So, in the spirit of pico-projects, I've started building that address book application I mentioned awhile ago and I want to start writing about it as I go.

First off, hopefully you'll notice the quick diagram I threw together in OmniGraffle. This is a sort of rough sketch of the loosely-joined architecture I want to explore with this thing.

• Data: This is where address book entries live.
• Model: A set of objects encapsulating the data, this is how address book entries will be accessed.
• REST API: Model objects exposed as resources identified by URI, serialized and deserialized as XML, and manipulated by GET / PUT / POST / DELETE methods.
• XSLT Filter: XML data produced by REST API calls can be first passed through XSL at a given URL before being served up as a response.
• HTML, CSS, JavaScript: Thanks to the XSLT filter layer, the XML vocabulary used to describe address book entries can be transformed into user interface presentation.
• HTTP: Everything happens via HTTP...
• Web Browser Client: ...and everything is viewed in a web browser.

Now, I call this a loosely-joined architecture because I want to stress that you should be able to swap out just about any part of this whenever you want.

Want the Data to be in MySQL? Fine. Want it to be in flat files? Fine. Just make sure the Model can cope while maintaining a consistent interface for the REST API. Want to change the user interface in the browser? Great-- ideally, all you have to do is change some XSLT files. I'm writing everything from the XSLT Filter down to the Model in Python. Don't like that? Fine. Rewrite it all in Perl, and hopefully everything from the XSLT up to the browser will still be useful to you.

At some point, you might even want to ditch the browser for a native desktop client. Fabulous! Just ignore everything past the REST API and HTTP, don't use any XSLT in the Filter, and use the API and XML directly.

I don't think any of this is particularly revolutionary-- although I thought it was when I first saw Amazon Web Services doing some of this, and I hope to throw a little GMail in as well. I hope that this will all be useful as I muddle through explaining what I'm doing. In the meantime, you can see me getting the stage set as I start checking things into my Subversion repository over here:

• http://www.decafbad.com/svn/trunk/hacks/abook/

shortname=abook1