How necessary it is to use "must-revalidate" in "Cache-Control: no-cache, must-revalidate"
cache-control, http, http-headers
Solution
`must-revalidate` should be specified by servers where it would be incorrect (and not just suboptimal) for a client to get a stale response. This applies to all requests with `max-stale` as you mentioned. It also applies, if a cache temporarily loses connectivity to the origin (a cache is allowed to return a stale entry with a `Warning` header in this case). That being said, I think you are right that this directive is not needed particularly in practice; it's seen most often in a situation where the origin wants to let a client cache a copy of the resource (for bandwidth conservation purposes) but always validate it before use, as in:
`Cache-Control: private, max-age=0, must-revalidate`
Problem
Often I use ``` Cache-Control: no-cache ``` or ``` Cache-Control: max-age=0 ``` The spec says `must-revalidate` is for `max-stale`... (the server issue `max-stale`?) So if for normal web servers, Apache, or Rails with Mongrels, then I think usually there is no `max-stale`, so `must-revalidate` is not needed?