Wednesday, 31 August 2011

Technical Investment vs Technical Debt

A subject that has been bothering me for a while is the concept of Technical Debt. It’s an interesting analogy, but I think it has become increasingly misused and misunderstood in the agile world. In this post I want to focus on the real meaning of Technical Debt and describe an alternative approach that worked on a current project.

What is Technical Debt?

In its simplest form, Technical Debt is work that you really should be doing but which you put off until later in order to meet some short-term delivery goal - it’s a compromise! For example, I might know that my JSON to Domain Object mapping approach is sub-optimal bit I decide to live with that for a while in order to complete a set of urgent features required for an important customer demo.

The key part of Technical Debt is that you MUST pay it back at some point in the future; and the sooner you do the better. To continue my example: as soon as the customer demo is complete, the fixing of the mapping approach should be prioritised as an urgent task.

How is it Misused?

One of the main problems I have encountered on many projects is that they misuse Technical Debt. The two most common misuses are:

  • Being far too willing to accept Technical Debt; and
  • Allowing Technical Debt to exist for too long (i.e. not paying it back quick enough)

Both of the above burden a project and, in my experience, they are allowed to happen because people don’t understand the true implications of Technical Debt correctly. There seems to be a general miscomprehension that Technical Debt can be captured as a story in the backlog that can be played at some time in the future much like any other story.

What I believe that most projects miss is the fact that having a Technical Debt story in the backlog not only requires that the debt is paid back in the future, but that its mere presence incurs an ongoing interest payment on the project. This interest payment comes in many different forms, some of which include:

  • increased difficulty understanding the code base
  • unnecessary complexity in the code
  • more difficult and time consuming refactoring
  • needing to write more code than necessary to add new features
  • slower build and test times

All of the above (and other forms) result in one major impact: reduced team velocity. So, any Technical Debt, besides being a story on the backlog that has to be played in the future, also reduces velocity. Too much of it and velocity grinds to a halt.

Cumulative Impact

Projects that tend to allow Technical Debt to be readily created and to persist also tend to allow multiple pieces of Technical Debt to be present at the same time. This has a cumulative effect in that features are built on one piece of debt and then new debt grows on these features and so on. After a while the ongoing interest payments increase exponentially and the effort required to repay each piece of Technical Debt increases significantly.

At this point a project tends to come to a halt completely because the interest payments of having the Technical Debts stop the project adding any new features, while the effort required to pay back the debt has become too huge. The project either fails or needs a reboot.

Is There An Alternative?

From time to time, any project must accept some compromise in order to meet some critical goal. I see no problem with this. However, what projects need to do is to ensure that these compromises are kept as small as possible and have the smallest possible payback cost. So, how do we ensure this? The solution I would propose is what I have coined as ‘Technical Investment’.

Using the debt/investment analogy for personal finances, we can see two approaches: I can borrow a load of money to buy what I need now and then have to deal with interest payments until I can pay the debt back; or, I can put some money away into an investment so that when I need to buy something I can do so without having to go into debt. I temporarily reduce the size of my investment to make the purchase but then I quickly replenish it and grow it again ready for another future purchase. Hopefully my investment also grows over time so I can buy more things or reduce my investment portfolio by less each time.

So, how does this Technical Investment work in agile software development? What we do is we dedicate a small portion of the team during each iteration to refactoring, cleaning, rewriting, upgrading libraries and so on. This may be in any area of the code, not just that directly touched by the current stories in play. We invest in the code base a small amount each iteration. Perhaps we might deliver one small story or feature less each sprint, but as the project progresses we avoid the large reductions of velocity that are inherent in a Technical Debt driven model.

So, Does It Work?

On a recent project we adopted the above approach (although somewhat informally, which is why I wanted to document it more formally in this post). We had a team of five developers working on weekly sprints. Each day we created two pairs to work on features, while the fifth team member was left free to invest in the code base. The lone person changed each day or so. What did we find?

Firstly we found that the need to incur Technical Debt (or draw down from our investment, to follow the analogy) was significantly reduced. Quite often the areas where we would have had to make a compromise to meet a sprint goal were already addressed and improved before making the compromise became necessary. We were able to preempt many areas of improvement before they became an issue.

Secondly, we found that in cases where a compromise was made (where we drew down from our investment) that we were able to quickly fix the compromise and get back to adding to our investment much sooner. We also avoided the problem of any cumulative compromises, because payback was smaller, which made correcting individual cases much simpler when we decided to accept them.

By working in this way we were able to have a continually improving code base, while still maintaining a high velocity of new features. The code base continually improved in quality and clarity throughout the project even though features were continually being added.

But Shouldn’t This Happen Anyway?

This is an interesting question. Yes, a good agile team should be continually refactoring and improving code as part of every story. However in practice, the pressure to complete features often reduces the amount of improvement that is done. It’s also often difficult to justify changes to code in an area that no-one is currently working on just because it needs some love: which may benefit some unknown work in the future.

Having a concept called ‘Technical Investment’ also gives a concrete thing that can be talked about with Product Owners. We can discuss the investments made during reviews and present them in a positive way, something that I have found much more difficult to do with debt.

I suspect that it may also be possible to measure and visualise the investment in a similar way to the measuring of velocity. This is certainly something I’d like to explore in the future as being able to show how investing in the code reduces future reductions in velocity is a great way to justify the practice.


Keeping the code base clean and building up good quality through continual Technical Investment on each iteration has many advantages over the use of Technical Debt to capture and deal with compromises. When applied, the need to compromise becomes less and the effort involved in fixing each compromise is significantly reduced. The chances of building up cumulative, velocity zapping compromises also lessens significantly. If you have a longer-running or larger project then continual Technical Investment is the only sure way to make sure the project doesn’t grind to a halt due to debt interest as some time in the future.

1 comment:

  1. Great post!

    I often take exactly the approach you are proposing: I might "hack" out some ugly/bad code from time to time when I am doing exploratory work on a problem I'm not sure how to fix, and I may even let the code linger for a short while. But the minute I start paying "interest" on the debt I created either by duplication, being slowed down or otherwise, I invest the time in sorting it out, because I know there is a long term pay off.

    I think the willingness to accept lingering technical debt in a lot of orgs stems from human natures want of instant gratification and tendency to underestimate the impact of deferred pain (debt and economy analogy: the way we have kicked the can ahead with the economy, years of not sorting the problems out while they are small lead to a painful rebalancing and deleveraging later on, in this case now).