Reuse is an Event. Sharing is a Journey.

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Reuse in Product Line Engineering

The idea of reusing systems and software engineering assets – such as requirements, designs, source code and test cases – has long been the leading candidate for discontinuous improvements in engineering efficiency and quality. The field of Systems and Software Product Line Engineering (PLE) has focused on lucrative opportunities for reuse within a family of similar products or systems.

Reuse in concept is simple, clear and powerful. Reuse in practice, however, has been messy, muddled and anemic. The problem is, the things about reuse that seem obvious don't work, and the things about reuse that work aren't obvious.

This has resulted in many different engineering organizations predictably taking the same ruinous path to failed reuse and PLE initiatives, while consistently overlooking the readily available path to reuse and PLE breakthroughs. Not only has this inhibited the field of systems and software PLE, but it has also unnecessarily stunted the growth and advancement of entire companies and industries.

This contrast in perspectives on reuse – the seemingly obvious things that don't work versus the things that do work that aren't so obvious – can be succinctly stated as: Reuse is an Event, Sharing is a Journey.

Reuse is an Event

Central to the least effective forms of reuse in PLE is the idea that reuse is an event. Unfortunately this is also the intuitive first impression most people have about PLE, and this misperception sticks. The classic example is when an organization creates a library of reusable core assets for a domain. The reuse event occurs during application engineering, when someone finds and reuses a core asset from the library for a new or enhanced product or system. High fives all around.

The reuse event provides 100% reuse on day one, but 0% reuse every day after that. After N different reuse events from a library asset, there are N copies of the asset, plus the one in the core asset library. Enhancements, variations or fixes on any of these N+1 copies results in divergence rather than reuse. To reconcile and consolidate this divergence requires a level of manual effort that is proportional to N2. That is, a change to any of the N+1 copies must be reconciled with the other N copies, or N+1 times N.

Now, the problem with treating reuse as an event becomes apparent. For N reuse events, the savings are proportional to a linear N, but thereafter throughout evolution and maintenance of a product family the costs incurred are proportional to a polynomial N2. Not a good balance, particularly considering that the timespan, effort and cost for the evolution and maintenance phase of a system typically dominates the upfront creation effort and cost.

Forms of reuse that fall into the anemic reuse-is-an-event category include clone-and-own (new systems built by cloning, modifying and maintaining copies of previous system assets), reusable asset libraries, the lego block software component analogy, and Software Product Line approaches based on separate domain engineering (building a library for a domain) and application engineering (copy-based reuse events from the library).

Sharing is a Journey

Key to the most effective forms of reuse in PLE is the idea that sharing is a journey. Reuse in this case is not an event experienced in isolation, but rather an odyssey embarked upon with others over time. Unfortunately this is a rather non-intuitive perspective that on first impression sounds like it might be more trouble than it's worth.

The sharing approaches to reuse in PLE are generative in nature, where shared PLE assets are automatically compiled, assembled or otherwise configured into systems through some type of abstract feature-based specification. The classic examples in PLE are feature-based product line configurators that select and configure from a supply chain of shared product line assets – such as requirements, design models, source code, documentation and test cases for a product or system family – based on the specification of features and capabilities that are needed or not needed for any particular product or system.

In contrast to reuse events, sharing is long term, and that is what enables success. Reconsider the previous example of maintaining N different systems that use the same asset, but this time through sharing rather than reuse events that create N different copies. Enhancements, variations and fixes are now done once in the shared, pre-generator form of the asset, so that the N users can all automatically regenerate the new and improved asset, using their previously created feature-based abstract specification.

Without the feature-based abstraction, automation would not be possible. Dramatically better than the N2 cost with copy-based reuse events, the evolution and maintenance of the shared asset is now centrally coordinated for all N users, so there is a linear N cost for the initial use of the asset and a linear N cost for evolution and maintenance. This provides very efficient lifetime amortization through cost sharing across the N products and it allows N to grow very large.

Asking the Right Question

In a recent PLE deployment, BigLever provided the following insight to aid the customer in understanding and applying this critical shift in perspective.

The customer group leading the implementation of BigLever's tools and methodology for their PLE assets and automated configuration were being asked by their management and customers for metrics on "how much reuse" they were achieving among their products. They were struggling to find a good way to express this. The problem with providing a meaningful answer was that they were being asked for metrics on reuse events, whereas the real benefit metrics were coming from their sharing journey.

Answering a different question provided a better answer and allows them to be mentors to their management and customers on the difference between reuse-is-an-event versus sharing-is-a-journey. Looking across their collection of shared assets, they can create a visually intuitive bar graph to illustrate how much of this asset collection is shared among all N systems in their product family (i.e., truly common), how much is shared among N-1 systems, among N-2, among N-3, and so forth, all the way down to how much of the shared asset collection is unique (today) to just 1 system in the family.

Shifting perspective from reuse-is-an-event on individual products to sharing-is-a-journey for the entire product line portfolio makes it easier to see how much of the engineering effort over time is shared, as well as how the effort is distributed and amortized across the entire product family.

What to Do?

The contrasting reuse perspectives have put the PLE industry into a bit of a quandary. Reusing systems and software engineering assets is always the best candidate for discontinuous engineering improvements that lead to discontinuous business benefits. However, the reuse-is-an-event approaches to PLE that seem obvious don't work, while the sharing-is-a-journey approaches to PLE that do work aren't obvious. There's no simple resolution for this quandary, other than to educate – by becoming mentors to promulgate these ideas within our organizations and industries, and to create new mentors who can do the same.