Tornado Auto Etag 机制

    为了研究缓存看了 tornado 里的 finish 函数


        def finish(self, chunk: Union[str, bytes, dict] = None) -> "Future[None]":
            """Finishes this response, ending the HTTP request.
            Passing a ``chunk`` to ``finish()`` is equivalent to passing that
            chunk to ``write()`` and then calling ``finish()`` with no arguments.
            Returns a `.Future` which may optionally be awaited to track the sending
            of the response to the client. This `.Future` resolves when all the response
            data has been sent, and raises an error if the connection is closed before all
            data can be sent.
            .. versionchanged:: 5.1
               Now returns a `.Future` instead of ``None``.
            if self._finished:
                raise RuntimeError("finish() called twice")
            if chunk is not None:
            # Automatically support ETags and add the Content-Length header if
            # we have not flushed any content yet.
            if not self._headers_written:
                if (
                    self._status_code == 200
                    and self.request.method in ("GET", "HEAD")
                    and "Etag" not in self._headers
                    if self.check_etag_header():
                        self._write_buffer = []
                if self._status_code in (204, 304) or (
                    self._status_code >= 100 and self._status_code < 200
                    assert not self._write_buffer, (
                        "Cannot send body with %s" % self._status_code
                elif "Content-Length" not in self._headers:
                    content_length = sum(len(part) for part in self._write_buffer)
                    self.set_header("Content-Length", content_length)
            assert self.request.connection is not None
            # Now that the request is finished, clear the callback we
            # set on the HTTPConnection (which would otherwise prevent the
            # garbage collection of the RequestHandler when there
            # are keepalive connections)
            self.request.connection.set_close_callback(None)  # type: ignore
            future = self.flush(include_footers=True)
            self._finished = True
            return future

    从代码中可以看出, 满足下面条件的请求:

    • self._headers_written 为不为True
    • http status 为 200GETHEAD 请求
    • 同时没有 Etagresponse header 的情况下

    tornado 会自动计算返回结果的 sha1, 并设置 Etag 若客户端支持 Etag 机制, 正确返回 If-None-Match, 就能节约一波流量, 美滋滋.