private String getOngoingBackupId() throws IOException {
    ArrayList<BackupInfo> sessions = systemTable.getBackupInfos(BackupState.RUNNING);
    if (sessions.size() == 0) {
      return null;
    }
    return sessions.get(0).getBackupId();
  }
Check if any ongoing backup. Currently, we only reply on checking status in backup system
table. We need to consider to handle the case of orphan records in the future. Otherwise, all
the coming request will fail.
@return the ongoing backup id if on going backup exists, otherwise null
@throws IOException exception
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/impl/BackupManager.java