17 April 2006 - 14:00Declarative programming and dynamic languages

I am following the dynamic vs static debate and its by-product, the debate of Ruby vs Java, not really understanding what the fuss is all about.

I think we could say that the language used in the XML descriptors of a package using declarative programming (such as an EJB XML descriptor, a struts-config.xml or bean-definitions.xml for Spring) fits the definition of a dynamic language.
You can change the behavior at runtime, you can create new classes and call them from different points in your program, you can even pass invalid objects to your program and find out that your program is not working because you didn’t XML-program (I have to admit ‘XML-program’ is an atrocious name) it correctly.

Declarative programming and the descriptors that enforce the contracts specified declaratively could be viewed as the place where a Java app would act in a dynamic way. The language of the application would be Java + various XML descriptors rather than only Java and this language could be viewed as dynamic.

So far declarative programming has been used mostly for infrastructure (EJB, Struts, etc…) but it would be interesting to see what results if it is being used for describing an application. I am thinking particularly about organizations that write programs that have to be easily customizable (such as a B2C shopping cart, a content-management system, etc…). It would be a good thing if I could describe this application in a declarative way so that I could describe and incorporate a customized business-process easily into the application. My app would be changed dynamically, customized behaviour would be created and passed to the appropriate components, etc… It would be written in Java and described and assembled in XML descriptors. It would be dynamic.

For example when the user presses the submit order button on an e-commerce site I should be able to specify in a declarative fashion that the DB will get updated, the credit card will be charged and the back-office system will know to send the product to the user and ask for replenishment. I want to add this and other functionality and turn it on/off at run-time.

To go back to the debate of Ruby vs Java, I think that Ruby will not get much penetration into enterprise development. The reason is that by now a huge investment has been made in JEE applications, an investment of the size of a few millions line of code that have been developed and, more importantly, tested and debugged. Assuming that you need to add some dynamic behavior to your product you could do so by applying declarative programming to your existing code base rather than re-writing the application once more in the language that happens to be popular. The debate of Ruby vs Java is a non-debate, it debates the current Ruby development environment and Java’s development environment as it was 4-5 years ago, i.e. Java running on its own. However, today most applications written in Java are usually run in some container that allows for loading behavior dynamically. Ruby vs Java + some frameworks would seem a more rational debate, however I have not seen anyone taking part in it.