In a nutshell, OData adds a definition language for database-like Entities (think SQL tables) to Atom and provides conventions for representing their properties (think SQL columns) in XML. Their definition language is based on ADO.NET, yes they could have chosen different bindings which would have made more work for their engineers, less for the rest of us and improved the chances of widespread adoption. But it is what it is (a phrase I seem to be hearing a lot of recently).
One of the OData-defined conventions is that data services can publish a metadata document which describes the entities that you are likely to encounter in the Atom feeds it publishes. This can be useful if you want to POST a new entry and you don't know what type of data property X is supposed to have. To get the metadata document you just get a special URL, for the sample data service published by Microsoft:
To see what this might look like in the real world you can also look at the $metadata document published as part of the Netflix OData service I used as the source of my examples last time.
Wouldn't it be nice to have a simple documented form of this information? The schema on which it is based even allows annotations and Documentation elements. I browsed the web a bit but couldn't find anyone who had done this so I wrote a little XSLT myself. Here is a picture of part of the output from transforming the Netflix feed
Now there is an issue here. One of the things that I've commented on before is the annoying habit of specification writers to change the namespace they use when a new version is published. I can see why some people might do this but when 90% of the spec is the same it just causes frustration as tools which look for one namespace have to have significant revisions just to work with a minor addition of some optional elements.
As a result, I've published two versions of the XSLT that I used to create the above picture. If somebody out there knows how I can do all of this in one XSL file without lots of copying and pasting I'd love to know.
The first xslt uses the namespace from CSDL 1.1 and can be used to transform the metadata from the sample OData service published by Microsoft. The second xslt uses the namespace from CSDL 2.0 and must be used to transform the metadata from Netflix. If you get a blank HTML file with just a heading then try the other one. When clicking on these links your browser may attempt to render them as HTML, you can "View Source" to see the XSLT as plain text.
Here is how I've used these files on my Mac:
$ curl http://services.odata.org/OData/OData.svc/\$metadata > sample.xml $ xsltproc odata2html-v1p1.xsl sample.xml > sample.html $ curl http://odata.netflix.com/v2/Catalog/\$metadata > netflix.xml $ xsltproc odata2html-v2.xsl netflix.xml > netflix.html
This transform could obviously be improved a lot, it only shows you the Entities, Complex Types and Associations though I am rather proud of the hyperlinking between them.