code: public void start() throws IOException {
      // Initialize UFS states
      for (AlluxioURI syncPoint : mSyncPathList) {
  
        MountTable.Resolution resolution = null;
        long mountId = 0;
        try {
          resolution = mMountTable.resolve(syncPoint);
          mountId = resolution.getMountId();
        } catch (InvalidPathException e) {
          LOG.info("Invalid Path encountered during start up of ActiveSyncManager, "
              + "path {}, exception {}", syncPoint, e);
          continue;
        }
  
        try (CloseableResource<UnderFileSystem> ufsResource = resolution.acquireUfsResource()) {
          if (!ufsResource.get().supportsActiveSync()) {
            throw new UnsupportedOperationException("Active Sync is not supported on this UFS type: "
                + ufsResource.get().getUnderFSType());
          }
          ufsResource.get().startSync(resolution.getUri());
        }
      }
      // attempt to restart from a past txid, if this fails, it will result in MissingEventException
      // therefore forces a sync
      for (long mountId: mFilterMap.keySet()) {
        long txId = mStartingTxIdMap.getOrDefault(mountId, SyncInfo.INVALID_TXID);
        launchPollingThread(mountId, txId);
  
        try {
          if ((txId == SyncInfo.INVALID_TXID)
              && ServerConfiguration.getBoolean(PropertyKey.MASTER_ACTIVE_UFS_SYNC_INITIAL_SYNC)) {
            mExecutorService.submit(
                () -> mFilterMap.get(mountId).parallelStream().forEach(
                    syncPoint -> {
                      try {
                        RetryUtils.retry("active sync during start",
                            () -> mFileSystemMaster.activeSyncMetadata(syncPoint,
                                null, getExecutor()),
                            RetryUtils.defaultActiveSyncClientRetry(ServerConfiguration
                                .getMs(PropertyKey.MASTER_ACTIVE_UFS_POLL_TIMEOUT)));
                      } catch (IOException e) {
                        LOG.warn("IOException encountered during active sync while starting {}", e);
                      }
                    }
            ));
          }
        } catch (Exception e) {
          LOG.warn("exception encountered during initial sync {}", e);
        }
      }
    }
docs: start the polling threads.