Delivering on software product is hard, delivering a line of products is harder. Mark Dalgarno and Danilo Beuche share some ideas that help.
Although the term "Software Product Line Engineering" is becoming more widely known, there is still uncertainty among developers about how it would apply in their own development context. In this article we tackle this problem by describing the design and automated derivation of the product variants of a Software Product Line using an easy to understand, practical example.
One increasing trend in software development is the need to develop multiple, similar software products instead of just a single product. There are several reasons for this. Products that are being developed for the international market must be adapted for different legal or cultural environments, as well as for different languages, and so must provide adapted user interfaces. Because of cost and time constraints it is not possible for software developers to develop a new product from scratch for each new customer and so software re-use must be increased. These types of problems commonly occur in portal or embedded applications, e.g. vehicle control applications [Ste04] but are also seen in desktop applications. Software Product Line Engineering (SPLE) offers a solution to these increasingly challenging, problems [Cle01].
The basis of SPLE is the explicit modelling of what is common and what differs between product variants. Feature Models [Kan90] [Cza00] are frequently used for this. SPLE also includes the design and management of a variable software architecture and its constituent (software) components.
This article describes how this is done in practice, using the example of a Product Line of meteorological data systems. Using this example we will show how a Product Line is designed and how product variants can be derived automatically.
Software product lines
Before we introduce the example, we'll take a small detour into the basis of SPLE. The main difference from 'normal', one-of-a-kind software development, is a logical separation between the development of core, reusable software assets (the platform) and actual applications. During application development, platform software is selected and configured to meet the specific needs of the application.
The Product Line's commonalities and variabilities are described in the Problem Space. This reflects the desired range of applications ('product variants') in the Product Line (the 'domain') and their inter-dependencies. So, when producing a product variant, the application developer uses the problem space definition to describe the desired combination of problem variabilities to implement the product variant.
An associated Solution Space describes the constituent assets of the Product Line (the 'platform') and its relation to the problem space, i.e. rules for how elements of the platform are selected when certain values in the problem space are selected as part of a product variant. The four-part division resulting from the combination of the problem space and solution space with domain and application engineering is shown in Figure 1.
Several different options are available for modelling the information in these four quadrants. The problem space can be described e.g. with Feature Models, or with a Domain Specific Language (DSL). There are also a number of different options for modelling the solution space for example component libraries, DSL compilers, generative programs and also configuration files [Cza00].
In the rest of this article we will consider each of these quadrants in turn, beginning with Domain Engineering activities. We'll first look at modelling the problem space - what is common to, and what differs between, the different product variants. Then we'll consider one possible approach for realising product variants in the solution space using C++ as an example. Finally we'll look at how Application Engineering is performed by using the problem and solution space models to create a product variant. In reality, this linear flow is rarely found in practice. Product Lines usually evolve continuously, even after the first product variants have been defined and delivered to customers.
Our example Product Line will contain different products for entry and display of meteorological data on a PC. An initial brainstorming session has led to a set of possible differences (variation points) between possible products: meteorological data can come from different sensors attached to the PC, fetched from appropriate Internet services or generated directly by the product for demonstration and test purposes. Data can be output directly from the application, distributed as HTML or XML through an integrated Web server or regularly written to file on a fixed disk. The measurements to make can also vary: temperature, air pressure, wind velocity and humidity could all be of interest. Finally the units of measure could also vary (degrees Celsius vs. Fahrenheit, hPa vs. mmHg, m / s vs. Beaufort).
Modelling the problem space
We will now convert the informal, natural-language specification of variability noted above into a formal model in order to be able to process it. Specifically, we will use a Feature Model. Feature models are simple, hierarchical models that capture the commonality and variability of a Product Line. Each relevant characteristic of the problem space becomes a feature in the model. A definition of the term 'feature' is given in Definition 1.
Feature models have a tree structure, with features forming nodes of the tree. Feature variability is represented by the arcs and groupings of features. There are four different types of feature groups: 'mandatory', 'optional', 'alternative' and 'or'. When specifying which features are to be included in a variant the following rules apply:
If a parent feature is contained in a variant,
- all its mandatory child features must be also contained ('n from n'),
- any number of optional features can be included ('m from n, 0 < = m<=n'),
- exactly one feature must be selected from a group of alternative features ('1 from n'),
- at least one feature must be selected from a group of or features('m from n, m>1').
Unfortunately, no single standard has yet been agreed for the graphical notation of feature models. However, in the literature, the graphical notation of the original Feature-Oriented Domain Analysis (FODA) method [Ste04] is common. However, this is representable with standard text tools and graph libraries only with difficulty. Therefore in this article a simplified notation has been used. Alternatives and groups of or features are represented with traverses between the matching features. In this representation both colour and box connector are used independently to indicate the type of group. Our notation is shown in Figure 2. Using this notation, our example feature model, with some modifications, is shown in Figure 3.
Each Feature Model has a root feature. Beneath this are three mandatory features - 'Measurements', 'Data Source' and 'Output Format'. Mandatory features will always be included in a product variant if their parent feature is included in the product variant. Mandatory features are not variable in the true sense, but serve to structure or document their parent feature in some way. Our example also has alternative features, e.g. 'External Sensors', 'Demo' and 'Internet' for data sources. All product variants must contain one and only one of these alternatives.
At this stage we can already see one advantage that feature modelling has over a natural-language representation - it removes ambiguities - e.g. whether an individual variant is able to process data from more than one source. When taking measurements any combination of measurements is meaningful and at least one measurement source is necessary for a sensible weather station, to model this we use a group of Or features. Usually simple optional features are used, such as the example of the Alarm. Further improvements can also be made by refining the model hierarchy. So the strict choice between Web Server output formats - HTML or XML - can be made explicit.
Feature models also support transverse relationships, such as 'requires' and 'mutually exclusive', in order to model additional dependencies between features other than those already described. So, in the example model, a selection of the 'Freeze Point' alarm feature is only meaningful in connection with the temperature measurement capability. This can be modelled by a 'Freeze Point' requires 'Temperature' relationship (not shown in the figure). However, such relations should be used sparingly. The more transverse relations there are, the harder it is for a human user to visualize connections in the model.
When creating a feature model it can be difficult to decide exactly how problem space variabilities are to be represented in the model. In this case it is best to discuss this further with the customer. It is usually better to base these discussions around the feature model, since such models are easier for the customer to understand than textual documents and / or UML models. Formalising customer requirements in this way offers significant advantages later in Product Line development, since many architectural and implementation decisions can be made on the basis of the variabilities captured in the feature model. In the example, the use of the output format XML and HTML can be clarified. The model explicitly defines that the choice of output format is only relevant for Web Server, a format selection is not possible for File or Text output. However, in the context of a discussion of the feature model it could be decided that HTML is also desirable for the on-screen (Window) representation and could also be applicable for file storage.
This results in the modified feature model shown in Figure 4.
We have added 'Plaintext' to the existing features; this was implicitly assumed for output to the screen or to a file. We have modelled the mutual exclusion of XML and screen display ('Text') using a (transverse) relationship between these features (not shown).
The previous discussion describes the basic feature model approach commonly found in the literature, but a number of people have extended this basic approach. To complement the so-called hard relations between features ('requires' and 'conflicts') the weakened forms 'recommends' and 'discourages' have been added to many feature model dialects. A few tools also support the association of named attributes with features. This allows numeric values or enumerated values to be conveniently associated with features e.g. the wind force required to activate the storm alarm could be represented as a 'Threshold' attribute of the feature 'Storm Alert'.
An important and difficult issue in the creation of feature models is deciding which problem space features to represent. In the example model it is not possible to make a choice from the available hardware sensor types (e.g. use of a PR1003 or a PR2005 sensor for pressure). So, when specifying a variant, the user does not have direct influence on the selection of sensor types. These are determined when modelling the solution space. If the choice of different sensor types for measuring pressure is a major criterion for the customer / users, then appropriate options would have to be included in the feature model. This means that the features in the problem space are not a 1:1 illustration of the possibilities in the solution space, but only represent the (variable) characteristics relevant for the users of the Product Line. Feature models are a user-oriented (or marketing-oriented) representation of the problem space, not the solution space.
After creating the problem space model we can use it to perform some initial analysis. For example, we can now calculate the upper limit on the number of possible variants in our example Product Line. In this case we have 1,512 variants (the model in Figure 3 only has 612 variants). For such a small number of variants the listing of all possible variants can be meaningful. However, the number of variants is usually too high to make practical use of such an enumeration.
Modelling the solution space
In order to implement the solution space using a suitable variable architecture, we must take account of other factors beyond the variability model of the problem space. These include common characteristics of all variants of the problem space that are not modelled in the feature model, as well as other constraints that limit the solution space. These typically include the programming languages that can be used, the development environment and the application deployment environment(s).
Different factors affect the choice of mechanisms to be used for converting from variation points in the solution space. These include the available development tools, the required performance and the available (computing) resources, as well as time and money. For example, use of configuration files can reduce development time for a project, if users can administer their own configurations. In other cases, using preprocessor directives (#ifdef) for conditional compilation can be appropriate, e.g. if smaller program sizes are required.
There are many possibilities for implementation of the solution space. Very simple variant-specific model transformations can be made with model-driven software development (MDSD) tools by including information from feature models in the Model-Transformation process. [Voel05] gives an example using the openArchitectureware model transformer. Aspect-oriented programming (AOP) can also be used as a means for the efficient conversion of variabilities in the solution space. Product Lines can also be implemented naturally using 'classical' means such as procedural or object-oriented languages.
Designing a variable architecture
A Product Line architecture will only rarely result directly from the structure of the problem space model. The solution space which can be implemented should support the variability of the problem space, but won't necessarily be a 1:1 correspondence with the architecture. The mapping of variabilities can take place in various ways.
In the example Product Line we will use a simple object-oriented design concept implemented in C++ . A majority of the variability is then resolved at compile-time or link-time; runtime variability is only used if it is absolutely necessary. Such solutions are frequently used in practice, particularly in embedded systems.
The choice of which tools to use for automating the configuration and / or production of a variant plays a substantial role in the design and implementation of the solution space. The range of variability, the complexity of relations between problem space features and solution constituents, the number and frequency of variant production, the size and experience of the development team and many further factors play a role. In simple cases the variant can be produced by hand, but automated tools in the form of Excel and / or small configuration scripts, and also model transformers, code generators or variant management systems will speed production.
One approach for modelling and mapping of the solution space variability is to use a separate Solution Model to model the solution space, to associate solution space elements with problem space features, and to support the automatic selection of solution space elements when constructing a product variant. This separation of concerns also has the advantage of allowing both models to evolve independently.
Solution models have a hierarchical structure, consisting of logical items of the solution architecture, e.g. components, classes and objects. These logical items can be augmented with information about 'real' solution elements such as source code files, in order to enable automatic production of a solution from a valid feature model configuration (more on this later). For each solution model element a rule is created to link it to the solution space. For example, the Web Server implementation component is only included if the Web Server feature has been selected from the problem space. To achieve this, a hasFeature('Web Server') rule is attached to the 'Web Server' component . Any item below 'Web Server' in the Solution model can only be included in the solution if the corresponding Web Server feature is selected.
In our example, an architectural variation point arises, among other possibilities, in the area of data output. Each output format can be implemented with an object of a format-specific output class. Thus in the case of HTML output, an object of type HtmlOutput is instantiated, and with XML output, an XmlOutput object. There would also be the possibility here of instantiating an appropriate object at runtime using a Strategy pattern. However, since the feature model designates only the use of alternative output formats, the variability can be resolved at compile-time and a suitable object can be instantiated using code generation for example.
In our example solution space a lookup in a text database is used to support multiple natural languages. The choice of which database to use is made at compile-time depending on the desired language. No difference in solution architectures can be detected between two variants that differ only in the target language. Here the variation point is embedded in the data level of the implementation.
In many cases managing variable solutions only at the architectural level is insufficient. As has already been mentioned above, we must also support variation points at the implementation level, i.e. in our case at the C++ source code level. This is necessary to support automated product derivation. The constituents of a solution on the implementation level, like source code files or configuration files which can be generated, can also be entered in the solution model and associated with selection rules.
So the existence of the Web Server component in a product variant is denoted using a #define preprocessor directive in a configuration Header file. In addition, an appropriate abstract variation point variable 'WEB SERVER' must first be created of the type ps:variable in the solution model. The value of this variable is determined by a Value attribute. In our case this value is always 1 if the variable is contained in the product variant. An item of type ps:flagfile can now be assigned to this abstract variable. This item also possesses attributes (file, flag), which are used during the transformation of the model into 'real' code. The meaning of the attributes is determined by the transformation selected in the generation step.
Separating the logical variation point from the solution makes it very simple to manage changes to the solution space. For example, if the same variation point requires an entry in a Makefile, this could be achieved with the definition of a further source element, of the type ps:makefile, below the variation point 'WEB SERVER'.
Deriving product variants
The solution model captures both the structure of the solution space with its variation points and the connection of solution and problem space. The direction of this connection is important since problem space models in most cases are much more stable than solution spaces; linking the solution space to the problem space is more meaningful than the selection of solution items by rules in the problem space. This also increases the potential for reuse, since problem space models can simply be combined with other (new, better, faster) solutions.
Now we have all the information needed to create an individual product variant. The first step is to determine a valid selection of characteristics from the feature model. In the case of some tools, the user is guided towards a valid and complete feature selection and for a large feature model this can reduce the time to create a complete and consistent selection by an order of magnitude. Once a valid selection is found, the specified feature list as well as the solution model serve as input for the production of a variant model. Then, as is described above, the rules of the individual model items are checked. Only items that have their rules satisfied are included in the finished solution.
Open issues in SPLE
So far we have highlighted some of the most common issues that will be encountered when working with Software Product Lines. In this section we highlight some additional issues.
Even with visualization support from specialist tools, the visual representation of very complex model structures is a not a completely solved problem. Larger feature models can have several hundred features and the solution space can have several thousands or more constituents. Thus it can be hard to understand the implications of modifications to these models just through use of model diagrams.
Issues around Product Line evolution are also very important. Evolution must be managed since changes that positively affect one or more variants could have a negative effect on other variants and these issues may only show up when variants are produced long after the changes have been implemented.
Finally, testing a Product Line also represents a significant challenge. Most Product Lines offer more potential variability than is in use at any one time, and testing all possible variants is usually impossible and in some cases a waste of time. Testing just those variants that are produced is already a difficult problem where there is a high number of variants. However, it is still necessary to co-ordinate testing with variant production in some way. One approach is to create test asset variants as one does for the (software) solution space variants - effectively creating a parallel test solution space that is driven from the Feature model. Reduction of test effort is still an open issue though for many Product Lines.
We've shown above how the variability of the problem space of a Product Line can be described very simply using feature models. Automated production of solution variants is the logical next step, for which we have shown one example. We have also highlighted some of the approaches and issues that need to be considered when using Software Product Lines. These are covered in more depth in [Bos00], a standard work on Software Product Lines. The authors also welcome comments and questions on any aspect of Software Product Lines Engineering. n
References and links
[Bos00] J. Bosch, Design and Use of Software Architectures: Adopting and Evolving a Product Line Approach, Addison-Wesley, 2000
[Cle01] P. Clements, L Northrop, Software Product Lines: Practices & Patterns, Addison-Wesley 2001 (see also www.sei.cmu.edu/productlines/framework.html
[Cza00] K. Czarnecki, U.W. Eisenecker, Generative Programming: Methods, Tools, and Applications, Addison-Wesley, 2000
[Kan90] K. Kang, et al., Feature Oriented Domain Analysis (FODA) Feasibility Study, Technical report CMU/SEI-90-TR-021, Software Engineering Institute, Carnegie Mellon University, 1990
[Ste04] M. Steger et al., 'Introducing PLA RK Bosch Gasoline of System: Experiences and Practices' in: Proc. of the Software Product Line Conf. 2004, S. 34-50
[Voel05] M. Voelter, Variantenmanagement in the context of MDSD, in: JavaSpektrum 5/05
Overload Journal #78 - Apr 2007 + Design of applications and programs
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