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

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

• 15 April 2005
• David Dugan

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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

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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

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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

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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

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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

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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

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eCGE Components
Component Language GUI Framework Design Methodology OS Supported
Original CGE C MS Visual Studios procedural MS Windows
CSPL parser C N/A command line application object-oriented any
Spring and Tree Algorithms C N/A selected by a menu item procedural any
Enhanced CGE Java AWT / Swing object-oriented any supporting Java 1.4
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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

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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

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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

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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

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CGE Evolution
First Iteration
Second Iteration
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CGE Evolution
Third Iteration
Fourth Iteration
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eCGE Design

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

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Legacy Design Approach
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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

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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).

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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.

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Layout using Random Algorithm Method

• Applying the Random Algorithm results in this
  arrangement.

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Layout using Manual Methods

• Manually laying out the elements provides a more
  readable layout.

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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)

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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

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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

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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/ )

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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

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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

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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
  committees questions.

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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

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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

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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

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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

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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

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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.

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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.

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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

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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)

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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

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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)

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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

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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