Viskit: Rapid Modeling of Event Graph Components

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Viskit Rapid Modeling of Event Graph Components

• Arnold Buss
• MOVES Institute

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Overview

• Introduction
• Event Graph Methodology
• Simkit Java Implementation
• XML Schema for Event Graph Representation
• Viskit, Visual Tool for Event Graph Construction
• XML Schema for DES Model Assemblies

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Introduction

• Barriers to use of simulation models
• Simulation Big and Costly
• Time to develop model
• Lack of rigorous methodology
• Lack of rapid-development tools

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Introduction (cont)

• Breaking the Barriers
• Event Graph Methodology
• Simkit Programming API
• Viskit Graphical Tool
• XML Schema for Component Representations

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Event Graph Methodology

• Pure Discrete Event Simulation (DES)
• Four Elements to DES Model
• Parameters
• State Variables
• Events (State Transition Functions)
• Scheduling Relationships Between Events

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Event Graph Methodology (cont)
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Simple Event Graph
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More Complicated Event Graph
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Event Graph Components

• Based on Listener Pattern
• Loosely Coupled Simulation Components
• Listener Event Graph Objects (LEGO)

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SimEventListener Pattern

• SimEvent Source dispatches its SimEvent to
  registered SimEvent Listeners
• All SimEvents (except Run) are heard by
  Listener
• Used for assembling models from components

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Example Multiple Server Queue
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Property Change Listener Pattern

• PropertyChangeEvents fired at every state
  transition
• Listener may register for specific or all
  PropertyChange Events
• Used for statistics, graphing, etc.

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Property Change Listener Example
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Adapter Pattern

• Used when event name change is desired in
  listener object
• When Event A in source occurs, Event B in
  Listener is triggered

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Adapter Representation
Is represented
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Simkit

• Java API for creating Event Graph models
• Open Source
• Effective for teaching DES
• Used in more than 30 NPS Masters Theses
• Implements Listener Event Graph Object pattern

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Simkit

• Based on solid Event Graph methodology
• Enables reasonably rapid construction of DES
  models
• NPS OR/MV students able to implement models in
  matter of a few months
• Enabler for implementing new features in DES
  models

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Recent NPS Theses using Simkit

• REMUS vehicle for mine detection (Allen, 2004)
• Repair and Sparing of Jet Engines (Schoch, 2003)
• Operational Availability of MMA (Margolis, 2003)
• Submarine Mine avoidance (Nawara, 2003)
• Dynamic Allocation of Fires and Sensors (Havens,
  2002)

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Simkit Relation to Event Graph
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Sample Code Snippet

• public class Server extends SimEntityBase
• . . .
• private RandomVariate serviceTime
• protected int numberInQueue
• protected int numberAvailableServers
• public void doArrival()
• numberInQueue numberInQueue 1
• firePropertyChange("numberInQueue",
  numberInQueue 1, numberInQueue)
• if (getNumberAvailableServers() gt 0)
• waitDelay("StartService", 0.0)
• public void doStartService()
• numberInQueue numberInQueue 1
• firePropertyChange("numberInQueue",
  numberInQueue 1, numberInQueue)
• numberAvailableServers
  numberAvailableServers 1
• firePropertyChange("numberAvailableServers
  ", numberAvailableServers 1,

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Listener Patterns in Simkit

• ArrivalProcess arrival new ArrivalProcess(. .
  .)
• Server server new Server(. . .)
• arrival.addSimEventListener(server)
• PropertyChangeListener numInQueueStat
• new SimpleStatsTimeVarying(numberInQueue)
• server.addPropertyChangeListener(numInQueueStat)
• PropertyChangeListener numAvailableServerStat
• new SimpleStatsTimeVarying(numberAvailableServer
  s)
• server.addPropertyChangeListener(
• numAvailableServerStat, numberAvailableServers)
  

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Viskit Motivation

• Simkit requires Java programming
• Need for non-programmer use
• Need for rapid development tool
• Organize reusable components
• Repository of existing models and components for
  reuse

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XML for Event Graph Components

• Data-centric representation of Event Graph
  Components
• XML is standard, open data format
• Language and Implementation independent
• Enabler for web services

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Event Graph/XML Translation
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Example XML Snippet
ltSimEntity name"SimpleServer" version"0.1"gt
ltParameter type"simkit.random.RandomVariate"
name"serviceTime"/gt ltParameter type"int"
name"totalNumberServers"/gt ltStateVariable
name"numberInQueue" type"int"/gt
ltStateVariable name"numberAvailableServers"
type"int"/gt ltEvent name"Run"gt
ltStateTransition state"numberInQueue"gt
ltAssignment value"0"/gt
lt/StateTransitiongt ltStateTransition
state"numberAvailableServers"gt
ltAssignment value"totalNumberServers"/gt
lt/StateTransitiongt lt/Eventgt . .
. lt/SimEntitygt
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XML Snippet (cont)
ltEvent name"Arrival"gt ltStateTransition
state"numberInQueue"gt ltAssignment
value"numberInQueue 1"/gt
lt/StateTransitiongt ltSchedule
condition"numberAvailableServers gt 0"
priority"1.0" delay"0.0" event"StartService"/
gt lt/Eventgt ltEvent name"StartService"gt
ltStateTransition state"numberInQueue"gt
ltAssignment value"numberInQueue - 1"/gt
lt/StateTransitiongt ltStateTransition
state"numberAvailableServers"gt
ltAssignment value"numberAvailableServers - 1"/gt
lt/StateTransitiongt ltSchedule
delay"serviceTime.generate()"
event"EndService"/gt lt/Eventgt
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Code Generator

• Generates Simkit Java code from XML document
• SimEntity XML document -gt SimEntityBase subclass
• Assembly XML document -gt instances of SimEntities
  and Listeners

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Viskit Event Graph Editor

• Even more rapid development of Event Graph
  Components
• Exposes Event Graph structure better than code
• Modeler draws Event Graph and fills in
  information
• Open Source

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Viskit Event Graph Editor
Scheduling/Canceling Edge
Create New Event
Self-Scheduling Edge
State Variables
Drawing Canvas
Simulation Parameters
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Server Component in Viskit
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Viskit Assembly Editor

• Assembly consists of
• Instances of SimEntities and PropertyChangeListene
  rs
• Listening connections

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Viskit Assembly Editor
Event Graph Components
Listener Connection Toolbar
New Instance Wizard
Assembly Canvas
PropertyChangeListeners
Execution Controls
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Example Viskit Assembly
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Summary

• Rapid DES modeling is possible
• Event Graph Methodology robust
• Simkit implementation mature, stable
• Viskit application accelerates development time
• Basic features and design of Viskit complete

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Ongoing Work and Improvements

• Improve user-friendliness of Viskit
• Add Experimental Design and execution features
• Execution on computer clusters
• Documentation, Help and Tutorial
• Incorporate features based on user feedback and
  testing

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Questions