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From "Adam Kocoloski (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-431) Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
Date Mon, 29 Aug 2011 17:10:37 GMT


Adam Kocoloski commented on COUCHDB-431:

I'm trying to digest this patch and finding it a bit difficult.  I understand the motivation
behind the use of the process dictionary, but I think the function names don't make it obvious
which ones are mutating the cors_headers.  Reading the patch I think that list is:


and the basic gist is, 1) reset cors_headers at the start of request processing, 2) if the
request method is OPTIONS, advertise the capabilities using the preflight_headers functions,
3) else, check the Origin against the list of allowed Origins specified in the _security object.

If I'm reading this correctly, the cors_headers that db_check_origin/2 sets on a successful
match are already set by set_default_headers/1 in the beginning of the request processing.
 I guess it doesn't hurt to be explicit.

What's the rationale for splitting the headers into SimpleHeaders and CouchHeaders in the
preflight_headers function?  It seems to be purely cosmetic.  I don't know what criteria were
used for including headers in that list, but I'm guessing X-Upondata-Api-Key was included
inadvertently :)  I don't recognize X-Couch-Auth-Key either.

I think this is one patch where we need a pretty thorough commit message describing the feature.

> Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
> -----------------------------------------------------------------------------------
>                 Key: COUCHDB-431
>                 URL:
>             Project: CouchDB
>          Issue Type: New Feature
>          Components: HTTP Interface
>    Affects Versions: 0.9
>            Reporter: James Burke
>            Assignee: Benoit Chesneau
>            Priority: Minor
>             Fix For: 1.2
>         Attachments: 0001-cors-support.-should-fix-COUCHDB-431-2.patch, 0001-cors-support.-should-fix-COUCHDB-431.patch,
0001-cors-support.-should-fix-COUCHDB-431.patch, 0001-cors-support.-should-fix-COUCHDB-431.patch,
A_0001-Generalize-computing-the-appropriate-headers-for-any.patch, A_0002-Send-server-headers-for-externals-responses.patch,
A_0003-Usably-correct-w3c-CORS-headers-for-valid-requests.patch, A_0004-Respond-to-CORS-preflight-checks-HTTP-OPTIONS.patch,
cors.html, test_cors2-1.tgz, test_cors2.tgz
> Historically, browsers have been restricted to making XMLHttpRequests (XHRs) to the same
origin (domain) as the web page making the request. However, the latest browsers now support
cross-domain requests by implementing the Access Control spec from the W3C:
> In order to keep older servers safe that assume browsers only do same-domain requests,
the Access Control spec requires the server to opt-in to allow cross domain requests by the
use of special HTTP headers and supporting some "pre-flight" HTTP calls.
> Why should CouchDB support this: in larger, high traffic site, it is common to serve
the static UI files from a separate, differently scaled server complex than the data access/API
server layer. Also, there are some API services that are meant to be centrally hosted, but
allow API consumers to use the API from different domains. In these cases, the UI in the browser
would need to do cross domain requests to access CouchDB servers that act as the API/data
access server layer.
> JSONP is not enough in these cases since it is limited to GET requests, so no POSTing
or PUTing of documents.
> Some information from Firefox's perspective (functionality available as of Firefox 3.5):
> And information on Safari/Webkit (functionality in latest WebKit and Safari 4):
> IE 8 also uses the Access Control spec, but the requests have to go through their XDomainRequest
object (XDR):
> and I thought IE8 only allowed GET or POST requests through their XDR.
> But as far as CouchDB is concerned, implementing the Access Control headers should be
enough, and hopefully IE 9 will allow normal xdomain requests via XHR.

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