def __init__(self, name, host='127.0.0.1', port=27017):
        self.name = name
        self.host = host
        self.port = port
def commit(self):
        context = self.pop()
        if self:
            # savepoint
            self[-1]['cbs'].extend(context['cbs'])
            self[-1]['dirty'] = self[-1]['dirty'] or context['dirty']
        else:
            # transaction
            for func, args, kwargs in context['cbs']:
                func(*args, **kwargs)