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From "Jason Smith (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-431) Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
Date Tue, 17 May 2011 03:09:48 GMT


Jason Smith commented on COUCHDB-431:

Thanks for getting the discussion started! This is why I don't think the patch is ready. Still,
the security object idea was mine but I later abandoned it.

CORS Overview

There is a "simple" query and a non-simple query. Simple queries use a simple method, and
only simple headers.

Simple methods are GET, HEAD, POST.
Simple headers are Accept, Accept-Language, Content-Language, Last-Event-ID, and Content-Type
(which must be application/x-www-form-urlencoded, multipart/form-data, or text/plain).

For a *simple* cross-origin XHR query, the browser just runs it immediately, with an "Origin"
header. If the response header Access-Control-Allow-Origin matches the request header "Origin",
then the browser passes the response back into Javascript.

For a non-simple cross-origin XHR query, the browser queries the URL with an OPTIONS method.
It sets an "Origin" header, plus Access-Control-Request-Method and Access-Control-Request-Methods.
If the server responds affirmatively, the browser runs the real query and then passes the
response back into Javascript. (The server can also indicate whether the resource allows logged-in
users (basic auth, cookie headers, etc.), or whether all queries must be anonymized by the

So the CORS policy is per *resource* (URL), but resources can only whitelist entire domains.
(Well, actually "origins" which is http or https, the DNS domain, and the port.)

Couch Discussion

About vhosting and rewriting, Couch just needs to return the right policy for the final resource
it processes.

My first attempt was to do direct domain-to-domain whitelisting.

Firstly, you need universal CORS to respond to misc. queries: / (to ping couch), /_all_dbs,
/_session, /_log, /_config, and also PUT /db.

A good use case is a Futon fork. You whitelist and then when you
go to that site, it can control every aspect of your couch just like old busted Futon.

The same technique would enable, something like:

1. You go to
2. You put your couch URL in a form field
3. The page pings your couch to check for CORS.
4. If no CORS, it pops up an iframe or target=_blank link to your couch Futon configurator.
You add the app to the whitelist.
5. The app keeps pinging your couch until it gets a pong (it was added to the whitelist)
6. With CORS, and with your admin session, the app can install itself on your couch (replicate,
set up vhosts and rewrites, etc.).

So you just installed a couchapp by following a link on Twitter, and all you had to do was
input one URL into your whitelist. ( of course has a superior UI for this).

That use case (and simplicity) is why I started with domain-to-domain permission. To open
up just one db, you can vhost db.mycouch:5984 -> mycouch:5984/db/ and make sure httpd.secure_rewrites
is true.

> 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
>            Priority: Minor
>         Attachments: 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
> 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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