Http code: What does 304 mean? How much do you know?

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1. http code 304 Not Modified

The HTTP status code 304 Not Modified is a no-content status code that indicates that the client sent a conditional request and the server found that the resource has not changed since the last request, so it returns this status code instead of retransmitting the same data.

When a client (usually a browser) requests a resource from a server, if the resource has been cached before, it will include some information in the request header, such as If-Modified-Since or If-None-Match, to ask the server whether the resource has been modified.

• If-Modified-Since: The client tells the server that it has not retrieved the resource since a certain date.
• If-None-Match: The client tells the server that it has a specific version of the resource (identified by the ETag) and asks whether this version is still valid.

If the server finds that the requested resource has not changed since the last request, it sends a 304 Not Modified response without including the actual content of the resource. This reduces bandwidth consumption and improves performance because the client can directly use its cached copy.

1.1. Example

1.1.1. Client request header example:

 GET /example.html HTTP/1.1 Host: www.example.com If-Modified-Since: Tue, 15 Nov 2022 12:45:26 GMT

1.1.2. Server response header example:

 HTTP/1.1 304 Not Modified Date: Tue, 28 Oct 2024 12:30:00 GMT Last-Modified: Tue, 15 Nov 2022 12:45:26 GMT Cache-Control: max-age=3600

In this example, the client provides an If-Modified-Since header indicating that it has a copy of the resource that has not been updated since the specified date. The server checks the resource and determines that it has not been modified since that date, so it returns a 304 Not Modified response.

For front-end developers, understanding the 304 Not Modified response can help optimize application performance, especially when network requests are frequent. Proper use of HTTP caching mechanisms can significantly reduce data transmission and improve user experience.