public String readBackupStartCode(String backupRoot) throws IOException {
    LOG.trace("read backup start code from backup system table");

    try (Table table = connection.getTable(tableName)) {
      Get get = createGetForStartCode(backupRoot);
      Result res = table.get(get);
      if (res.isEmpty()) {
        return null;
      }
      Cell cell = res.listCells().get(0);
      byte[] val = CellUtil.cloneValue(cell);
      if (val.length == 0) {
        return null;
      }
      return new String(val, StandardCharsets.UTF_8);
    }
  }
Read the last backup start code (timestamp) of last successful backup. Will return null if
there is no start code stored on hbase or the value is of length 0. These two cases indicate
there is no successful backup completed so far.
@param backupRoot directory path to backup destination
@return the timestamp of last successful backup
@throws IOException exception
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/impl/BackupSystemTable.java