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Rewriting from scratch? Yeah, it's a bad idea.
Friday, December 2, 2011 | Permalink

We all know the feeling. We have code that's been lying around for a long time, collected a bit of dust, gotten a bit obsolete perhaps, or gotten twisted up and hacked around. Code that we're just not very happy with. It's a mess. It's ugly. It's fundamentally broken. It's not future proof. It's probably still working and doing its thing for the most parts, but we're not happy with it. So we're bringing up that big "rewrite" idea.

I'm probably just rehashing common knowledge here, but rewriting software from scratch is almost always a bad idea. Yet it is so appealing. And smart people fall into the trap all the time. And we never learn. No amount of experience seems to take the sheen out of the imagined perfect cleaned up code that would result from such an effort.

There are others that have written on this topic before, for instance here. And I could add another example to the list, the Orca driver back when I was at ATI. It was decided that the old OpenGL driver sucked. It was a mess. Nvidia had been producing higher quality drivers, but from my perspective we were catching up and was actually in a pretty decent shape at that point. Of course, I wasn't a driver writer so what did I know? When I heard about the complete rewrite of the OpenGL driver, I was quite skeptic but positive at the same time. But before long I started questioning the wisdom of going down this path. Initially both drivers were maintained in parallel. Fewer bugs were fixed and very little happened in terms of features, but there was hope for the better once the new driver was in good shape. But things were taking longer than expected. Don't it always? Eventually the whole team worked on the new driver. The old driver came to a complete halt. No updates. Month after month. Nvidia was pumping out new cool extensions on a regular basis like always. The new driver wasn't ready to ship. Fun stuff, especially when the next hardware generation came around, like they do on regular basis, and gosh, perhaps we need an OpenGL driver for it? The new one didn't have nearly all the features of the old one, nor was it higher performance. Of course, once the driver was in good enough shape that it could run Doom3 it was all fun again. Soon enough they had optimized it so that it showed a decent performance gain over the old driver. So it was all worth the effort, yay! Except of course, Doom3 was important and the Orca driver wasn't ready for prime time, so some of these optimizations were backported into the old driver, and would you know it, now the old driver was faster again!
Now, I wasn't a driver writer and I didn't have full insight in all the events surrounding this driver, so this story is mostly my causal observation from an ISV relations point of view. Someone on the team might have seen it differently. But still, I believe ATI lost at least a year, if not two, in their OpenGL support. By now that's ancient history, and I was happy that AMD shipped OpenGL 4.2 drivers on release date. But I bet there are people on the team now complaining that the code is a mess.

Anyway, my intention wasn't really to tell that story, but instead tell a story of my own. Some of you may remember my old announcement that I started working on Framework4. It's depressing when I look it up and I realize that was three years ago, and it's still not done! Gosh, I thought it was only two years ago, but it's three. So what prompted me to rewrite my framework? It's not like Framework3 wasn't working. I've been pumping out oodles of demos using it. It's just that it was a bit ugly. It was getting kind of obsolete. DX11 was around the corner and OpenGL was getting a serious overhaul and finally started deprecating things, and I wanted to make this big move into the new age. I had rewritten my framework before, this was my third one. I had great visions about new clean code, using standard coding conventions, with nicer interfaces. In the beginning it was fun of course. I was coding up cool stuff, new SIMD optimized vectors classes, compile time hashes, modern interfaces matching DX11 and OpenGL 3.0+, poking around a little with compute shaders. Soon though I wanted to make a real demo using it. Pretty soon I realized the framework had a pretty long way to go before I could ship a demo using it. So I started fixing up the rendering stuff. I was pretty much just writing code that wasn't that much different from what I was doing in Framework3. "I'll optimize it later anyway, I just want it to work for now." And then came the boring parts. My GUI system. At first I wanted something new, but found that I had no particular visions about what a better one might look like. So I just copied over the old one from Framework3, changed the code to standard conventions, fixed up all the rendering to Framework4 style. Boooooring, but eventually up and running. And I kept running into all those small things, things that I haven't implemented yet, things that separate a running prototype from a releasable demo. So I got kind of bored at the whole thing, and ended up spending less and less time on the project. Of course there are other factors at play here too. The last couple of years have been rather life changing for me. Went from single to having a girlfriend and ultimately married and now I am a father of a two months old baby. I rarely find myself spending whole days coding anymore. Even if I'm free the whole day, it's not like I can sit down 8 hours straight coding like I sometimes used to. I haven't exactly stopped coding though, but whenever I have had a demo idea recently, I've been implementing it in Framework3.

I'm still poking around in Framework4 at times though. Actually, just recently I did some work to bring the OpenGL part into the 4.2 era. But I haven't been able to let go of the feeling that somehow I took the wrong path after all. Perhaps I should've just upgraded Framework3 instead. Back when DX10 was new I opted to just make a D3D10 renderer instead of doing the big rewrite. I felt it was too much work. I suppose it was one of my wiser moves. I had to bend things a little, change a few interfaces to shoehorn it into the Framework3 paradigm, but I got it working. I had to update a bunch of old demos though, but that was actually not that much work. So I've been thinking a little lately, what is it actually that I really wanted with a new framework? And the answer is pretty much this: DX11 rendering. So after contemplating the move for a while I finally gave it a shot yesterday. I implemented a DX11 renderer in Framework3. Basically I just copied the DX10 renderer and search-replaced "D3D10" with "D3D11" and then crunched through all the compile errors until I had a fully working DX11 renderer. This exercise took about an hour. An hour!!! I spent one hour and now I have a fully working DX11 renderer in Framework3. It's ahead of where I am with Framework4.

So now I'm not sure what I will do going forward. For sure I'll replace the DX10 renderer with the DX11 one and let all old DX10 demos run through DX11 API, but with a D3D10 feature level. Perhaps I should just refactor Framework3 to fix all those things I've been unhappy about. Surely I'll have to update all old demos too, but chances are that that's not much work this time around either. Or maybe I'll just copy Framework3 to a new folder and call it Framework4, and then start refactoring things from there?

Moral of the story: Rewriting from scratch is almost never the right way to go. It's easy to overestimate the benefits and underestimate the effort necessary to get there.

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Vilem Otte
Tuesday, December 13, 2011

Rewriting whole code is definitely evil. The best way is - copy old project, change messy stuff to clean stuff, remove deprecated stuff and add new features.

Of course don't forget commenting a lot, as fixing some 8 year old bug when you've no comments in the code can stop your work for days, not even weeks.

default_ex
Tuesday, December 13, 2011

Why not just adopt a better cleanup regime. I've been using procrastitracker for awhile now, and at the end of every month I scrape it for any entries for the project's I've worked on. Then it's just going through the files one-by-one to clean up the ugly. Has turned out to be a worthwhile exercise, some of the best optimizations I've done so far have come from the clean up period when I came in with the mindset of just making it consistent with the rest of the codebase.

Jerry
Saturday, January 14, 2012

Wait 10 years.

Overlord nailed it:
"In my experience rewriting parts of the code on a continuing basis and having a plan for doing so is the key to having all your stuff up to date"

Sparc
Tuesday, January 31, 2012

Very true!
I have experienced this enough very early in my career that my Engine still continues to evolve supporting Dx 8, 9 and 11 and GL 2, 3 Code-paths but I never reinvented the wheel, I just kept updating.
The secret is never version your Code-base. Versioning is only for external users and fixed stable builds

LeSnip3R
Monday, February 13, 2012

I feel your pain; I have been through that same process myself.

However every time I re-write my framework I port part of the old oneS to the new one.
So essentially large parts of the new frmework comes from the previous ones; a bit like a genetic algorithm applied to coding

If something was working and well tested before, there's no point tossing it away; instead I import it and modify it to fit the new coding style/trend I'm going for.

What's important is that new framework shows an improvement over the old one.

David C. G.
Monday, May 21, 2012

I recently took over a rewrite of an engine from DX9 to DX11. The previous programmer had decided to take the opportunity to re-architect the entire thing. When I took over, it was well overdue and not even rendering on screen. After many patches and fixing up it was finally working mostly. Now all I had to do was fix a bunch of difficult bugs. A pattern emerged. Many of these bugs were fixed by reverting to methods/code used in the previous version. The previous version encapsulated a lot more than was plainly obvious. It contained a lot of hard won working bug free solutions to problems. Also, to top it off the main feature of the new architecture, deferred rendering contexts performed much slower than the DX9 version. A complete rewrite again to re-introduce a rendering thread brought the performance back in line. The only things that went right with the new code base were bits and pieces that code have easily just been refactored into the DX9 codebase in the first place because that's where we ended up with essentially a refactored DX9 code base with DX11 improvements. This probably took 5 times the effort and cost of doing it 'right' the first time.

Yanko
Saturday, September 13, 2014

i have xp in converting apllications from perl/plsql to java/vb/plsql.... it's never fun...
building something new completelly from scratch is fun..

will you be doing OpenGL4+ in Framwork4?

do you have a any tutorials anywhere online?
for noobs in opengl4-c++..

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