If the content hasn't changed since the time indicated in the If-Modified-Since header, the Google data service returns a 304 (Not Modified) HTTP response.

如果自if - Modified - Since标头中指示的时间后内容未发生更改，那么Google数据服务将返回一个304 (Not Modified) HTTP响应。

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The client also reads the Last-Modified response header and sends back the date value in an If-Modified-Since header to ask the server if the resource has changed.

客户端还读取Last - Modified响应Header，并在if - Modified - Since Header中发回日期值，以向服务器询问资源是否已更改。

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The browser sends the server an If-Modified-Since GET request in such scenarios, and the server can very efficiently return Not-Modified if nothing more recent is available.

在这种情况下，浏览器向服务器发送一个if - Modified - Since GET请求，如果没有最近的内容可用，则服务器能非常高效地返回Not - Modified。

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