Perhaps more concretely, the thredds server
currently supports access controls such as
passwords and client-side keys. How would
CORS affect those?
=Dennis Heimbigner
Unidata
Roberto De Almeida wrote:
Hi, guys!
In 2006 I wrote an implementation of an OPeNDAP client in Javascript called
jsdap (https://code.google.com/p/jsdap/). At the time Javascript was still
a toy language and the XML HTTP Request (XHR) was unable of handling binary
data, but I managed to hack a full client that worked in all major browsers
(including IE by injecting vbscript!). And while it was written more as a
proof-of-concept the client is actually used in some data portals like
http://www.ifremer.fr/oceanotronPortal/. (A Node.js OPeNDAP server was also
added 3 years ago.)
Fast forward 7 years and we now have a lot of new technologies on the
table: a new XHR object with support for binary transfers, typed arrays and
WebGL. I've been playing again with using Javascript as an OPeNDAP client,
in particular to display real time information from OPeNDAP servers. I have
set up a small OPeNDAP server on one of my VPS streaming the system load
information:
http://vps.dealmeida.net:5000/.dds
http://vps.dealmeida.net:5000/.das
This is an infinite dataset (try "curl http://vps.dealmeida.net:5000/.asc";),
and it will keep streaming the data at one record per second until the
connection is broken. Keep in mind that this is a regular OPeNDAP Sequence,
and nothing was changed in the specification to make this work.
Nevertheless, I'm not aware of OPeNDAP clients that can access the stream
other than the development version of Pydap.
On another machine I have a widget displaying the information on a real
time graph: http://dealmeida.net/opendap-streaming/
You can see how everything was implemented on this Mercurial
repository<http://code.dealmeida.net/opendap-streaming/src/356dde80f6e55603c2ab7e581244015663504fda?at=demo>.
The
data is displayed by fetching the .dods response and parsing it. We still
need a few hacks to do this, but only because the data is being streamed
(Mozilla handles it nice; Chrome cannot stream binary data, so it still
fetches it as string). Handling regular OPeNDAP datasets should be pretty
straightforward with the new XHR, and I plan to rewrite jsdap as soon as I
have some free time.
*Now to my request:* the only reason that the demo works -- having a page
in one host displaying data from an OPeNDAP server on another -- is because
I
enabled<http://code.dealmeida.net/pydap/commits/4c2d38b5822ba8f5f61e83bcb23230a2ca7e5da1>
CORS <http://en.wikipedia.org/wiki/Cross-origin_resource_sharing> on Pydap.
By default, now all DODS, DAS and DDS responses from Pydap have the
following additional headers:
Access-Control-Allow-Origin: *
Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type
These headers (the first one, actually) allow the responses to be accessed
through XHR from any host. As far as I know there is no downside in doing
this. Even if your server use cookies for authenticating access to certain
datasets the cookies *will not* be sent unless
the Access-Control-Allow-Credentials header is set (and set to true), which
would allow other sites to "steal" your data and download it by
impersonating a logged user.
My request is that all OPeNDAP servers enable CORS from any host by default
today, at least in the DODS, DAS and DDS responses; and if not by default,
at least as an option. This way, by the time Javascript matures enough so
that its performance on the browser becomes comparable to desktop
applications we can start building rich web applications that use all the
data available through OPeNDAP.
Some resources
About CORS: http://en.wikipedia.org/wiki/Cross-origin_resource_sharing /
https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS
Security concerns:
https://code.google.com/p/html5security/wiki/CrossOriginRequestSecurity
Thank you,
Rob
