An object oriented design mistake illustrated

November 9, 2007

Being written in Python, DWiki uses a certain amount of object oriented design (not huge amounts; it has basically no inheritance hierarchy among its classes, although they have various interrelations). In particular, it has a Page class that represents a particular real page at the model level (not the presentation level, which is where template processing and so on happens).

Here is a trick question: is a page's URL a property of the page? (That is, should the Page class have a url method or property.)

It certainly seems intuitive that a page's URL is a property of the page, and when I started writing DWiki I started out trying to code things up this way. It turned into a disaster, because the intuitive answer is wrong. In a web server environment, it turns out that a page's full absolute URL must be built from a combination of data from three objects:

  • the page itself is what knows its path relative to the root of the model (technically the model's data store).
  • the configuration is what knows the URL of the model's root, to which the page's path is added to get the absolute URL.
  • only the request knows what the web server's hostname and port is, because that comes from the Host: header in the request itself.

(A DWiki instance doesn't require you to talk to it with any particular hostname and port, because I dislike burning that sort of system-level information into what is effectively application configuration.)

One of the reasons I didn't realize this up front before I started coding DWiki was that I hadn't done enough work with HTTP and the web to really be aware of all of the issues; in fact I was so ignorant that I didn't realize I was ignorant, so I forged on in blithe confidence and ignored various warning signs.

(And I have to admit that I made some terrible howlers in this area even after I had DWiki and WanderingThoughts going. Writing DWiki has been not merely educational, but repeatedly educational.)

Written on 09 November 2007.
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Last modified: Fri Nov 9 23:53:41 2007
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