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From "Christopher Lenz (JIRA)" <j...@apache.org>
Subject [jira] Reopened: (COUCHDB-257) HTTP caching headers don't provide expected behaviour
Date Wed, 16 Sep 2009 07:19:57 GMT

     [ https://issues.apache.org/jira/browse/COUCHDB-257?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Christopher Lenz reopened COUCHDB-257:
--------------------------------------


Oops, turns out I got this wrong. As Curt explains on the mailing list, adding an Expires
header set to the past, in combination with the "Cache-Control: must-revalidate" header we're
already sending, just forces the user agent to not apply any heuristics to determine freshness
and always revalidate with the server. This is the behavior we want.

Furthermore, according to Curt, max-age=0 or Expires=Date is insufficient because at least
with some user agents there's a one second window where you could still get the cached entity
without revalidation.

In light of this I retract my objection. Sorry for the noise!

> HTTP caching headers don't provide expected behaviour
> -----------------------------------------------------
>
>                 Key: COUCHDB-257
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-257
>             Project: CouchDB
>          Issue Type: Bug
>          Components: HTTP Interface
>    Affects Versions: 0.8.1, 0.9
>         Environment: Server: Ubuntu Hardy on x86. Client: Windows XP (32-bit).
>            Reporter: Vinay Sajip
>            Priority: Minor
>         Attachments: caching-header-patch.diff, expires.patch
>
>
> The HTTP caching headers currently put out cause IE (for example) to not display information
correctly in Futon. It's easy to reproduce: I open windows in Firefox and IE simultaneously,
do an update using Firefox (e.g. add a new document) and refresh the IE window. The updated
document count is not shown. If I clear the browser cache and try again, the updated information
is displayed.  The HTTP header put out is
> Cache-Control: must-revalidate
> which seems to me insufficient - for IE, at least. Is there way of configuring these
headers, to for example 
> Cache-Control: no-cache
> Pragma: no-cache
> Expires: some date in the past, or the same value as the Date: header
> Christopher Lenz has said about this that "This is due to extra-aggressive (and against
the HTTP spec) caching   that IE does on XMLHTTPRequests. A patch would need to do user agent
sniffing to conditionally add the  "cache: false"  parameter to the jQuery ajax() invocations
in   jquery.couch.js (and maybe elsewhere). I wouldn't want to add this for all user agents,
as it basically circumvents any caching for AJAX  requests (even for not-craptastically-broken
implementations), and  thus would add quite a bit of unnecessary overhead."
> To this, I would comment that I don't believe a patch to the client-side code in Futon
would be sufficient. There are other clients out there, some of which will be on Windows and
so by default use the (acknowledgely broken) Microsoft stack. In my view it is more important
to err on the side of correctness than performance - so I believe the headers generated server-side
need to change, as well as perhaps Futon client-side changes.
> I note that handle_uuids_req in couch_httpd_misc_handlers.erl uses the no-cache/Expires
scheme I mention.

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