Modeling and Monitoring of Hierarchical State Machines in Scala

Hierarchical State Machines (HSMs) are widely used in the design and implementation of spacecraft flight software. However, the traditional approach to using HSMs involves graphical languages (such as UML statecharts) from which implementation code is generated (e.g. in C or C). This is driven by the fact that state transitions in an HSM can result in execution of action code, with associated side-effects, which is implemented by code in the target implementation language. Due to this indirection, early analysis of designs becomes difficult. We propose an internal Scala DSL for writing HSMs, which makes them short, readable and easy to work with during the design phase. Writing the HSM models in Scala also allows us to use an expressive monitoring framework (also in Scala) for checking temporal properties over the HSM behaviors. We show how our approach admits writing reactive monitors that send messages to the HSM when certain sequences of events have been observed, e.g., to inject faults under certain conditions, in order to check that the system continues to operate correctly. This work is part of a larger project exploring the use of a modern high-level programming language (Scala) for modeling and verification.