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From "Jon Moore (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HTTPCLIENT-1789) 200 Response with Vary header does not invalidate cached 404 response without
Date Mon, 28 Nov 2016 16:15:00 GMT

    [ https://issues.apache.org/jira/browse/HTTPCLIENT-1789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15702356#comment-15702356
] 

Jon Moore commented on HTTPCLIENT-1789:
---------------------------------------

I see what you're saying, and agree this behavior is surprising. I believe the issue is that
the original `404` response did not have a `Vary` header on it; that's not a case I believe
we contemplated when writing the caching module originally.

Let me go back and look at the implementation to see how hard this might be to fix, although
if you have a patch or pull request ready to go I'd be happy to review that too.

I'm going to adjust the priority of the issue to `Minor`; this is typically how we classify
cases where the caching module could cache something but doesn't, or could return a cached
entry but doesn't (the rationale being that the response returned to the client is still a
semantically correct one).


> 200 Response with Vary header does not invalidate cached 404 response without
> -----------------------------------------------------------------------------
>
>                 Key: HTTPCLIENT-1789
>                 URL: https://issues.apache.org/jira/browse/HTTPCLIENT-1789
>             Project: HttpComponents HttpClient
>          Issue Type: Bug
>          Components: HttpCache
>         Environment: Tested with the org.apache.httpcomponents.httpclient 4.3.6 OSGi
bundle distributed by Eclipse Orbit: <http://download.eclipse.org/tools/orbit/downloads/drops/R20160520211859/>
>            Reporter: Andreas Sewe
>
> While implementing my own {{HttpCacheStorage}} I noticed the following problematic cache
revalidation behavior. FYI, this behavior also occurs with {{BasicHttpCacheStorage}} (created
through {{CachingHttpClients.createMemoryBound()}}), so it is not caused by my {{HttpCacheStorage}}
implementation. Consider this sequence of requests and responses:
> * {{GET /something HTTP/1.1}}
> * {{Accept: application/json}}
> * {{404 Not Found HTTP/1.1}}
> * {{Cache-Control: max-age=60}}
> This response is cached under the key {{/something}}. After 60 seconds, another {{GET}}
request is performed and send over the network, as the cached {{404}} response is stale.
> * {{GET /something HTTP/1.1}}
> * {{Accept: application/json}}
> * {{200 OK HTTP/1.1}}
> * {{Vary: Accept}}
> * {{Cache-Control: max-age=120}}
> This response is cached under the key {{\{Accept:application/json\}/something}} and key
{{/something}}’s {{variantMap}} is updated to refer to this key. After another 60 seconds,
a third {{GET}} request is performed which again performs *network I/O* – even though it
IMHO should not.
> * {{GET /something HTTP/1.1}}
> * {{Accept: application/json}}
> * {{200 OK HTTP/1.1}}
> * {{Vary: Accept}}
> * {{Cache-Control: max-age=120}}
> This re-validation occurs because a stale {{404}} response for {{/something}} was cached
– although its {{variantMap}} contains a fresh, selectable {{200}} response.
> FWIW, [RFC 7234|https://tools.ietf.org/html/rfc7234#page-9] has this to say about the
subject:
> {quote}
>    The stored response with matching selecting header fields is known as
>    the selected response.
>    If multiple selected responses are available (potentially including
>    responses without a Vary header field), the cache will need to choose
>    one to use.  When a selecting header field has a known mechanism for
>    doing so (e.g., qvalues on Accept and similar request header fields),
>    that mechanism MAY be used to select preferred responses; of the
>    remainder, the most recent response (as determined by the Date header
>    field) is used, as per Section 4.
> {quote}
> According to this, the {{200}} response should have been selected, as its {{Date}} is
newer than the {{404}}'s responses. Instead, another request for {{/something}} is send to
the server, even though the most recent cache entry is still fresh.



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