XRX and MDA

Summary

This posting describes how using three technologies (XForms, REST and XQuery or XRX) you can dramatically transform your organizations development methodology. You can move true model-driven architecture (MDA) that dramatically reduces the temptation to duplicate code.

In January of 2007 I was working on a complex real-estate forms project for a state agency in Minnesota. Our job was to get 87 counties across the state of Minnesota to agree on the over 250 data elements that describe real estate transactions. We had spend months developing a complex XML Schema and web forms with over 50 one-to-many relationships. And out next task was to be able to save the form

data into a structure that could be quickly queried by any of these 250 data elements. But the prospect of "shredding" the documents up into 50 distinct SQL INSERT statements and then reconstituting the documents with 50 distinct SQL SELECT statements was going to require doubling our small team of four to at least eight developers. We didn't have the budget or schedule to do this. After chatting with Kurt Cagle he suggested saving the data to a database that supported XQuery. Both the open source eXist native XML database and our corporate standard, DB2 supported XQuery. It turned out that we could indeed save the entire document with a single command and still perform complex queries on any element in any document. Our project could proceed with a small team and stay on schedule.

And then an interesting thing happened. Our team started to understand that if we saved additional metadata in these native XML data stores we could accelerate our project even further. Every pick-list in every form could dynamically call REST-enabled web services to get their values. XML Schemas could be stored in the databases and be queried. We had found a simple and elegant solution to the pervasive problem of where to store the model in a model-driven development project.

In the past I had been on large teams of developers that attempted to use model-driven development. But although we started out each project with idealistic goals, each time we were faces with a classic option: copy, paste and edit the model or write a transform. The copy and paste solution was quick but it would have to be redone each time the model changes. The transform took longer to write but then could be rerun each time the model changed.

Like most developers, I always was over-optimistic that I had done my homework and done accurate and detailed modeling of the problem. I thought the models were stable and would not change very often. And I was almost always wrong. Models do change and sometimes in totally unexpected ways.

But by storing our models in a native XML database an interesting started to happen. We could quickly transform the models (stored in XML) into other artifacts with simple XQueries. It became far less tempting to commit the sins of copy/paste/edit when it only took five minutes to create another transform. And most remarkably, each of these transforms was another REST enabled web service that could be reused by many web clients or other development tools.

So how do you get started? You might try the following steps.

1. Download the exist database from http://www.exist-db.org
2. Use a WebDAV interface and copy some XML Schema documents into a collection
3. Write a few small XQuerys that pull metadata out of your XML Schemas. For example pull out all the enumeration values out of an element.
4. Use those XQueries to drive some aspects of your development such as an XForms selection list.
5. Compare the amount of code you wrote with any other MDA system. If you don't have at least a 10 to 1 savings I would be very surprised.

What do you think? Do you have a more efficient way to query your model? Let me know!

- Dan