Software Reuse by Ivar Jacobson

February 8, 2004

Many organizations are trying to achieve software reuse. Many of them think that they will achieve reuse if they just put their components in some sort of repository. Then they just expect others to reuse their valuable assets. I never cease to be surprised that people still do that. We have said for at least a decade that you don’t get any substantial reuse by just posting components. It seems as if people, instead of learning from others experience, always have to make the same mistakes as others over and over again.

The book I wrote with Martin Griss and Patrik Jonsson on Software Reuse is very explicit about how to achieve reuse. The subtitle of the book is: Architecture, Process and Organization for business success.

It all starts with Architecture. You design an architecture which identifies which components are reusable and which are not, thus a layered architecture. Usually two layers on top of middleware are enough. The top layer contains application components, which are not designed to be reusable. The second layer contains the domain components intended for reuse by several applications in the top layer. The only practical way to get a domain component is by designing an architecture in which the component has a clear role. Without this architectural setting, a component will never really be reused.

Process comes next. The way you develop the domain components is different from how you develop application components. They are documented for reuse. We suggested in our book that you design a façade for each domain component. This façade contains an extract of the models of the component, an extract defining how the component can be reused. In case the domain is big enough (e.g. banking, telecom, airline), the façades can through careful design evolve into a more powerful tool – a domain-specific language (DSL). Instead of designing on top of the interfaces of the domain components, expressed through their façades, you will design using a domain specific language.

Domain components also come with a kit containing tools for its reuse, examples of how to reuse it, and other things that help in getting it reused.

Developing application components is different. Application components don’t require to be documented for reuse. Instead they are designed using domain components and through interaction with other application components.

Finally Organization. In order to get successful reuse, you also need to develop an organization which maps 1:1 to the architecture. This means that for each component in the architecture there is a corresponding responsibility in the organization. Higher level components are the responsibility of a subsystem-owner. Lower level components are the responsibility of an individual developer. Without such a simple mapping between architecture and organization it will be practically impossible to get good reuse. Such an organization will become an expert organization – people will become experts in a subject matter. It grows people that want to create quality products. I call such an organization an architecture-based organization as opposed to a project-based organization. A project-based organization has a line manager for each project. The manager should only care about his own project. Such an organization is counterproductive to reuse.

In an architecture-based organization, the project managers are not line managers. Instead they own the budget for their project. Using this budget they can buy resources from the line organization (that is the subsystem-owners with domain expert developers) to get their projects done in time. The line organization can work with several projects concurrently and they have to prioritize them based on delivery dates. Yes, there may be conflicts, but these are usually not large. The total costs are substantially lower because the domain components are developed and maintained by experts. Thus in the case of conflicts, more resources can be added to remove the conflict, and still the costs are much lower than with a project-based organization.  

The approach I just presented also has the advantage that you grow the reusable components as you work with real projects. You will start with an empty architecture, but you fill it as you go. You don’t sit there and speculate about what might be reusable; instead you design the components in the architecture that are first needed. They have probably not stabilized for the first project, so they may have to grow through several projects before they really become reusable without redesign.

To achieve reuse is not easy. People have tried simple solutions like class libraries and component libraries into which component designers post their components hoping that other people will reuse them. We have seen very little reuse coming this way. Managers have thought it was the lack of incentives so they have stimulated all kinds of initiatives to get reuse. This is ignorant. You will get reuse, but only by standing on top of an integration of A, P and O, that is Architecture, Process and Organization. In another postcard I will tell you how you can move to an architecture-based organization from a project-based organization without revolution. It is really fascinating to see how easy it can be done, given that you are smart. However, that is not a problem, since you are smart, aren’t you?  

3 Comments
  1. vijaynarayanan | June 21, 2009 at 4:55 pm Reply

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    Thanks for the insightful Ivar. Related to what you pointed out, architecture that defines roles for different components, facilitates integration among domain-specific and domain-agnostic capabilities plays a critical role in systematic reuse.

  2. Ivar | March 3, 2008 at 1:19 am Reply

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    Hey, your conclusions goes to far.
    DSL’s can be created by using UML. There are many such good examples.
    Now, I don’t want you to believe I think everything with UML is good. See my blog and go to the UML category. What I think is really good is what we call Essential UML.
    Moreover, DSL’s are not new. They have been there for more than twenty years. Like everything in software we swing from one position to the other. DSL’s were very popular in the telecom space back in the late 80s and they came as a revolt to a unifying approach called SDL. SDL was created by the telecom industry in 1976 and it can best be described as a predecessor to UML. Ten years later we got UML. Maybe it is time for another swing to DSL’s. I would welcome it, if we did it well.
    The problem with DSL’s has been to guarantee consistencies between models of the same system. The problem is called DSL fragmentation. This is a problem that has fascinated many computer scientists butI don’t know if they have found a solution yet. I doubt it.
    I will use what works. I know UML works very well in many cases. I am sure some DSLs also works in some domains. We just need to relax and not be so fundamentalistic about these things.