Thursday, July 5, 2012

The Misuse of Reuse

The software industry has been pursuing reuse for at least four decades. The approach has changed over that time. It started with structured programming promising reusable snippets of code. We then moved to object-oriented programming promising reuse through inheritance. Today we are focusing on service-oriented architectures promising reusable services that can be written once and then used by multiple applications.

For forty years we have been pursuing reuse and for forty years we have been failing. Perhaps it is time to reexamine the goal itself.

Let's start by reviewing the arguments in favor of reuse. They are quite simple. And, as we will soon see, they are quite flawed.

The argument goes as follows. Let's say we have three systems that all make implement the same function, say Function 1. This situation is shown in Figure 1.

Figure 1. Three Systems Implementing Function 1

It seems fairly obvious that implementing Function 1 three times is an ineffective use of resources. A much better way of implementing these three systems is to share a single implementation of Function1, as shown in Figure 2.

Figure 2. Three Systems Sharing A Single Function 1

In general, if there are S systems implementing Function 1 and it costs D dollars to implement Function 1 that the cost savings from reuse is given by

D * (S - 1)

If D is  $10000 and S is 5, then reuse should save us $40,000. Right? Not so fast.

In order to evaluate the claim of cost savings through reuse, we need to apply some principles of IT Complexity Analytics. IT Complexity Analytics tells us that the complexity of Function 1 is exponentially related to the number of systems using the Function. This is because each system is not using the exact same function, it is using some variant of the same function. Function 1 needs to be generalized for every possible system that might someday use it, not only those we know about, but those we don't know about. This adds considerable complexity to Function 1. 

If the size of the circle reflects the complexity of the functionality, then a much more realistic depiction of the reuse scenario is shown in Figure 3. 

Figure 3. Realistic Depiction of Sharing Functionality

Since system cost is directly related to system complexity (one of the axioms of IT Complexity Analytics) we can say that in most cases, the theoretical cost savings from reusing functionality is overwhelmed by the actual cost of the newly introduced complexity.

However, the situation is even worse than this. Not only is the cost savings from reuse rarely achieved, but a number of additional problems are introduced. 

For example, we now have a single point of failure. If the system implementing Function 1 fails, all three of our systems fail. 

We have also compromised our security. As IT Complexity Analytics predicts, the overall security of a system is inversely related to its complexity. The more complex a system is, the lower its inherent ability to maintain security. 

And we have created a highly inefficient system for running on a Cloud. The extra cloud segments we will need to pull in to support our reuse will dramatically increase our cloud costs.

Given all of the problems we have created, we most likely would have been better off not attempting to create a reusable function in the first place. 

Now I should point out that I am not totally opposed to reuse. There are situations in which reuse can pay dividends. 

In general, a reuse strategy is indicated when the inherent complexity of the functionality being shared is high and the usage of that functionality is relatively standard. In these situations, the complexity of the functionality dominates over the complexity of the sharing of the functionality. But this situation is unusual. 

When should you pursue reuse? It all comes down to complexity. Will your overall system be more complex with or without shared functionality? This requires a careful measure of system complexity with and without the proposed sharing. If you can lower system complexity by sharing, do it. If you can't, don't. 


Roger Sessions writes about the topic of Organizational Complexity and IT. If you would like to get email notifications about new posts, use the widget on the right.