Package ptolemy.domains.sr.kernel

Class SRDirector

  • All Implemented Interfaces:
    java.lang.Cloneable, Executable, Initializable, SuperdenseTimeDirector, PeriodicDirector, Changeable, Debuggable, DebugListener, Derivable, ModelErrorHandler, MoMLExportable, Moveable, Nameable
    Direct Known Subclasses:
    FMIMADirector, MetroIISRDirector
    public class SRDirector
extends FixedPointDirector
implements PeriodicDirector
    A director for the Synchronous Reactive (SR) model of computation. The SR model of computation has a notion of a global "tick" of a clock, and at each tick of the clock, each port either has a value or is "absent." The job of this director is determine what that value is for each connection between ports. An iteration of this director is one tick of this global clock.

    Execution proceeds as follows. The director checks each actor to determine whether it is strict or not by calling its isStrict() method (here, "strict" means that all inputs must be known before the actor can specify any outputs). By default, actors are strict. Strict actors are fired only once in an iteration. Their inputs are all known (and may absent) when prefire() is invoked. If prefire() returns true, the fire() and postfire() are invoked exactly once.

    Specialized actors may be non-strict, meaning that they are able to produce outputs even their inputs are not known. Such actors must conform to certain requirements in order to ensure determinacy. First, such actors should check input ports by calling their isKnown() method before calling hasToken() to determine whether the port is "absent." Only if both isKnown() and hasToken() return true should the actor call get() on that port. A non-strict actor may be prefired and fired repeatedly in an iteration if some of the inputs are unknown. Once an actor is fired with all its inputs known, it will not be fired again in the same iteration. A composite actor containing this director is a non-strict actor.

    Each actor's fire() method implements a (possibly state-dependent) function from input ports to output ports. At each tick of the clock, the fire() method of each non-strict actor may be evaluated multiple times, and each time, it must implement the same function. Thus, the actors are required to conform with the strict actor semantics, which means that they do not change their state in the prefire() or fire() methods, and only change their state in postfire(). This helps ensure that the actor is monotonic. Montonicity implies three constraints on the actor. First, if prefire() ever returns true during an iteration, then it will return true on all subsequent invocations of prefire() in the same iteration(). In subsequent iterations, inputs may become known, but once they are known, the value of the input and whether it is present cannot change in subsequent firings in the same iteration. Second, if either prefire() or fire() call clear() on an output port, then no subsequent invocation in the same iteration can call put() on the port. Third, if prefire() or fire() call put() on an output port with some token, then no subsequent invocation in the same iteration can call clear() or put() with a token with a different value. These constraints ensure determinacy.

    If synchronizeToRealTime is set to true, then the postfire() method stalls until the real time elapsed since the model started matches the current time. This ensures that the director does not get ahead of real time. However, of course, this does not ensure that the director keeps up with real time. Note that this synchronization occurs after actors have been fired, but before they have been postfired.

    The SR director has a period parameter which specifies the amount of model time that elapses per iteration. If the value of period is 0.0 (the default), then it has no effect, and this director never increments time nor calls fireAt() on the enclosing director. If the period is greater than 0.0, then if this director is at the top level, it increments time by this amount in each invocation of postfire(). If it is not at the top level, then it refuses to fire at times that do not match a multiple of the period (by returning false in prefire()). If it fires, then in postfire() it calls fireAt(currentTime + period) in postfire() to request the next firing.

    Note that currentTime, as usual, means local time. If this director is inside a modal model, then that current time may lag behind the time of the environment in which the modal model executes because time does not elapse while a mode is inactive. In fact, time may bounce around, since if a startTime is specified and the modal model has a reset transition, then local time may be reinitialized to the start time during execution. Moreover, if the startTime of this director inside a modal is greater than the start time of the enclosing director, then local time may actually be ahead of environment time.

    This behavior gives an interesting use of SR within DE or Continuous. In particular, if set a period other than 0.0, the composite actor with this SR director will fire periodically with the specified period.

    If period is greater than 0.0 and the parameter synchronizeToRealTime is set to true, then the prefire() method stalls until the real time elapsed since the model started matches the period multiplied by the iteration count. This ensures that the director does not get ahead of real time. However, of course, this does not ensure that the director keeps up with real time.

    Since:
    Ptolemy II 2.0
    Version:
    $Id$
    Author:
    Paul Whitaker, Edward A. Lee, Contributor: Ivan Jeukens, Haiyang Zheng
    Pt.AcceptedRating:
    Green (pwhitake)
    Pt.ProposedRating:
    Green (pwhitake)

    • Field Detail

      • period

        public Parameter period
        The time period of each iteration. This parameter has type double and default value 0.0, which means that this director does not increment model time and does not request firings by calling fireAt() on any enclosing director. If the value is set to something greater than 0.0, then if this director is at the top level, it will increment model time by the specified amount in its postfire() method. If it is not at the top level, then it will call fireAt() on the enclosing executive director with the argument being the current time plus the specified period. In addition, if it is not at the top level, then it refuses to fire at times that do not match a multiple of the period (by returning false in prefire()).

    • Constructor Detail

      • SRDirector

        public SRDirector()
           throws IllegalActionException,
                  NameDuplicationException
        Construct a director in the default workspace with an empty string as its name. The director is added to the list of objects in the workspace. Increment the version number of the workspace.
        Throws:
        IllegalActionException - If the name has a period in it, or the director is not compatible with the specified container.
        NameDuplicationException - If the container already contains an entity with the specified name.

      • SRDirector

        public SRDirector​(Workspace workspace)
           throws IllegalActionException,
                  NameDuplicationException
        Construct a director in the given workspace with an empty name. The director is added to the list of objects in the workspace. Increment the version number of the workspace.
        Parameters:
        workspace - The workspace for this object.
        Throws:
        IllegalActionException - If the name has a period in it, or the director is not compatible with the specified container.
        NameDuplicationException - If the container already contains an entity with the specified name.

      • SRDirector

        public SRDirector​(CompositeEntity container,
                  java.lang.String name)
           throws IllegalActionException,
                  NameDuplicationException
        Construct a director in the given container with the given name. The container argument must not be null, or a NullPointerException will be thrown. If the name argument is null, then the name is set to the empty string. Increment the version number of the workspace.
        Parameters:
        container - Container of the director.
        name - Name of this director.
        Throws:
        IllegalActionException - If the director is not compatible with the specified container.
        NameDuplicationException - If the name collides with an attribute in the container.

    • Method Detail

      • fireAt

        public Time fireAt​(Actor actor,
                   Time time,
                   int microstep)
            throws IllegalActionException
        Request a firing of the given actor at the given absolute time, and return the time at which the specified will be fired. If the period is 0.0 and there is no enclosing director, then this method returns the current time. If the period is 0.0 and there is an enclosing director, then this method delegates to the enclosing director, returning whatever it returns. If the period is not 0.0, then this method checks to see whether the requested time is equal to the current time plus an integer multiple of the period. If so, it returns the requested time. If not, it returns current time plus the period.
        Overrides:
        fireAt in class Director
        Parameters:
        actor - The actor scheduled to be fired.
        time - The requested time.
        microstep - The microstep (ignored by this director).
        Returns:
        Either the requested time or the current time plus the period.
        Throws:
        IllegalActionException - If the operation is not permissible (e.g. the given time is in the past).
        See Also:
        Director.fireAtCurrentTime(Actor), Director.fireContainerAt(Time)

      • prefire

        public boolean prefire()
                throws IllegalActionException
        Return true if we are ready to fire, meaning that either (1) we are at the top level, (2) the period parameter is zero (in which case we are always ready to fire, or (3) the period parameter is nonzero, the current time matches the next expected execution time, and the current microstep is 1. If this actor is enclosed within a director that does not understand microsteps, then we ignore the microstep value. This method will also synchronize to real time, if appropriate.
        Specified by:
        prefire in interface Executable
        Overrides:
        prefire in class FixedPointDirector
        Returns:
        true If current time is appropriate for a firing.
        Throws:
        IllegalActionException - If the period parameter cannot be evaluated.

      • postfire

        public boolean postfire()
                 throws IllegalActionException
        Call postfire() on all contained actors that were fired on the last invocation of fire(). Return false if the model has finished executing, either by reaching the iteration limit, or if no actors in the model return true in postfire(), or if stop has been requested. This method is called only once for each iteration. Note that actors are postfired in arbitrary order.

        If the period parameter is greater than 0.0, then if this director is at the top level, then increment time by the specified period, and otherwise request a refiring at the current time plus the period.

        Specified by:
        postfire in interface Executable
        Overrides:
        postfire in class FixedPointDirector
        Returns:
        True if the Director wants to be fired again in the future.
        Throws:
        IllegalActionException - If the iterations or period parameter does not contain a legal value.