Toc is an experimental programming language based on occam that combines CSP-based concurrency with integrated specification of timing requirements. In contrast to occam with strict round-robin scheduling, the Toc scheduler is lazy and does not run a process unless there is a deadline associated with its execution. Channels propagate deadlines to dependent tasks. These differences from occam necessitate a different approach to programming, where a new concern is to avoid dependencies and conflicts between timing requirements. This paper introduces client-server design patterns for Toc that allow the programmer precise control of timing. It is shown that if these patterns are used, the deadline propagation graph can be used to provide sufficient conditions for schedulability. An alternative definition of deadlock in deadline-driven systems is given, and it is demonstrated how the use of the suggested design patterns allow the absence of deadlock to be proven in Toc programs. The introduction of extended rendezvous into Toc is shown to be essential to these patterns.
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