code: public void launchPollingThread(long mountId, long txId) {
      LOG.debug("launch polling thread for mount id {}, txId {}", mountId, txId);
      if (!mPollerMap.containsKey(mountId)) {
        try (CloseableResource<UnderFileSystem> ufsClient =
                 mMountTable.getUfsClient(mountId).acquireUfsResource()) {
          ufsClient.get().startActiveSyncPolling(txId);
        } catch (IOException e) {
          LOG.warn("IO Exception trying to launch Polling thread {}", e);
        }
        ActiveSyncer syncer = new ActiveSyncer(mFileSystemMaster, this, mMountTable, mountId);
        Future<?> future = getExecutor().submit(
            new HeartbeatThread(HeartbeatContext.MASTER_ACTIVE_UFS_SYNC,
                syncer, (int) ServerConfiguration.getMs(PropertyKey.MASTER_ACTIVE_UFS_SYNC_INTERVAL),
                ServerConfiguration.global()));
        mPollerMap.put(mountId, future);
      }
    }
docs: Launches polling thread on a particular mount point with starting txId.
  
  @param mountId launch polling thread on a mount id
  @param txId specifies the transaction id to initialize the pollling thread