When the time comes and you feel adventurous, you may want to migrate from your current development approach to a (software) product line approach. There is a lot to reflect on before and while migrating. A few simple checks can tell you a lot in terms of where you stand and what your migration options are. This article will concentrate on discussing product relation patterns and their influence on product line migration approach decisions. The Product Forest Your products are developed mostly independent from each other but have a set of functionalities they share. The problem domain of these products is also very similar, for instance all products are intended to control air conditioning systems (in different ways). In this case your products form what I call a Product Forest. Sharing of basic libraries like UI widgets or mathematical function collections (anything which does not contain shared application logic) might happen already as a basic form of reuse. This kind of pattern is often seen when a company runs several projects for different clients in a similar domain independently and concurrently. In the figure below you see how the products (P0 to P3) appear over time and how they evolve due to changes or bug fixes (P0-P1). Another example for a product forest are earlier releases of Microsoft Office. The included applications Excel, Word, and PowerPoint shared some basic concepts and were basically sold as a set of closely related products, but came from very different code bases. You could see and feel in many places in these tools where the applications behaved similar but not identical. This (slightly) deviating behaviour is one of the major problems you will run into with a product forest. Over time it gets harder and harder to ensure consistent behaviour over the range of products. This in turn increases cost of support and maintenance (since each product needs individual care and knowledge). But this could reduce customer satisfaction also, since they expect similar behaviour since they got the product from the same company but dont get it. The Product Bush Much stronger bonds between products can be found in a Product Bush. In a product bush your products have shared ancestors and are created via clone-and-own. A new product starts basically as a copy of an existing one and is developed (read tweaked) until it fits the new and changed requirements. What is wrong with this pattern? As with the product forest, in some cases nothing at all. Since you just have to pick the right ancestor for your new product (the one which can be evolved into the new product with least cost / time / ) this often sounds like a good plan and might even be one. Doing a copy does not involve much extra cost. The only tooling you need is called copy and that is for free. So clone is the cheap part here. However, while this approach might feel good in the beginning, later the own part might strike back at you. For instance when a customer of P0 wants a combination of features of the older P0 and the newer P1. If you ever tried to merge code of systems which both had some time of independent evolution on their back, you know what I mean. This is most often a non-trivial task up to the point that its cheaper to re-implement the desired combination of functionalities. If the product bush has been growing for long enough, integrating bug fixes, which affect several products, becomes a nightmare too. If this scenario is used in your organization and you wonder why it is called a bush, just draw the relationship between your projects/products on a sheet of paper The Product Gang The next level of product relationship is the Product Gang. Your products are build from sets of reusable assets by way of configuration and customization. Reuse means that for a new product you take the reusable assets and put them together in a way that they can be used for developing the missing piece for the new product. Changes to the reusable assets (here called the Platform PF) do not directly affect a product until the application developer decides to pull in the new version of the PF. The products of the product gang are now sharing more / most assets and are technically derived from the platform assets. Hence products are now in fact instances of the platform, and I start calling them Product Variants (PV0-PV3) instead of products as in the earlier figures. Highly configurable application frameworks, products build on top of reusable components, you name it. Basically a lot of the current reuse based development approaches belong here. One might say, this looks pretty good to me, what is the catch here? Well, while the Product Gang certainly is much better than the two previous ones when it comes to reuse, there is still something missing to make it a real Software Product Line. Just like in a real-world gang, there is a strong link between all the members. However, there are also secrets: No one is really disclosing information about himself/herself to the other members of the gang. And this is the fundamental challenge of this approach: because there is no or not much knowledge about the members of the gang, effects of changes to the shared assets are hard to estimate and also hard to coordinate. For example, if you have to change an interface method in a component, knowing that it is never actually called by any other application than yours would be nice. Since gang members evolve separately (in separate branches or even different version control repositories), there is no easy way to find out whether this is the case or not. Basically, when there is not enough knowledge about the usage of provided reusable elements in product gang members, the assumption must be very conservative. I. e. in this case you must assume that more or less every member might use it in its existing form. That in turn makes changes very complicated. Also when a change is introduced, it would be useful to verify that it works for all members which use it. But to do so before releasing a change one has to run tests on at least a selection of gang members (or make sure the change works in all possible cases by other means). However, this is often impossible due to slightly different build processes for the different products, the inability to access the other projects repositories, or simply lack of time. This reduces over time reuse in favour of product individual solutions, since for those the effects are local only. The Product Family At last we arrive at a pattern which is more or less the ultimate goal of software product line development: the Product Family. The main difference to the previous scenario, the product gang, is the availability of knowledge about members of the family (PV0-PV3) within the whole development organization. The product family approach also includes continuous care for all relevant products during the evolution of the product family assets. At all points in time all (currently as relevant categorized) members of the family are involved when making changes to the shared assets. This, of course, is not only a technical process but also involves communication between the individuals responsible for the assets and the product (variants) to discuss/prioritize/reject changes. The bidirectional link between reusable core assets and the products which instantiate them is the key difference between Software Product Line reuse and traditional reuse approaches. Of course this does not come for free. Those of us who have a large family know what I mean From here we go All of the mentioned patterns have their use cases; none is bad as such. But when it comes to product lines there can be only one, the product family. The important question is: how to migrate from any of the other patterns to the Product Family. There are basically two scenarios. One I call the Separate Product Merger, because individual products, often fairly independently developed, are the starting point. What is shared among the existing products is the membership to a common problem space, but the solution spaces are either entirely independent or at least deviated to a degree so that merging the solution space would be to much effort. However, using one or more of the products solution assets as a base for the new product line should at least be considered. For instance pick the (two) best product(s), the most simple one, or the most complex one. With a product bush or product gang, reuse is already happening. Thats why I call the other scenario simply Reuse Improvement. Since both problem space and solution assets are closely related a lot of the existing assets can be used in the new product line. Often one of the goals of the improvement scenario is to be able to create all existing products more or less unchanged from the new product line assets. In this scenario there is a choice what is analyzed first. The often simpler and quicker approach is to start with solution space variations (e.g. by comparing and merging on code level, more on this in a later article) and take the road to the problem space variation from there. The other approach starts with the problem space going only in a second step to the solution space, much like a software product line development when it starts from the scratch. In this case it is more complicated to provide an easy mapping between problem space and solution space because problem space variations might not so easily map to the existing variations in solution space. However, this second approach is more or less mandatory for the Separate Product Merger scenario.