A software product line (SPL) consists of a family of software systems that have some common functionality and some variable functionality (Parnas 1979, Clements 2002, Weiss 1999). Software product line engineering involves developing the requirements, architecture, and component implementations for a family of systems, from which products (family members) are derived and configured. The problems of developing individual software systems are scaled upward when developing software product lines because of the increased complexity due to variability management. This chapter gives an overview of designing software product line architectures using the PLUS (Product Line UML-based Software engineering) method. The topic is covered in considerable detail in the author's book on this topic (Gomaa 2005a).

SPL technology is particularly valuable for developing real-time embedded product families, in which some external devices, such as sensors and actuators, may be optional (such as light or turntable in a microwave oven SPL) in some family members or there may be variants (one-level on/off heating element or multi-level high/medium/low/off heating element) that are used by different family members. To manage the variability of SPL architectures and implementations necessitates developing feature models to determine what the variability is and developing variable software architectures consisting of kernel, optional, and variant components to determine how the variability is mapped to the design.

As with single systems, a better understanding of a software product line can be obtained by considering the multiple views, such as requirements models, static models, and dynamic models of the product line. A visual modeling language such as UML helps in developing, understanding, and communicating the different views. A key view in the multiple views of a software product line is the feature modeling view. The feature model is crucial for managing variability and product derivation as it describes the product line requirements in terms of commonality and variability, as well as defining the product line dependencies. Furthermore, it is necessary to have a development approach that promotes software evolution, such that original development and subsequent maintenance are both treated using feature-driven evolution.

Section 15.1 describes the software process model for SPL Engineering. Section 15.2 presents the problem description for the SPL example used in this chapter. Section 15.3 describes requirements modeling for SPLs, in particular use case modeling and feature modeling for SPLs.