If-Modified-Since vs. If-None-Match
The usual browser conditional request looks like this:
GET /i/yahoo.gif
HTTP/1.1 Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f" HTTP/1.1
But what is the relation between If-Modified-Since and If-None-Match. OR? AND? So if any of them "fails", a new version will be sent?
Solution 1:
http://www.ietf.org/rfc/rfc2616.txt
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates
An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since or If-Unmodified-Since header field) and one or more entity tags (e.g., in an If-Match, If-None-Match, or If-Range header field) as cache validators, MUST NOT return a response status of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in the request.
-
An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity tags as cache validators, MUST NOT return a locally cached response to the client unless that cached response is consistent with all of the conditional header fields in the request.
- Note: The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non-redundant information as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative assumptions about the validators they receive.
Solution 2:
Note that the newer "RFC 7232 Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests" has a section on precedence which changes how this behaviour is specified. This effectively says that If-None-Match
takes precedence over If-Modified-Since
. See:
3. When If-None-Match is present, evaluate the If-None-Match
precondition:
* if true, continue to step 5
* if false for GET/HEAD, respond 304 (Not Modified)
* if false for other methods, respond 412 (Precondition Failed)
4. When the method is GET or HEAD, If-None-Match is not present, and
If-Modified-Since is present, evaluate the If-Modified-Since
precondition:
* if true, continue to step 5
* if false, respond 304 (Not Modified)
5. When the method is GET and both Range and If-Range are present,
evaluate the If-Range precondition:
* if the validator matches and the Range specification is
applicable to the selected representation, respond 206
(Partial Content) [RFC7233]
6. Otherwise,
* all conditions are met, so perform the requested action and
respond according to its success or failure.
See here how step 3. causes an evaluation of the If-None-Match
pre-condition, and if it is present then you never go to step 4. which is the step that evaluates If-Modified-Since
.