The Legacy Code Lifecycle

If you have worked for more than couple of years as a programmer then you’ve seen a legacy system. If you are lucky then you have only dabbled with them and not been drawn into the epicentre of a full-blown legacy code project. If you’ve been unlucky then your very first job was maintaining an old legacy system that made you question your decision to choose this career.

Inception of a Legacy SystemFrankenstein

So what do I mean when I say Legacy Code? There are lots of definitions: old code, somebody else’s code, bad code etc. But in the extreme cases nearly everyone recognizes legacy code. Have you ever made a change to a system that should have taken one day but took five? Or made a seemingly simple change that rippled through the system introducing a bug in a totally different module?

I think that the concept of legacy code is tightly coupled to how we as programmers write code. To illustrate this, let’s start with the lifecycle of a project gone wrong.

First Phase – The Land of Milk and Honey

A company decides to build a new system. The developers rejoice as they get to work on a shiny, new greenfield project. This is developer paradise. If the original team is reasonably competent then this phase is a joy to work in. It is easy to add new features and the customers are happy as their requests are fulfilled within a couple of days. The developers get to use the latest, shiny technology and enjoy the feeling of fast flowing development.

However, it is very possible to sabotage the whole system from the very start. An example is the prototype trap; first the team hacks together a prototype which the customer loves, then the team decide to build on the prototype instead of throwing it away. And with that decision made, they are well on their way to building a spaghetti code jungle.

Second Phase – Getting Fatter

Life is not too bad. The features are not flying out the door at the same speed anymore but the customer is reasonably happy. The code might be starting to get a bit clunky, the controllers/code-behind classes are not as skinny anymore. Class size is growing as the classes accumulate more methods and more lines per method. The bug count increases as new features involve changing existing code and not just adding new code. It is high-time to set up bug tracking and a process for change requests.

Third Phase – A Bump in the Road

It is common for programmers to change jobs regularly and in most companies the key people will eventually leave for greener pastures. This means that the two or three developers that built the system and know every nook and cranny are gone. And so is their knowledge. New developers come in and do not manage to grasp the structure and logic of the system. Perhaps it is not even their main task and they are just helping out occasionally.

The era of quick fixes has begun. The new developers never went through the first phase and don’t remember when the code base was a thing of beauty. They see a chunky, inelegant code base and they want to get in and out as quickly as possible. Maybe the customer is putting the pressure on or they just want to get back to their other (greenfield) project.

Fourth Phase – The Big Project

Good news! The customer is delighted with the system. It is saving/earning them buckets of money. They have loads of ideas for new features and want them implemented pronto.

But this is where everything goes wrong. The current team of developers have allowed the code base to rot by applying quick fixes and not understanding the original vision for the system. There is not much structure left and new code has been dumped in inappropriate places. The team are trying to build on a foundation of sand and it is now the bug-fix death march begins.

They try to estimate how long a new feature is going to take but this is extremely difficult due to no-one knowing how big the ripple effect will be. So they triple their estimate to be on the safe side. The problem is this feature will never be done. They might get to 90% done but that last 10% is unattainable. In an entangled code base most large changes introduce bugs. Fixing those bugs introduces new bugs. It is near impossible to complete this project without bugs and meet a deadline.

This is where it turns into a death march. The team has a deadline to meet and just throws code at the bugs in a desperate attempt to get everything done on time. People work long hours, the code quality worsens all the time. In the end, the customer gets a bug-ridden Big Ball of Mud. The customer is not happy and crisis talks begin about the future of the project.

Fifth Phase – What do we do now?

At this stage, you have a burnt out team (or maybe no team at all) and a system that is a maintenance nightmare . What happens now? Throw away the code (and the money invested) and rewrite it? Limp along for years and spend your days fixing all the bugs and taking an eternity to add new features? This system might only be a few years old and already could be thrown on the scrap heap.

A lot of us have seen and experienced projects like this and agree that this system qualifies as legacy code. But I favour Michael Feathers’ much tougher definition:

Code without tests is bad code. It doesn’t matter how well written it is; it doesn’t matter how pretty or object-oriented or well-encapsulated it is. With tests, we can change the behavior of our code quickly and verifiably. Without them, we really don’t know if our code is getting better or worse.

So yes, my example qualifies as legacy code but so does most of code I’ve written during my career as a programmer. Code without tests inevitably rots and becomes harder to maintain. It might never get to the fifth phase but will require a lot of care and attention to stay at the second phase and not get out of control.

In my experience, writing unit tests is the key to changing the lifecycle of a project and to start improving code quality and reducing the bug count. But how can you learn unit testing and TDD if you work with a legacy system? If you are stuck with a legacy system and want to make programming fun again then get started by reading my next article on TDD and Legacy Code.