304s

The 304s refer to a combination of a HTTP status code and a related HTTP request, specifically HTTP/1.0. When a web server receives an HTTP request, but finds no resource matching the request's URL, it returns a response indicating that the requested resource cannot be found. This is known as a 304 response from the server, in which the server returns an empty response body, but lists the headers that would be returned in a successful request.

In order to get a 304 response, a browser or client first sends a request for a specific resource. If the request indicates that the browser or client wants to only receive data if it has changed (via the If-Modified-Since or If-Unmodified-Since headers), the server will then return the headers of what the server would return in case of a successful request. Since the server indicates that the requested resource has not changed, it will return a 304 status code in conjunction with an empty body.

A 304 response includes some additional information: the Last-Modified and ETag headers. These headers are used for tracking changes, enabling the web browser cache to decide whether the response has been changed, and therefore should be updated or left unchanged. The server-side configuration can determine whether changes to resources should be immediately notified to the users or whether intermediate, falsified status codes should be given to them, including further delays.