Introduction to Model Conceptualization and Design

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COMP8700 Agent-Directed Simulation
Introduction to Model Conceptualization and
Design
Dr. Levent Yilmaz MSNet Auburn MS
Laboratory Computer Science Software
Engineering Auburn University, Auburn, AL
36849 http//www.eng.auburn.edu/yilmaz
Acknowledgements Thanks to Dr. Bernard Zeigler
and Dr. Gabriel Wainer for sharing their DEVS
lecture materials
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Aim

• The aim of this lecture is to overview the
  conventional conceptual and design models as well
  as fundamentals, principles, and the conceptual
  framework underlying the DEVS formalism.

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Modeling system dynamics

• Interested in modeling systems dynamic behavior
  ¾ how it organizes itself over time in response
  to imposed conditions and stimuli.
• Predict how a system will react to external
  inputs and proposed structural changes.

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Modeling techniques classification

• Conceptual Modeling informal model.
• Communicates the basic nature of the process
• Provides a vocabulary for the system (ambiguous)
• General description of the system to be modeled

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Domain Modeling

• Partitions and illustrates the important domain
  concepts.
• A classic object-oriented analysis activity.
• What are the objects of interest in the this
  domain?
• their attributes?
• their relationships?
• IMPORTANT Not software objects, but a visual
  dictionary of domain concepts.

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A Domain Model Does Not Represent Software
Objects

• A model of domain concepts, not of software
  objects.
• A visual dictionary of important words in the
  domain.
• Uses UML static structure diagram notation.

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How to Make a Domain Model

• List the candidate conceptual classes using the
  Conceptual Class category list
• Draw them in a domain model
• Add the associations necessary to record
  relations
• Add the necessary attributes to fulfill the
  information requirements.

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Partitioning the Domain Model
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Finding Domain Concepts

• Candidate lists (Conceptual category list
  textbook page 134)

• Linguistic Analysis Identify the nouns and noun
  phrases in textual descriptions.
• Care must be applied with this method a
  mechanical noun-to-class mapping isnt possible,
  and words in natural languages are ambiguous.
• Specification Design a library catalog system.
  The system must support the registration of
  patrons, checking books in and out patrons,
  adding and removing of books, and determining
  which patron has a book.

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Approaches

• Abbott and Booch suggest
• Use nouns, pronouns, noun phrases to identify
  objects and classes
• Singular ? object, plural ? class
• Not all nouns are really going to relate to
  objects
• Coad and Yourdon suggest
• Identify individual or group things in the
  system or problem
• Ross suggest
• People, places, things, organizations, concepts,
  events
• Danger signs class name is a verb, is described
  as performing something

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Associations
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Multiplicity
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Focus on Important Associations

• Use Common associations list (page156 of your
  textbook)
• Name an association based on TypeName VerbName
  TypeName format

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Domain Model Adding Attributes

• An attribute is a logical data value of an
  object.
• The values of attributes of an object at any time
  during run-time execution constitutes the state
  of that object.
• UML Attribute Notation
• Relate with associations, NOT attributes

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Attributes

• Show only simple relatively primitive types as
  attributes.
• Connections to other concepts are to be
  represented as associations, not attributes.

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Do Not Use Attributes To Relate Concepts

• Why not?

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Formal Modeling

• Advantage of Formal Methods
• Correctness and completeness ? Testing
• Communication means ? Teamwork
• Formalism
• Communication convention
• Formal specification in unambiguous manner
• Abstraction (representation) Manipulation of
  abstraction
• Formal model - Formal specification

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Declarative models

• System states (representing system entities)
• Transitions between states
• State-based declarative models
• Example States number of persons waiting in
  line
• Transitions arrival of new customers/departure
  of serviced ones

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Declarative models (cont.)

• Event-based declarative models
• Arcs represent scheduling.
• Event relation from arrival of token i to
  departure of token i.

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Functional models

• Black box.
• Input signal defined over time
• Output depending on the internal function.
• Timing delays discrete or continuous
• Example inputs customers arriving
• Outputs delayed output of the input customers

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Spatial models

• Space notions included
• Relationship between time and space positions
• Example customers moving through the server.

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THE DEVS FORMALISM DEVS Discrete Event System
Specification
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Basic Entities and Relations in Modeling and
Simulation
Experimental Frame
Source
Simulator
System
behavior database
Modeling Relation
Simulation Relation
Model
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DEVS Modeling Simulation Framework

• DEVS Discrete Event System Specification
• Provides sound formal MS framework
• Supports full range of dynamic system
  representation capability
• Supports hierarchical, modular model development

(Zeigler, 1976/84/90/00)
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The DEVS Framework for MS

• Separates Modeling from Simulation
• Derived from Generic Dynamic Systems Formalism
• Includes Continuous and Discrete Time Systems
• Provides Well Defined Coupling of Components
• Supports
• Hierarchical Construction
• Stand Alone Testing
• Repository Reuse
• Enables Provably Correct, Efficient, Event-Based,
  Distributed Simulation

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Formalism transformation
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DEVS Formalism

• Discrete-Event formalism time advances using a
  continuous time base.
• Basic models that can be coupled to build complex
  simulations.
• Abstract simulation mechanism

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Atomic model definition

• Behavioral models

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DEVS Atomic models

• Atomic DEVS lt S, X, Y, ??int ,??ext , ?, ta gt
• X external input event set
• Y external output event set
• S sequential state set
• ??int internal transition function
• ?ext external transition function
• ? output function
• ta time advance function

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DEVS Atomic models (cont.)

• ta S ? R0,??
• Q (s,e) s ??S, 0 ? e ? ta(s) total state
  set, e elapsed time
• ??int S ? S
• ??ext X Q ? X
• ? S ? Y

?
S
??int
Y
X
R
??ext
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Atomic model Discrete Event Dynamics
External Event Transition Function (?ext)
transforms state and an input event into another
state (e.g., receiving a faulty device, put it
into a queue to await its turn for repair.)
Internal Event Transition Function (?int)
transforms state into another state after time
has elapsed (e.g., there are 10 parts available
and broken part requires 7 of them, after
fixing broken part, 3 parts will remain.)
Time Advance Function (ta) maps a state into a
duration (e.g., how long to fix a device
once processing has started.)
Output Function (?) maps a state into an output
(e.g., number of parts available falls below a
minimum number, issue an order to restock.)
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DEVS atomic models semantics
DEVS lt X, S, Y, dint , dext , ta, l gt
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Atomic model example ping-pong

• AMplayer_A lt S, X, Y, ??int ,??ext , ?, ta gt
• X Ball_B
• Y Ball_A
• S A, D
• ?int (A) D
• ?ext (Ball_B, D) A
• ta(A) thinking_time
• ta(D) INFINITY
• ?(A) Ball_A

Ball_A
A
D
Ball_B
Ball_B
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Dynamic behavior of DEVS models
M
out
in
event
x1
y1
x2
t
S
s2?ext((s0,e),x1)
s2
s1?int(x2)
s1
s0
t
e
ta(s2)
ta(s0)
ta(s1)
t
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Atomic model example Processing Server
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Coupled models

• Structural models (multicomponent)

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Hierarchical vs. Incremental modelling
GEN-BUF-PROC
BUF-PROC
out
in
out
in
out
out
GEN
BUF
PROC
done
A
B
C
Incremental A and B connect
ABC
BC

• Petri Net incremental
• DEVS hierarchical

A
B
C
Hierarchical A and BC connect
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Coupled models formal specification

• CM lt X, Y, D, Mi, Ii, Zij, select gt
• X is the set of input events
• Y is the set of output events
• D is an index for the components of the coupled
  model, and
• " i Î D, Mi is a basic DEVS model (that is, an
  atomic or coupled model), defined by
• Mi lt Ii, Xi, Si, Yi, dinti, dexti, tai gt
• Ii is the set of influencees of model i (that is,
  the models that can be influenced by outputs of
  model i), and " j Î Ii, Zij is the i to j
  translation function.
• Finally, select is the tie-breaking selector.

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Coupled DEVS example
lt GEN-BUF-PROC model gt
out
in
out
in
out
out
GEN
BUF
PROC
done

• GEN-BUF-PROC lt X, Y, GEN, BUF, PROC, Ii,
  Zij, SELECT gt
• X ? Y out
• I(GEN) BUF
• I(BUF) PROC
• I(PROC) BUF, self
• Z(GEN)BUF Z(BUF)PROC
• Z(PROC) BUF Z(PROC)self.
• SELECT (GEN, BUF, PROC) GEN (BUF,
  PROC) BUF

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Closure Under Coupling
DN lt X , Y, D, Mi , Ii , Zi,j gt
Every DEVS coupled model has a DEVS Basic
equivalent
DEVS lt X, S, Y, dint, dext, dcon, ta, l gt
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Input/output ports concepts

• Components (D)
• couplings
• Internal Couplings (IC)
• External Input Couplings (EIC)
• External Output Couplings (EOC)

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Coupled DEVS example
lt GEN-BUF-PROC model gt
out
in
out
in
out
out
GEN
BUF
PROC
done

• GEN-BUF-PROC lt X, Y, GEN, BUF, PROC, EIC,
  EOC, IC, SELECT gt
• X ?
• Y out
• EIC ?
• EOC (PROC.out, GEN_BUF_PROC.out)
• IC (GEN.out, BUF.in), (BUF.out, PROC.in),
  (PROC.out, BUF.done)
• SELECT (GEN, BUF, PROC) GEN (BUF,
  PROC) BUF
  ?

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DEVS Simulation Protocol
Coupled Model
coordinator
simulator tN
simulator tN
simulator tN
Component tN tL
Component tN tL
Component tN tL
After each transition tN t ta(), tL t
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DEVS Simulator Protocol
tL 0 tN ta()
simulation cycle
initialize
tL tN tN tN ta()
tL time of last event tN time of next event