Monday, January 27, 2003

XML Pipeline Definition Language

In February of 2002, Sun Microsystems submitted their XML Pipeline Definition Language to the W3C. See

I took the time to read the specification because I am interested in alternative ways to create composite web services. I found this spec hard to read - in a Sun kind-of-way, but here are some thoughts:
- XML PDL goes beyond web services, it is about processing xml documents in a pipeline manner, thus it may have significant relevance outside of the web service world.
- XML PDL uses a declarative approach to defining inputs and output. It makes the assumption that if I produce X and you consume X, then the producer will automatically feed the consumer. There is no need to write the actual web service call - it is implied. If multiple participants could produce X or consume X, it cries foul - and declares it an error. This is rather odd, but will work when you define a well scoped process that the pipeline works within.
- XML PDL is expressed in XML and can be implemented in your favorite language (C, C++, Java, etc.)

All in all, I gotta pass on this one as a real player in the web services world. Although, I do see the need to steal some of the concepts. I liked the concept of creating a 'pipeline scope' and performing implicit invocations. Now, this won't work in many environments, but I do see it being valuable in ad-hoc scripting environments, but with this said, you don't want to do ad-hoc scripts in an xml dialect IMHO. If XML PDL finds a home, it will likely be found behind the scenes in environments like web service platforms and web service scripting language implementations.

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