Wednesday, October 10, 2007

Metadata Web Services

I have been using the eXist native XML database/web server for about nine months now and it is starting to change the way I think about metadata management.

My latest project for a financial institution requires us to quickly build XForms to manage various metadata as well as data. What I am finding is that my old method of storing metadata in XML files on a file system and then transforming the metadata using Apache Ant was a complex process. My new approach is to store the XML directly in eXist. This used to be a little bit hard since I thought that you had to use the eXist web interface to upload each XML file and ant scripts to backup your eXist database.

This all changed when I was shown how to use the Microsoft Windows WebDAV tool. Copying files to eXist and backing up the entire data store is just a drag and drop using Windows.

Now an entire new set of metadata web services are becoming much easier to build. Take the simple task of building a pick list of enumerate values for a form. XForms allows you to use the select1 control and specify an itemset using an XPath expression. I can now just load the data elements into an instance and grab the values and labels directly from the enumerations in the metadata registry files.

The only drawback to this is the fact that you load more metadata (like the full definitions) then you need in building the form. But once again eXist comes to the rescue. It is just a few lines of code to create a little web service (using XQuery) that you pass a code table to that returns just the label/value pairs. Using this method the selection list are always up to date and don't require any "batch" updates.

What I am learning from this is that in the past, metadata management was usually an after thought. Something that the coding standards people used to enforce database column naming conventions. But with metadata being stored in eXist metadata becomes part of application services. Building apps is just assembling forms that pull metadata from the registry in real time.

If you are concerned that the metadata registry server will be overloaded with requests for information each time forms load, we should remember that these services are RESTful. The results can also be cached so they don't have to be regenerated. I still have more to learn about how to make these services fast but since metadata is small it can usually always exist in RAM and disk I/O is very limited.

All of these developments are just small pieces of the puzzle at putting well-managed metadata at the core of your enterprise development methodologies. It is really the heart of the model-driven enterprise.

Let me know if you are creating metadata web services. I would like to know what things you feel are useful to your users.

- Dan

3 comments:

somegeek said...

Do you any sample metadata webservice using XQuery for a simple input field in an XForm? Thanks.

Dr. Data Dictionary said...

Can you be a little more specific? What are you looking for as far as inputs and output? For example given a data element what is its label?

- Dan

Trudy said...

Good words.