eCGE (Enhanced Petri Net Graphical Editor) : A Multi-Platform Petri Net Editor

1. eCGE (Enhanced PetriNet Graphical Editor) : A Multi-Platform Petri Net Editor 15 April 2005 David Dugan

2. Overview • Role of Petri Nets in software development and testing • How CGE implements Stochastic Petri Nets • History of CGE and how eCGE enhances capability • Design of eCGE • Test Cases • Future plans and conclusion

3. Verification of design requirements • Through formal testing or simulation of the environment • Very costly and time consuming • Difficult to go thru all possible execution paths • Difficult to simulate timing problems • Does not catch problems until well into coding phase • Performance modeling though a special graphical language formalism • Temporal specifications for time-critical systems • Probabilistic specifications to describe selection among different possible events • Performance modeling through graphics

4. Petri Nets - Graphical Modeling of Critical Timing Applications • Flow charts, block diagrams, state diagrams do not show timing or probability of events • Firing time in Petri Nets is the delay time before a transition can take place • Immediate (Deterministic) – Fixed delay time • Timed (Stochastic) – Random or asynchronous time

5. eCGE Benefits • eCGE provides a graphical means to specify a Stochastic Petri Net Model to model the dynamic as well as static aspects • Output of eCGE can be input into the Stochastic Petri Net Package to obtain numerical results for the model using Markov analysis

6. eCGE Goals • Model a system quickly and inexpensively • Produce a tool that does not require extensive training • Be able to use the model to communicate to customers

7. CGE Background • Original work by Norb Gravelle proof of concept that CSPL coding language can be generated from a graphical interface instead of a text editor • Work by Wen Wei extended this tool to include graphical layout algorithms • Spring Algorithm • Tree Algorithm

8. eCGE Components

9. My Contribution • Combined work by Gravelle, Wei, and CSPL parser into a single application • Added parser to import CSPL files • Designed the application using object-oriented analysis and design techniques for future enhancements • Designed an interface for future development of graph layout algorithms • Added a Random graph layout algorithm

10. CSPL (C Based Stochastic Petri-net language) • Specification language for Stochastic Petri Net package - CSPL - can be used to specify complex system behaviors with Petri nets • CSPL Conversion Process • Reads in CSPL file to determine elements and attributes of model • CGE Language adds graphical information for each model element

11. Enhancements to CGE I • Ability to read in a CSPL file and display it graphically • Edit and display an imported CSPL file • Parameters for a model are input through dialog boxes • Specify the properties of a place, transition, or arc • Interface for defining guard function • Read and save a model in CGE format

12. Enhancements to CGE II • Consistency checking of parameters insures a syntactically correct model • Improved maintainability of design through complete redesign of code based on object oriented design • Rewrote all code in JAVA to enhance portability to other platforms • Added a Random Graph Layout Algorithm

13. CGE Evolution First Iteration Second Iteration

14. CGE Evolution Third Iteration Fourth Iteration

15. eCGE Design • Redesign and integration of legacy CGE components into eCGE • Gravelle’s Original CGE • Wen Wei Graphical Layout algorithms • CSPL parser • Used Object Oriented Design to integrate these components into a single application

16. Legacy Design Approach

17. eCGE Classes • High-Level Class Chart for eCGE • An appendix in the thesis describes the design and implementation details • Class charts • Design and implementation details of key components

18. Case Studies • Case # 1 - Connected Cyclic Reliability (CCR) model for the anti-lock braking system • Analyzed the mean time to failure for the braking system of a vehicle • Case # 2 - How the layout affects understanding and finding problems in a Petri net model (race condition).

19. Case Study # 1 – CCR Model • Model input from CSPL language and translated into CGL language • Layout (Spring and Tree) algorithms proved inadequate to handle the reformatting of graphical layout of a complex model • eCGE requires further enhancements to resolve the layout of complex models, as shown in the following figures.

20. Layout using Random Algorithm Method • Applying the Random Algorithm results in this arrangement.

21. Layout using Manual Methods • Manually laying out the elements provides a more readable layout.

22. Case Study 2 • The use of layout is important in understanding the structure of a model and detecting potential problems • Types of possible problems: • Conflict • Confusion (analogous to a race condition)

23. Example of how a race condition “conflict” can be in Petri Net • p = places, t = transitions, 1 = tokens • Figures 3a and 3b represent confusion as to which path program will take based on which token fires first

24. CSPL Code for prior example of conflict (race condition) • The following segment shows the equivalent CSPL code for the previous example • The textual version shows the elements, but not the graphical structure, of the model

25. Future Enhancements of eCGE • Enhance graph layout algorithms to handle more complex structures • Be able to import and translate other Petri Net modeling tools file formats • Add a scroll bar to document window to increase work space • Be able to group elements to move them • Incorporate an algorithm to minimize arc crossings (such as the various graphviz layout algorithms, see http://www.graphviz.org/ )

26. Conclusions I • eCGE has integrated a number of separate programs which represent a foundation for using Stochastic Petri Nets in industry • Mechanizing and simplifying the development of a model by using a graphical, rather than textual, interface • Being able to visualize time critical events via the Petri net diagrammatic language using eCGE facilities and layout algorithms • Including visualization of a large library of legacy (textually based) CSPL models

27. Conclusions II • Increased maintainability of the code using object-oriented design makes it easier to further enhance the application • Made programs/components of CGE platform independent through use of JAVA to encourage usage on a variety of platforms

28. Questions & Answer Session Mr. Dugan has a severe speaking disability (WSU has certified) and for the purposes of the defense we asked him to write the answers to the committee’s questions.

29. Ordinary Petri Net • A biparte graph that consists of • A set of Places • A set of Transitions • A set of directed Arcs • Arcs connect Places to Transitions • Input Arc - Directed Arc from Place to Transition • Output Arc - Directed Arc from Transition to Place

30. Ordinary Petri Net Formal Definition • PN = (P,T,A) • P= {p1, p2,….p(n)} • T= {t1,t2,….t(m)} • A = {a1, a2, …, a(o)} - arcs

31. Generalized Stochastic Petri Net (GSPN) • Time is associated with Transition • Transitions are immediate and timed • Immediate - Fire immediately when enabled and the logical structure is modeled, but not the timing • Timed -Fire after a random,exponentially distributed enabling time • Example of timed - A finite time delay until a hardware device is ready to accept the command before issuing it • Timed - Used with devices that cannot respond at the program execution rate • Example of Immediate - Usual case in programming where transitions are made at the same rate as program execution

32. Markings • Tangible - Only timed transitions are enabled in a marking • Vanishing - At least one immediate transition is enabled in a marking. May be removed in the analysis to reduce time and space complexity • Absorbing • Applies to a place which has no output Arcs • Transition firing adds a token to this place • However, the token can no longer be used to enable a transition

33. Consistency Checking • Drawing Window • Shows an error message for an illegal operation • For example, conecting a place to a place • Dialogue Boxes • Grays out boxes or input fields(allows operator no input) if operation is illegal or undefined • Example - Transition Dialogue Box cannot have a Guard Function if no functions have been defined

34. Spring Algorithm • Places and transitions are like weights on springs • Starts from the first arc • Simulates a mechanical system consisting of springs (arcs) and nodes (places and transitions). From the initial configuration or ring positions, the system oscillates until it stabilizes at a minimum-energy configuration. It has been noted that in such a configuration, all the edges typically have relatively uniform length and nodes not connected tend to be far apart.

35. Tree Algorithm • The goal of this algorithm is to produce a graph that is planar, straight-lined, and upward (that is, a ‘parent’ node is above its ‘children’). Vertices at the same level are horizontally aligned. The first place in the place list is chosen to be the root of the tree. • The space (vertical distance) between each level is uniform. • The separation distance between two consecutive vertices on the same level is kept to a minimum. • The overall width of the graph is as small as possible and still be readable.

36. Other ways of laying out graphs • Reference Article: Empirical Layout of Aesthetics-based Graph Layout by Purchase, Carrington, and Allder • Planar Grid Drawing Algorithm • Force Directed Algorithm • Goals of Graph layout program • Minimize edge crossings • Orthogonality • Information Flow (connected nodes should be close together) • Minimize edge bends

37. Weaknesses of Graphical Approaches • Reference Article: Empirical Layout of Aesthetics-based Graph Layout by Purchase, Carrington, and Allder • No one type of layout works for every graph • Graph layout requires a significant computational power • History of the flow chart • How to show a large graph in a drawing window: • Add scroll bars • Re-size the model (zoom in and out)

38. Understanding • A graphical representation of a Petri Net tends to be at a higher level of abstraction than a textual representation • affects response time • Easier to see errors • Shows the structure of the model

39. Extensibility • Interface for graph layout algorihm described in section A.3 of thesis • Adding a graph layout algorithms requires the following steps: • Create a new subroutine containing the implementation (class document_class) • Add a new menu item in the Algorithms menu bar (class algorithms_menu) • Add code so that when the menu item is selected, the correct subroutine is called (affects classes document_manager, document_internal_frame, and document_panel)

40. Ease of Use • Goal is to minimize the amount of effort required to use the tool such as • Minimizing keystrokes • Logically organizing the layout of dialog boxes

41. OO Design Maintainability • The system architecture uses fine grained, self contained components that can readily be changed • Avoids shared data structures and global variables (minimizes data coupling) • Each class is self-contained - all relevant operations and data are contained within the class