Topologically configurable systems as product families

Railway interlocking systems are used as a case study to investigate about topological variability, that is, when prod- ucts of a family differ for the multiplicity of deployed features and especially for their connections. In the case of railway interlocking systems, each deployed system is configured on the basis of a description of the track layout the system is going to control: the configuration parameter is indeed a graph that reflects the topology of the track layout. One key aspect of this kind of systems is safety: the strict adherence to safety regulations makes configuration of these systems a critical issues, with specific impacts on the choice of variability model adopted. We review from a product line engineering perspective the different ways that are used in practice to encode this kind of variability and we discuss the main advantages, especially from the point of view of safety certification, that may be obtained by means of a product line approach. We conclude that indeed several research issues are still to be investigated in the particular case of interlocking systems, and we end by asking whether any other complex system may benefit from such results as well.