Does the `Expires` HTTP header needs to be consistent across multiple cold-cache requests?

I'm implementing a custom web server of a kind. And am looking into adding an Expires header support. However, I'm a little uns开发者_StackOverflow中文版ure of how exactly to implement it.

If multiple cold-cache requests are being made to the same unchanged resource on the server and the server returned different Expires header (say it uses relative time to calculate the exact value of the Expires date e.g. +6 hours from the request time), does that invalidate the cache on all the proxy servers in-between as well? Or is it impossible to happen (per the spec)?

Does the Expires HTTP header needs to be consistent across multiple cold-cache requests?

Ok, never mind, found the relevant information under the Cache Revalidation and Reload Controls section of the HTTP Spec

Basically, you can serve all the different validators you want but you must be aware that in such case proxies may have a set of different validators from their own cache and from various user agents communicating with the proxy. They may choose to send one to you and that might not be the correct or the most optimal one for the end-users. However, a "best approach" has been suggested in the spec.

I suppose this should covers Expires headers as well as ETags, Cache-Control and whatnot.

Here's the relevant excerpt, in case anyone's interested:

When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client has supplied its own validator in the request, the supplied validator might differ from the validator currently stored with the cache entry. In this case, the cache MAY use either validator in making its own request without affecting semantic transparency. However, the choice of validator might affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with 304 (Not Modified), then the cache can return its now validated copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate cache can compare the returned validator with the one provided in the client's request, using the strong comparison function. If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Otherwise, it returns the new entity with a 200 (OK) response. If a request includes the no-cache directive, it SHOULD NOT include min-fresh, max-stale, or max-age.