1. An HTTP request for a static txt file returns a 304 response the first time and a 200 response the second time. See Figure 1.
2. Reference URL: https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Status/304 . HTTP 304 Not Modified indicates that there is no need to retransmit the requested content, meaning the cached content can be used. This typically occurs with safe methods such as GET or HEAD, or when the request includes the headers: If-None-Match or If-Modified-Since. The request headers for the first 304 response include: if-modified-since: Thu, 27 May 2021 06:10:50 GMT. See Figure 2.
3. Reference URL: https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Headers/If-Modified-Since . If-Modified-Since is a conditional request header; the server only returns the resource with a 200 status code if the requested resource has been modified since the given date and time. If the resource has not been modified since that time, the server returns a 304 response without a message body, and the Last-Modified header will contain the last modification time. Unlike If-Unmodified-Since, If-Modified-Since can only be used in GET or HEAD requests.
4. The request headers for the second 200 response do not include: if-modified-since: Thu, 27 May 2021 06:10:50 GMT. See Figure 3.
