scn);
      }
docs: Sets the low water mark of a source
  
  @param source - the source
  @param scn    - the water mark value