The Economics of Reuse

If you need the same functionality in two projects, you should reuse code between them, right? Or should you? For as long as there has been a profession of software engineering, we have tried to achieve more reuse. But reuse has both a benefit and a cost. Too often, the cost is forgotten. In this article, I examine the economics of reuse.

True story: One of the earliest projects to embrace object-oriented programming in the 1990s did so with the goal of maximizing reuse. The team responsible for creating the company wide framework used the following formula for calculating the value of their work:

[Value of reuse] = [numbers of uses of framework] * [value of the framework to reusers] – [cost of developing the framework]

This formula is obviously correct, but this is where they went horribly wrong: The organization said [value of framework to reusers] = [cost of developing framework]. In other words: The more expensive it was to create, the more valuable it was to use.

We have clearly progressed beyond this thinking. A more updated formula would say: [value of framework to reusers] = [cost of developing the feature in question]. But even this is too optimistic.

No library comes for free to its users. At the very least, you have to discover the features and learn about the details. The cost of reusing depends on many factors, such as the quality of the framework and the documentation and also upon the type of feature. A complex algorithm with a simple interface is cheap to use, while most domain-specific frameworks require relatively much work to reuse. We can express this as a reuse value factor, likely between 90% and 50%. For most cases, my guess would be at about 75%.

So we have:

[value of reuse] = [number of users] * ([cost of feature] * [reuse cost factor]) – [cost of developing the reusable component]

What about the other important factor: [cost of developing the reusable component]?

It’s easy to assume that the cost of developing a feature in a framework is equal to that of developing the feature in an application, but on further analysis shows that this is far from true. A reusable component needs more documentation, it needs to handle more special cases and it has a slower feedback cycle. This cost is actually substantial and may mean that it costs between 150% to 300% or more to develop a feature for reuse. Personally, I think the reusability cost factor lies around 300%. And the lower this number, the higher the cost factor of reuse is likely to be, because that may mean we skimped on documentation etc.

A revised number would be:

[value of reuse] = [number of users] * ([cost of feature] * [reuse value factor]) – [reusability cost factor] * [cost of feature]

Or

[value of reuse] = [cost of feature] * ([number of users] * [reuse value factor] – [reusability cost factor])

The more complex formula actually lets us make a few predictions. Let’s say we assume a reuse value factor of 75 % (meaning that it requires 1/4 of the effort to reuse a library rather than creating the feature from scratch) and a reusability cost factor of 300 % (meaning that it requires three times the effort to create something that’s worth reusing). This means:

[value of reuse] = [cost of feature] * ([number of users] * 75% – 300%)

This equation breaks even when [number of users] = 4. That means that to get any value from your reused component, you better have five or more reusers or you have to find a way to substantially improve the [reuse value factor] or [reusability cost factor]. Very smart people have failed to do this.

Improving the value:

• Increase the number of reusers: Simple enough, but when you do, you risk that the [reuse value factor] goes down as the framework doesn’t suit everybody equally well.
• Reduce the cost of reusing the library: This means investing in documentation, improving your design, improving testing to reduce the number of bugs, handle bug reports and feature requests faster from your reusers – all of which increase your cost reusability cost factor.
• Reduce the extra work in making the library reusable: The most important way to reduce the cost of developing for reuse is to choose the right kind of problem to solve. Problems with a small surface and big volume are best. That means: Easy to describe, hard to implement. Sadly, most of the juiciest fruit was picked years ago by the standard library in your programming language and by open source frameworks.

On a global scale, reuse has saved the software industry tremendous amounts. In an organization, it can be hard to get the same effect. Reuse comes at a cost to the reuser and to the developer of the reusable library. How do you evaluate and improve your [reuse value factor] and your [reusability cost factor]?