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From "Jonathan Moore (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HTTPCLIENT-982) Could we get a way to know if the response has been served from the cache or not ?
Date Tue, 24 Aug 2010 14:02:16 GMT

    [ https://issues.apache.org/jira/browse/HTTPCLIENT-982?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12901889#action_12901889
] 

Jonathan Moore commented on HTTPCLIENT-982:
-------------------------------------------

Can I ask what the use case is for specifically knowing whether a response was served from
the (local) cache or not? The HTTP caching mechanism has been defined to maintain semantic
transparency, with the notion that a client perhaps can tell whether a response case straight
from an origin server or whether it was returned by a cache, and Age headers are the existing
protocol mechanism for this:

"The presence of an Age header field in a response implies that a response is not first-hand."

http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.3

Note that there may be other (proxy) caches in between the client and the origin server that
may be returning a cached response as well, not just the local cache.

The HTTP RFC has a very explicit definition of semantic transparency for a proxy, and we actually
have unit tests that verify this behavior with the CachingHttpClient. Note that adding additional
end-to-end headers (where extension headers are end-to-end headers) does not break semantic
transparency, and, as above, various intermediate proxies may well have done that to the original
response sent from the origin.

I'm really just bringing this up to make sure you're not expecting behavior from the local
cache that may not be supported by compliant HTTP/1.1 caches in general.

Ok, all that being said, let's get down to use cases: why do you need to know if a *specific*
response was returned by the cache or not? I think the answer to this can help us understand
where the best place to deliver this would be.


> Could we get a way to know if the response has been served from the cache or not ?
> ----------------------------------------------------------------------------------
>
>                 Key: HTTPCLIENT-982
>                 URL: https://issues.apache.org/jira/browse/HTTPCLIENT-982
>             Project: HttpComponents HttpClient
>          Issue Type: New Feature
>          Components: Cache
>    Affects Versions: 4.1 Alpha2
>            Reporter: Vianney Carel
>            Priority: Trivial
>
> Is there a way to know if the response has been served from the cache or not ?
> That's an information which might be useful for monitoring the activity of the cache.
> If there's no current way, maybe a flag could be added in the request context whenever
the response comes from the cache ... ?

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