Wednesday, November 8, 2017 5:12 PM
Conditional requests are those where the browser can ask the server if it has an updated copy of the resource. The browser will send some information about the cached resource it holds and the server will determine whether updated content should be returned or the browser’s copy is the most recent. In the case of the latter an HTTP status of 304 (not modified) is returned.
Though conditional requests do invoke a call across the network, unmodified resources result in an empty response body – saving the cost of transferring the resource back to the end client. The backend service is also often able to very quickly determine a resource’s last modified date without accessing the resource which itself saves non-trivial processing time.