Feature code is often scattered across a software system. Scattering is not necessarily bad if used with care, as witnessed by systems with highly scattered features that evolved successfully. Feature scattering, often realized with a pre-processor, circumvents limitations of programming languages and software architectures. Unfortunately, little is known about the principles governing scattering in large and long-living software systems. We present a longitudinal study of feature scattering in the Linux kernel, complemented by a survey with 74, and interviews with nine Linux kernel developers. We analyzed almost eight years of the kernel's history, focusing on its largest subsystem: device drivers. We learned that the ratio of scattered features remained nearly constant and that most features were introduced without scattering. Yet, scattering easily crosses subsystem boundaries, and highly scattered outliers exist. Scattering often addresses a performance-maintenance tradeoff (alleviating complicated APIs), hardware design limitations, and avoids code duplication. While developers do not consciously enforce scattering limits, they actually improve the system design and refactor code, thereby mitigating pre-processor idiosyncrasies or reducing its use.

As the complexity of avionic systems increases, the aerospace industry is turning to product-line engineering and model-based development to better manage complexity and reduce cost. This paper describes a method and a pattern catalog for modeling avionics product lines in SysML, a standard systems modeling language. The method is designed to satisfy aerospace systems and software development standards, and the patterns provide guidance for expressing variability in SysML. The paper also reports on the experience in applying the method and the patterns to model families of propeller controllers and fuel controllers for turbo engines.