def get_last_tid(self):
        try:
            spec = {'tid': {'$gt': 0}}
            sort = [('tid', -1)]
            row = self.db.docs.find_one(spec, sort=sort)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return row['tid']
        return 0
def update_snapshot_status(self, snapshot_id, status, progress):
        """Update the specified snapshot's status."""
        post_body = {
            'status': status,
            'progress': progress
        }
        post_body = json.dumps({'os-update_snapshot_status': post_body})
        url = 'snapshots/%s/action' % str(snapshot_id)
        resp, body = self.post(url, post_body)
        return resp, body