code: private void handleMasterCommand(Command cmd) throws IOException, ConnectionFailedException {
      if (cmd == null) {
        return;
      }
      switch (cmd.getCommandType()) {
        // Currently unused
        case Delete:
          break;
        // Master requests blocks to be removed from Alluxio managed space.
        case Free:
          mAsyncBlockRemover.addBlocksToDelete(cmd.getDataList());
          break;
        // No action required
        case Nothing:
          break;
        // Master requests re-registration
        case Register:
          mWorkerId.set(mMasterClient.getId(mWorkerAddress));
          registerWithMaster();
          break;
        // Unknown request
        case Unknown:
          LOG.error("Master heartbeat sends unknown command {}", cmd);
          break;
        default:
          throw new RuntimeException("Un-recognized command from master " + cmd);
      }
    }
docs: TODO(calvin): Evaluate the necessity of each command.