What can you do to a legacy codebase that will hav

Question

As you work in a legacy codebase what will have the greatest impact over time that will improve the quality of the codebase?

• Remove unused code
• Remove duplicated code
• Add unit tests to improve test coverage where coverage is low
• Create consistent formatting across files
• Update 3rd party software
• Reduce warnings generated by static analysis tools (i.e.Findbugs)

The codebase has been written by many developers with varying levels of expertise over many years, with a lot of areas untested and some untestable without spending a significant time on writing tests.

Answer 1

I'd say 'remove duplicated code' pretty much means you have to pull code out and abstract it so it can be used in multiple places - this, in theory, makes bugs easier to fix because you only have to fix one piece of code, as opposed to many pieces of code, to fix a bug in it.

Answer 2

The single biggest thing that I've done to the legacy code that I have to work with is to build a real API around it. It's a 1970's style COBOL API that I've built a .NET object model around, so that all the unsafe code is in one place, all of the translation between the API's native data types and .NET data types is in one place, the primary methods return and accept DataSets, and so on.

This was immensely difficult to do right, and there are still some defects in it that I know about. It's not terrifically efficient either, with all the marshalling that goes on. But on the other hand, I can build a DataGridView that round-trips data to a 15-year-old application which persists its data in Btrieve (!) in about half an hour, and it works. When customers come to me with projects, my estimates are in days and weeks rather than months and years.

Answer 3

I'd say it largely depends on what you want to do with the legacy code...

If it will indefinitely remain in maintenance mode and it's working fine, doing nothing at all is your best bet. "If it ain't broke, don't fix it."

If it's not working fine, removing the unused code and refactoring the duplicate code will make debugging a lot easier. However, I would only make these changes on the erring code.

If you plan on version 2.0, add unit tests and clean up the code you will bring forward

Answer 4

Good documentation. As someone who has to maintain and extend legacy code, that is the number one problem. It's difficult, if not downright dangerous to change code you don't understand. Even if you're lucky enough to be handed documented code, how sure are you that the documentation is right? That it covers all of the implicit knowledge of the original author? That it speaks to all of the "tricks" and edge cases?

Good documentation is what allows those other than the original author to understand, fix, and extend even bad code. I'll take hacked yet well-documented code that I can understand over perfect yet inscrutable code any day of the week.

Answer 5

As a parallel to what Josh Segall said, I would say comment the hell out of it. I've worked on several very large legacy systems that got dumped in my lap, and I found the biggest problem was keeping track of what I already learned about a particular section of code. Once I started placing notes as I go, including "To Do" notes, I stopped re-figuring out what I already figured out. Then I could focus on how those code segments flow and interact.

Answer 6

Add unit tests to improve test coverage. Having good test coverage will allow you to refactor and improve functionality without fear.

There is a good book on this written by the author of CPPUnit, Working Effectively with Legacy Code.

Adding tests to legacy code is certianly more challenging than creating them from scratch. The most useful concept I've taken away from the book is the notion of "seams", which Feathers defines as

"a place where you can alter behavior in your program without editing in that place."

Sometimes its worth refactoring to create seams that will make future testing easier (or possible in the first place.) The google testing blog has several interesting posts on the subject, mostly revolving around the process of Dependency Injection.