Practical causality handling for synchronous languages
The synchronous principle is a well-established paradigm for reconciling concurrency with determinism. A key is to establish at compile time that a program or model is causal, which basically means that there exists a schedule that obeys the rules put down by the language. This rules out surprises at run time; however, in practice it can be rather cumbersome for the developer to cure causality problems, in particular as programs/models get more complex. We here propose to tackle this issue in two ways. Firstly, we propose to enrich the scheduling regime allowed by the language to not only consider data dependencies, but also explicit scheduling directives that operate on statements or coarser scheduling units. These directives may be used by the developer, or also by model-to-model transformations within the compiler. Secondly, we propose to enhance programming/modeling environments to guide the developer in finding causality issues. Specifically, we propose dedicated causality views that highlight data dependencies involved in scheduling conflicts, and structure-based editing to efficiently add scheduling directives. We illustrate our proposals for the SCCharts language. An Eclipse-based implementation based on the KIELER framework is available as open source.
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