Class PetriNetDirector

  • All Implemented Interfaces:
    java.lang.Cloneable, Executable, Initializable, Changeable, Debuggable, DebugListener, Derivable, ModelErrorHandler, MoMLExportable, Moveable, Nameable

    public class PetriNetDirector
    extends Director
    This implementation of a PetriNet Director is an extension of the PetriNet Director created by Wang and Lee located in the experimental domain. This director supports an iteration count that can be used to stop execution, even if transitions are ready to fire. The director also supports a PetriNetDisplay actor that prints token counts of all the Places (at each iteration) after execution is finished. The documentation for the original Wang and Lee director is provided below and includes a definition of PetriNet.

    This domain implements the basic Petri Net model where Places and Transitions form a bipartite graph and enabled Transitions can fire randomly. It also allows Transitions to be replaced by any other Actors in Ptolemy. It implements two forms of Hierarchical and compositional Petri nets. The first form of hierarchical and compositional Petri net semantics comes from the fact that a Transition can contain a sub-Petri-net which is invisible to the director of the container of the Transition. The second form of hierarchical and compositional Petri net semantics comes from a new Actor called PetriNetActor which is a collection of Places and Transitions, and those Places and Transitions are visible to the director of the container of the PetriNetActor. The users can choose which form of models to use, and/or mix them together.

    The basic Petri net is a directed, weighted, bipartite graph consisting of two kinds of nodes, called Places and Transitions, where arcs are either from a Place to a Transition or from a Transition to a Place. In graphical representation, Places are drawn as circles, Transitions as bars or boxes. Arcs are labeled with their weights (positive integers). Labels of unity weight are usually omitted. Multiple arcs can exist between a Place and a Transition. A marking assigns to each Place p an nonnegative integer k, we say that p is marked with k tokens.

    Please note here the term token is used differently from the general Ptolemy term token. Here a token is really the integer 1. k tokens is represented by the integer k.

    A Transition t is said to be enabled if each input Place p of t is marked with at least the sum of w(p, t) tokens, where w(p, t) are the weights of the arcs from p to t.

    An enabled Transition may or may not fire. When there are multiple enabled Transitions, any of them can fire randomly. A firing of an enabled Transition t removes w(p, t) tokens from each input Place p of t, and adds w(t, p) tokens to each output Place p of t, where w(t, p) and w(p, t) are the weights of the arcs from and to the Transition respectively.

    A Transition without any input Place is called a source Transition, and one without any output Place is called a sink Transition. Note that a source Transition is unconditionally enabled, and that the firing of a sink Transition consumes tokens, but does not produce any.

    Many variations of Petri net exist in the literature including: hierarchical Petri nets, colored Petri nets, timed Petri nets, fuzzy Petri nets, stochastic Petri nets, compositional Petri nets, and many of the combinations.

    As pointed out earlier, in Ptolemy we implement the basic Petri net model plus two kinds of hierarchical and compositional Petri nets. This is made possible by defining the PetriNetActor. The PetriNetActor is a directed and weighted graph just like the basic Petri Net model. However, a PetriNetActor graph G = (V, E) contains three kinds of nodes: Places p_i, Transitions t_i, and PetriNetActors PA_i, i.e., V = {p_i} union {t_i} union {PA_i} , where each PA_i itself is again defined as a PetriNetActor. Places are assigned with non-negative integer markings. The default marking is 0. A Place is implemented by the atomic actor Place. A PetriNetActor is a TypedCompositeActor contains Places, Transitions and/or PetriNetActors.

    Each node of V is called a component of the PetriNetActor G. Therefore the vertex set V of G is also called the component set of the PetriNetActor G. The concept of component is a key difference between the basic Petri net model and the hierarchical Petri net model defined here. In Ptolemy term, a component is an element in the entityList of the PetriNetActor. A PetriNetActor consists of components. A component can be a Place, a Transition, and a PetriNetActor component. A component can be enabled and fires if it is a Transition or it is a PetriNetActor component that contains other Transitions. When the firing method _fireHierarchicalPetriNetOnce() fires, it chooses one component to fire.

    The definition of PetriNetActor defines one form of hierarchical and compositional Petri nets. It defines a hierarchical Petri net since the PetriNetActor G can contain other PetriNetActors PA_i as components. It defines a compositional Petri net since two PetriNetActors PA_1 and PA_2 of V can be connected through their ports to form a bigger Petri net G.

    The second form of Hierarchical and compositional Petri net comes from the fact that a Transition can be any TypedCompositeActor in Ptolemy domains, which can have its own firing director. The content of the Transition is invisible to the director of the container of the Transition. Therefore it is possible to have a Transition contains other Places and Transitions and has a PetriNetDirector as the local director for the Transition.

    The set of Transitions of the PetriNetActor G, or the Transition set of G, is defined to be the union of the Transitions t_i with the sets of Transitions of each PetriNetActor component PA_i. A member of the Transition set of G is therefore contained in G itself in which case the Transition is also a component of G, or it is contained in some PetriNetActor component PA_i. Therefore a Transition is a different concept from a Component in PetriNetActor graph. The method findTransitions() returns the Transition set of G.

    A component has ports through which connections to other components are made. A Place or a Transition each has one input port and one output port, where multiple connections can be made. In our model, a PetriNetActor component can have multiple ports. A PetriNetActor component PA_j can be connected to Places p_i, Transitions t_i, or other PetriNetActor components PA_i through ports. A Place p_i can be connected to Transitions t_i, or to ports of PetriNetActor components PA_i. A Transition t_i can be connected to Places p_i or to ports of PetriNetActor components PA_i.

    One restriction is that a port of a PetriNetActor component PA_i is either connected to Places or to Transitions, but not both. Another restriction is that a Place (Transition) is not allowed to be connected to another Place (Transition) through ports. Though no verification of these two conditions is checked, any violation of these two conditions will be reported by appropriate methods during the execution.

    Multiple arcs can exist between any two components. The arcs can be marked by an nonnegative integer as their weights. Weight 1 can be omitted. The method _getWeightNumber(arc) obtains the weight assigned to the arc. If no weight is assigned, the default weight is 1.

    For a Transition t, all Places p that can reach t through ports or without ports are the input Places of t. All Places that can be reached from t through ports or without ports are the output Places of t. Given a Transition t, the methods _findBackwardConnectedPlaces() and _findForwardConnectedPlaces() find the input and output Places of the Transition respectively.

    A Transition t is enabled or ready in the PetriNetActor if for each input Place p of t, the marking of p is bigger than the sum of the weights of all arcs connecting p to t. The method isTransitionReady(transition) tests whether the given Transition is enabled/ready or not.

    If a Transition t is enabled and t is contained in a PetriNetActor component PA_i, then the PetriNetActor component PA_i is also an enabled component. On the other hand, for any PetriNetActor component PA_i, if it contains an enabled Transition, then the PetriNetActor component PA_i is enabled. The method PetriNetActor.prefire() tests whether there is any enabled Transitions contained in the PetriNetActor component.

    An enabled Transition may or may not fire. For the given PetriNetActor G, all its enabled components including Transitions t_i and PetriNetActor components PA_i are collected together in a list returned by _readyComponentList(). Suppose the list has n components of t_i and PA_i, each component has 1/n probability to be chosen to fire. The method _fireHierarchicalPetriNetOnce() chooses one component from the list to fire.

    If an enabled Transition is chosen to fire, the method fireTransition() is called to fire the Transition and update the input and output Places of the Transition. The firing of the Transition is determined by its own director, if there is one, otherwise no action is needed. For each input Place of the Transition, its marking has to be decreased by the weight of each arc connecting the Place to the Transition. For each output Place, the marking will be increased by the weight of each arc connecting the Transition to the Place.

    If a PetriNetActor component PA_i is chosen to fire, the director then recursively repeats the same procedure for PA_i as for the top level PetriNetActor G.

    Finally, the firing of the hierarchical Petri net is continued until there is no more Transitions and components to fire, or it goes to infinite loop. Currently no action is taken for infinite loops.

    Other form of firing sequence can be defined as well. We could randomly fire all the deeply contained Transitions. We could randomly fire the components in each hierarchy. [1] T. Murata, "Petri nets: properties, analysis and applications", Proceedings of the IEEE, VOl. 77, NO. 4, April 1989, pp. 541-579. [2] J. L. Peterson, "Petri Net Theory and the modeling of systems", Prentice Hall, 1981.

    Ptolemy II 8.1
    Yuke Wang, Edward A. Lee and modified by Zach Ezzell
    • Field Detail

      • resetOnEachRun

        public SharedParameter resetOnEachRun
        If true, this parameter specifies that the random number generator should be reset on each run of the model (in the initialize() method). It is a boolean that defaults to false. This is a shared parameter, meaning that changing it somewhere in the model causes it to be changed everywhere in the model.
      • seed

        public SharedParameter seed
        The seed that controls the random number generator that determines which component is fired.

        This is a shared parameter, meaning that all instances of PetriNetDirector or derived classes in the same model share the same value. This parameter is used for testing so that a model has predictable results and can be compared against known good results.

        A seed of zero is interpreted to mean that no seed is specified, which means that each execution of the model could result in distinct data. For the value 0, the seed is set to System.currentTimeMillis() + hashCode(), which means that with extremely high probability, two distinct directors will have distinct seeds. However, current time may not have enough resolution to ensure that two subsequent executions of the same model have distinct seeds. For a value other than zero, the seed is set to that value plus the hashCode() of the full name of the director. This means that with high probability, two distinct director will have distinct, but repeatable seeds.

        This parameter contains a LongToken, initially with value 0.

    • Method Detail

      • attributeChanged

        public void attributeChanged​(Attribute attribute)
                              throws IllegalActionException
        If the attribute is seed then create the base random number generator.
        attributeChanged in class Director
        attribute - The attribute that changed.
        IllegalActionException - If the change is not acceptable to this container (not thrown in this base class).
      • clone

        public java.lang.Object clone​(Workspace workspace)
                               throws java.lang.CloneNotSupportedException
        Clone the director into the specified workspace. This calls the base class and then creates new ports and parameters.
        clone in class Director
        workspace - The workspace for the new object.
        A new director
        java.lang.CloneNotSupportedException - If a derived class contains an attribute that cannot be cloned.
        See Also:
        NamedObj.exportMoML(Writer, int, String), NamedObj.setDeferringChangeRequests(boolean)
      • findTransitions

        public java.util.LinkedList findTransitions​(TypedCompositeActor container)
        Find all Transitions of the given container, i.e., the Transition set of the container, which is supposed to be a PetriNetActor. A Transition can be contained in the top level PetriNetActor, or its PetriNetActor components. This method searches for Transitions recursively for each PetriNetActor component.
        container - The container where the Transitions are contained.
        the list of Transitions contained by the container.
      • fire

        public void fire()
                  throws IllegalActionException
        Fire enabled components of the PetriNetActor by calling the method _fireHierarchicalPetriNetOnce(), one at a time until there is no more enabled components to fire. The enabled component can be an enabled Transition or an enabled PetriNetActor component. It is the job of the method _fireHierarchicalPetriNetOnce() to find all enabled components if there is any, to choose which enabled component to fire, and to update markings of related Places when a component fires.

        A description of the firing is sent to any actors that implement the PetriNetDisplayer interface. If this director has a debug listener, then the description is also sent to those listeners.

        Specified by:
        fire in interface Executable
        fire in class Director
        IllegalActionException - If the method _fireHierarchicalPetriNetOnce() throws exceptions, which can happen if the method isTransitionReady() or fireTransition() throws exceptions.
      • fireTransition

        public void fireTransition​(TypedCompositeActor transition)
                            throws IllegalActionException
        Fire an enabled Transition.

        The transition argument to this method must be an enabled Transition. If the given Transition is Opaque, then it fires the Transition first, otherwise no action is taken for the Transition; the method then updates the markings of the input Places and output Places of the Transition. The update of the marking is done one relation at a time. The input Places and output Places are found by the methods _findForwardConnectedPlaces() and _findBackwardConnectedPlaces() respectively.

        transition - The transition to be fired.
        IllegalActionException - If the method _getWeightNumber() throws an exception.
      • isTransitionReady

        public boolean isTransitionReady​(TypedCompositeActor transition)
                                  throws IllegalActionException
        Test whether a given Transition is enabled or not.

        A Transition is enabled if for each of the input Places, the marking of the Place is bigger than the sum of weights of edges connecting the Place to the Transition. The Transition itself is any TypedCompositeActor. The Transition can be a component of a PetriNetActor, or it is contained in some PetriNetActor component.

        This is one of the key methods for hierarchical Petri Nets. It is equivalent to the prefire() method for a Transition. The method first finds all the input Places of the Transition by calling the method _findBackwardConnectedPlaces(), and sets the temporary marking of the Places equal to the real marking; then it enumerates all the arcs connecting Places to the Transition and decreases the temporaryMarking of the Places reachable from the arc. If after all arcs have been enumerated and the temporaryMarking of all input Places are greater than 0, then the Transition is ready to fire, otherwise it is not ready to fire. The reason that we use a temporaryMarking here is to keep the initialMarking of the places unchanged when we test a Transition is ready or not.

        transition - Transition to be tested to be enabled or not.
        true or false The tested transition is ready to fire or not.
        IllegalActionException - If the method "_getWeightNumber" throws exceptions, which can happen if the arcs are not assigned to some other value other than integers.
      • postfire

        public boolean postfire()
                         throws IllegalActionException
        Return false, indicating that the director does not wish to be scheduled for another iteration. FIXME: This is provisional since there is currently no way to stop the execution of a Petri net model, so we just run once.
        Specified by:
        postfire in interface Executable
        postfire in class Director
        IllegalActionException - Not thrown in this base class.