'When a new message queue node is in, firstly will add the address
to the known queue nodes, then a backup for this node will be created.'

def add_node(self, addr):

    if (addr in self.addrs):
        return
    self.addrs.append(addr)
    backup_store_dir = os.path.join(self.dir_, BACKUP_STORE_FN)
    backup_node_dir = addr.replace(':', '_')
    backup_path = os.path.join(backup_store_dir, backup_node_dir)
    self.backup_stores[addr] = Store(backup_path, size=(512 * 1024), mkdirs=True)
