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Keyword reference for priority

Question 49 (2008):

Submission reference: IN1705

How can we implement a structured priority alternation of processes? For example, I have 3 processes A, B and C. I want A to have the first priority, then B and finally C. Can we nest the PRI ALT constructs to achieve this, as follows:

  PRI ALT
    PRI ALT
      x ? any
        A()
      y ? any
        B()
    z ? any
      C()

?

Answer 49:

Priorities given by a PRI ALT relate to the guards rather than the processes they are defending. So, it does not make sense to say you "want A to have the first priority". It would make sense to say you "want channel x to have the first priority", etc.

The code fragment you give does give priority first to channel x, then to channel y and, then, to z. But there is no need to nest the PRI ALTs! A single PRI ALT can have any number of guarded processes. So:

  PRI ALT
    x ? any
      A()
    y ? any
      B()
    z ? any
      C()

does the same job. See slides 124 to the end of the choice slides for the rules about nesting ALTs.

Keywords: priority