@classmethod
def instance(cls, key, client_id, server):
    """Get an existing instance of the request, if any, from server.active_requests,
        otherwise make a new one."""
    if (key, client_id) in server.active_requests:
        return server.active_requests[key, client_id]
    else:
        inst = cls(key, client_id, server)
        server.active_requests[key, client_id] = inst
        return inst
