def sse(self, request):
        """SSE."""
        response = Response(status=200)
        response.content_type = 'text/event-stream'
        response.text = ''
        active_request_id = request.GET.get('request_id')
        client_last_request_id = str(request.headers.get('Last-Event-Id', 0))
        if self.history:
            last_request_id = next(reversed(self.history))
            if not last_request_id == client_last_request_id:
                data = []
                for _id in reversed(self.history):
                    data.append([
                        _id, self.history[_id].json, 'active' if active_request_id == _id else ''])
                if data:
                    response.text = U_SSE_PAYLOAD.format(last_request_id, json.dumps(data))

        return response