lib/storage/dataset-clients.js
function diff(datasetClientJson, localRecords, next) {
      var recordsDiff = diffRecords(syncUtil.convertToObject(localRecords), syncUtil.convertToObject(records));
      return next(null, datasetClientJson, recordsDiff);
    }
function diff(datasetClientJson, localRecords, next) {
      var recordsDiff = diffRecords(syncUtil.convertToObject(localRecords), syncUtil.convertToObject(records));
      return next(null, datasetClientJson, recordsDiff);
    }
to reduce the number of db operations, we try to compute only the records that need to be changed here.