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Guards and Effects

Guards and Effects are used to control the flow of the simulation and to execute additional actions or effects during the course of a simulation.

Guards and Effects

Concept

Detail

See also

Guards

Guards are conditional statements that are evaluated whenever the simulator has to determine the path to take next. Guards typically have these characteristics:

  • Defined on transitions and control flows to govern how simulation proceeds
  • Written in Javascript
  • Can refer to variables defined during simulation
Dynamic Simulation with Javascript

Adding Guards

Guards are defined on the Transition or Control Flow in the 'Properties' dialog for the selected connector. A Guard is typically a piece of Javascript that will evaluate to either True or False. For example, here is a conditional statement that refers to a current variable (Balance) being greater than zero. Note the use of the prefix this to indicate that the variable is a member of the current Class context.

Evaluation Semantics

During execution the Simulator will examine all possible paths forward and evaluate any guard conditions. This evaluation could establish that:

  • A single valid path forward evaluates to True; the Simulator will follow that path
  • Two valid paths exist; the Simulator will block, waiting for some manual input via the console window to resolve the deadlock
  • No valid path exists; the same response as when two paths are found - the Simulator waits for the user to change the execution context using the console window
  • No paths evaluate to True but a default (unguarded path) exists; the Simulator will take the unguarded path

Effects

Effects are defined behaviors that are executed at special times:

  • On entry to a state
  • On exit from a state
  • When transitioning from one state to another (transition effect)

Effects can either be a section of Javascript code or a call to another Behavior element in the current simulation.

Javascript Effects

A Javascript effect might resemble this example, in which the Balance variable is decremented:

Call Behavior Effects

In this example the effect is a call behavior effect. In this case, it calls into a model the Activity named Decrement Balance that is defined elsewhere. The simulation will then enter into that diagram/behavior and continue to execute until returning to the point at which the effect was invoked.

Order of Execution of Effects

In complex simulations that might involve transitioning out of deeply nested states into other deeply nested states in a different parent context, it is important to consider these rules concerning the order of execution:

  • All exit actions (effects) encountered leaving a nested context are executed in order of most deeply nested to least deeply nested
  • All actions (effects) defined on transitions are executed next
  • Finally, all entry effects are executed from the least deeply nested context to the most deeply nested

So the basic rule is: all exit actions, followed by all transition actions, and finally all entry actions.

Note on Javascript Variables

Javascript variables to be accessed and referred to during Simulation execution belong to either:

  • sim e.g. sim.pedestrianwaiting - typically used for global simulation variables, or
  • this e.g. this.CustomerNumber - typically used to refer to owning Class attributes

This is important to let the Javascript engine know you are referring to a Simulation variable and not a simple local variable used during, for example, basic calculations. You can create Simulation variables of arbitrary scope and depth - for example, this.customer.name is a legitimate qualified name.

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