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NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
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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
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