Saturday, August 1, 2009

Will code for XML

When working on my hackweek project, the KDE SDK, I had to deal with some XML parsing for operating REST web services. To make this easier I made use of my old project to automatically generate XML parsing code from a
schema. Yesterday Tobias blogged about the new XML Schema features in Qt 4.6, and wrote about the need to generate code from schemas. Having this thought come up twice in only a few weeks can't be an accident. So I brushed up the Kode Home Page today, which has a solution for this problem.

Kode is a project to handle C++ code generation, especially in the context of XML parsing, with a special focus on web services. It's so important these days to be able to easily operate XML based web services, and handle all kind of other XML data conveniently, that we really need great support for this use case. In C++ this is usually relatively hard, mainly because of the static nature of the language, but with code generation this problem can be solved.

The tool which has developed over the last five years, admittedly with long periods of inactivity, is kxml_compiler. It can take an XML schema and generate C++ classes with a nice native Qt API which deal with representing the data and all the XML reading and writing stuff. This saves a lot of boring and annoying coding of trivial code.

When I started this project, I was convinced that an accurate and complete formal description was the key to this mechanism. Nowadays I'm not that convinced anymore. I found it much more practical to go for a 80% solution instead of striving for perfection. As result I implemented the support for using XML examples as a schema description. This makes it easy to just rip off examples from informal REST API documentations, or to use instances of an existing format, and let code be generated from that, which deals with this data. It works surprisingly well, and doesn't have a lot of overhead. Obviously it's lacking in some areas, and the implementation is still far from being production ready, but it is a useful tool and it solves the problem for quite some cases. It accommodates quite well for the needs of the rapidly changing world of Web 2.0 and its APIs.

The code lives in KDE playground. If you are interested in working on it let me know.

1 comment:

  1. Very interresting. kxml_compiler seems to be very useful. Will surely try it in my future projects.


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