@Override
  public ModifyColumnResponse modifyColumn(RpcController controller, ModifyColumnRequest req)
    throws ServiceException {
    try {
      long procId = server.modifyColumn(ProtobufUtil.toTableName(req.getTableName()),
        ProtobufUtil.toColumnFamilyDescriptor(req.getColumnFamilies()), req.getNonceGroup(),
        req.getNonce());
      if (procId == -1) {
        // This mean operation was not performed in server, so do not set any procId
        return ModifyColumnResponse.newBuilder().build();
      } else {
        return ModifyColumnResponse.newBuilder().setProcId(procId).build();
      }
    } catch (IOException ioe) {
      throw new ServiceException(ioe);
    }
  }
Checks if the specified snapshot is done.
@return true if the snapshot is in file system ready to use, false if the snapshot is in the
        process of completing
@throws ServiceException wrapping UnknownSnapshotException if invalid snapshot, or a wrapped
                         HBaseSnapshotException with progress failure reason.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterRpcServices.java