304 Not Modified

The HTTP 304 is a response code servers use when they don’t see a need to transfer a resource.

304 Not Modified is a response to conditional HEAD and GET requests. The most common use-case is caching. For example, a browser can request a CSS file only if it has changed since the previous request; otherwise, a browser returns a cached copy stored locally.

Clients and servers use two sets of headers to enable this:

• Last-Modified and If-Modified-Since - date and time when the resource was last updated
• ETag and If-None-Match - a unique hash identifying the resource

Servers usually include Last-Modified and ETag headers in their responses (both can be used interchangeably). When clients send subsequent requests to the server, they include the values in those headers in the If-Modified-Since and If-None-Match headers, respectively.

When used in combination, If-None-Match takes precedence over If-Modified-Since because it’s more accurate to compare and track hashes rather than timestamps.

GET /voodoo.css HTTP/2
If-Modified-Since: Sun, 27 Jan 2021 08:28:37 GMT
If-None-Match: "1fd5d0a5-30d"

If the file hasn’t been modified, the server returns 304 Not Modified without a body.

HTTP/2 304 Not Modified

304 Not Modified is the equivalent of the 200 OK response if it hadn’t been for the conditional request.