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Ok, I read the article and I totally agree with it. Ovid, you should NOT completely rewrite a fully functional, in-production application. So go ahead and re-architect that broken, non-scaling app with the knowledge that you may fully adhere to Joel's statements... once it is a bug-fixed, reliable, documented piece of work. =) Not many, if any, of the arguments for just incremental code improvements he states apply to your inherited app.
The way I'd go about not rewriting something like this is exactly how Joel states: moving code around while not affecting what it does until it can be segmented and replaced. But from what you say, a *lot* of this puppy was implemented incorrectly, undocumented, and the over-all architecture isn't sound.
The few times I've had to rework systems like that, I've tried to rewrite some of the base and try to adapt the existing modules to work within that, then slowly work over each part, breaking them into more logical parts, cleaning up the code or just rewriting as necessary... but these were on the order of around 5k-10k lines total (including whitespace, comments, etc). Nothing HUGE, by any means. =/
My $.02, but I feel like I'm just being a "yes" man. =P
In reply to Re: (OT) Rewriting, from scratch, a huge code base