Code base of a large project is getting worse over time. I hope there are lucky exceptions, but in general it is true for most projects. The reasons are quite obvious:
- More and more features. It leads to increased complexity.
- Shortcuts and hacks to support "We need this fancy search till August. Period!" features
- Developers rotation. New developers don't know all the fundamental decisions and ideas behind the architecture. Knowledge gets lost with transition inevitably.
- Development team growth. More people - less communication. Less communication - bad decisions. It leads to code duplication, hacks to make something work without deep understanding of the underlying conditions etc.
Suddenly you can't add new features easily, you can't make significant changes easily, you have tons of technical debts and development team is close to bankruptcy. You want to change that and have just two options: refactoring or rewriting everything from scratch.
Refactoring and Punctuated Equilibrium
Punctuated Equilibrium is a theory in biology, but it is applied to other complex adaptive systems like societies as well. It states that the system has almost no changes in significant period of time and then changes very rapidly. Then it acquires the equilibrium state again. Sounds familiar? Sure, refactoring is almost the same.
Any change in the system introduces chaos in the short term. The goal of refactoring is to eliminate chaos, but often you increase it initially. When you are working on changes you make system less stable, but as the change is completed, the system can be more ordered. That is not always true for software development. If you use branches, this makes changes safer. Still significant changes increase the risk of new bugs.
The real danger of refactoring are local optimums. With a full rewrite you may have a better architecture than with refactoring. Solution? If you have a vision of final architecture, use it and try to make a path from current architecture to the new architecture. In general simplicity should emerge from refactoring.
Rewrite and Chaos
When you rewrite from scratch, you add such a large portion of chaos that it is hard to predict the final result. You have a new singularity that will explode to the new product universe. But are you certain that it will be better than the previous universe? How many the 'same bugs' will you fix in the new product version?
However, rewrite may look faster. I mean you may release a new version faster with rewrite, but most likely with more bugs and less stable.
I think we may expect something like that:
The green line shows how chaos changes with refactoring. After each refactoring there is a small increase of chaos, but then the system becomes stable and chaos decreases. You see that the final release is quite late, but keep in mind that there have been many releases before, so customers benefit earlier.
Black line is how chaos changes with a full rewrite. We have the old system during rewrite, so chaos is constant. After the public release chaos increases significantly. Quite many new (and old) bugs and quirks are expected, so stabilization period is longer. But the release itself is faster. The reason is that there is no burden behind. For example, there is no need to support all the places when you do a change, while in refactoring it is required to keep system working and stable all the time, and it demands additional effort.
Adaptation vs. Revolution
There is another angle to this problem. Refactoring is adaptation, while full rewrite is revolution. Again, revolution is a chaotic beast. You may slowly adapt the product for new external conditions or make one revolutionary rewrite.
So, Rewrite or Refactor?
I do think there is no universal correct answer to this question. If time to market is paramount, if you feel that you'd lose business if a new version will not be published in 6 months, you may try a full rewrite. But beware side effects! Quality may drop significantly and long stabilization period may hurt existing customers.
In most cases refactoring is preferable. Slow pace, high quality, constant improvements, happy customers. I like it more. But rewrite is so magnetic...