Friday, May 29, 2009

Antichrist Structure

Last time I mentioned how I'll post some articles discussing how some of the key technologies (like the Dojo Toolkit, PostgreSQL, Twisted, Zope, Python, and Repoze) are used by Antichrist Watch. In this post I'll discuss some of the general architecture behind the site, along with motivations behind some of the design decisions I made. In future posts I'll follow up with specifics related to some of the key technologies employed.

I've already covered the originally planned core features of the site, so I won't repeat them here. What's important for this discussion is that those features require fast access to live data on votes, tallies, and comments, and the ability to display that data in a number of ways, and that everything be done cleanly according to current industry standards (that is, using decent semantic HTML backed by appropriate CSS and RDF that'll not cause the respective W3C validators to choke). Obviously an AJaX design is called for, and the individual toolkits and frameworks used must be flexible enough to support reasonably standards-compliant code and mark-up. This may sound trivial on the surface, but oftentimes toolkits, frameworks, and even design tools have their own axes to grind and introduce deliberate incompatibilities or proprietary extensions in an attempt to promote their own stuff or denigrate others' stuff.

PostgreSQL for the back-end database was an easy choice. It performs well, has great support for standard SQL (and even XML) built-in, and has advanced capabilities like triggers, rules, and stored procedures that can be used to help ensure data integrity. I'm a big believer in keeping code that enforces data integrity as deep within a Web application as practically possible; the closer it is to the front-end the more space there is for something to go wrong. With PostgreSQL I was able to get most of the code that handles data integrity embedded in the database itself, so not only is there not even a theoretical way for a crafty user to bypass constraints, there's not even a theoretical way for misbehaving middleware to bypass them. I'll discuss this in more detail later.

The Dojo Toolkit for the front-end JavaScript will be surprising to some as many believe its recent (post 1.0) versions cannot be used within a site that validates. This is incorrect. What is true is that Dojo's Dijit widgets can not be used in a validating site via mark-up without modifying Dojo's parser. This is a completely different statement, as there are other (and better) ways to apply Dijit widgets to a site. I'll discuss my use of the Dojo Toolkit in quite a bit more detail later. Most of the site's code is actually Dojo code, so I suspect I'll dedicate a couple of posts to it.

Of course, the JavaScript needs to be tied into some mark-up, and for this I used a single Zope Page Template designed to output clean XHTML. This would be trivial for a typical Zope site, but in this case it's certainly not typical as only scattered Zope technologies are being used. I'll get back to this in a bit.

The Dojo Toolkit has the concept of an abstracted data store that it leverages heavily for most of its widgets. It has numerous concrete implementations of this store that make binding it to various data sources fairly trivial. The one it includes for dealing with relational databases requires data to adhere to a particular format that would be awkward to generate directly in PostgreSQL, so it was necessary to create some code to fetch data from PostgreSQL and format it for Dojo. Since I had already built a great deal of intelligence into the PostgreSQL database itself, it was actually fairly simple to create this middleware, and during development I tested out a couple of different approaches (including raw Python and PHP) before settling on one designed around Twisted Python. It makes good use of Twisted Enterprise and the psycopg2 database adapter (including its splendid DictCursor) to make database access fast, efficient, and comprehensible. I also tied the rendering of the main Zope Page Template into this Twisted app by virtue of Repoze Chameleon in lieu of making it a typical Zope site. This whole layer is probably also worthy of its own post.

That's a quick run-through of what it takes to reveal the Antichrist. I'll be posting more details on these individual components later; please feel free to point out areas of particular interest.

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