Parallel and Distributed Simulation

Object-Oriented Simulation

Outline

• Process-Oriented Simulation (quick review)• Generalized Simulation Architecture (review)• Object-oriented simulation

– Object-oriented vs. Object-based– Is-A and Has-A class hierarchies– Object Interactions

• Additional simulation concepts– Indirect event exchange via group communication– Event retraction

Event vs. Process Oriented Views

State variables

Integer: InTheAir;Integer: OnTheGround;Boolean: RunwayFree;

Event oriented view

Entities modeled by event handlers

Arrival Event

{

…

}

Landed Event

{

…

}

Departure Event

{

…

}

State variables

Integer: InTheAir;Integer: OnTheGround;Boolean: RunwayFree;

Process oriented view

Entities modeled by processes

Aircraft 1

{

ArriveLandDepart

}

Aircraft 2

{

ArriveLandDepart

}

Aircraft n

{

ArriveLandDepart

}

Simulation PrimitivesPrimitives needed to advance simulation time• AdvanceTime(T): advance T units of simulation time

– Also called “hold”– E.g.: AdvanceTime(R) to model using runway R units of

simulation time

• WaitUntil(p): simulation time advances until predicate p becomes true– Predicate based on simulation variables that can be modified by

other simulation processes– E.g.: WaitUntil(RunwayFree) to wait until runway becomes

available for landing

• Other combinations– WaitUntil(p,T): Wait up to T units of simulation time for predicate p

to become true– Not used in the air traffic example

Execution Example

OnTheGround

Simulation Time

StateVariables

RunwayFree

InTheAir

0 1 2 3 4 5 6 7 8 9 10 11

true

0

0

R=3G=4

false

1 2 1

1

0

2

true

1 0

Flight 11 InTheAir := InTheAir+1;

2 WaitUntil (RunwayFree);

3 RunwayFree := FALSE;

4 AdvanceTime(R);

5 RunwayFree := TRUE;

6 InTheAir := InTheAir-1;

7 OnTheGround:=OnTheGround+1;

8 AdvanceTime(G);

9 OnTheGround:=OnTheGround-1;

Flight 21 InTheAir := InTheAir+1;

2 WaitUntil (RunwayFree);

3 RunwayFree := FALSE;

4 AdvanceTime(R);

5 RunwayFree := TRUE;

6 InTheAir := InTheAir-1;

7 OnTheGround:=OnTheGround+1;

8 AdvanceTime(G);

9 OnTheGround:=OnTheGround-1;

Process-oriented Simulation• Is usually built over event oriented simulation

mechanisms (event list, event processing loop)• typically simplifies model development and modification• requires threading (e.g., co-routine) mechanism• Incurs additional complexity and computation overhead to

suspend and resume simulation processes

Overall Simulation Architecture

Network

SMP 1 SMP m

Processor 1 Processor n

Simulator 1 Simulator n

LP 1 LP p

Object 1 Object i

State variables

Process1

Processs

EventHandlers Methods

LP = Logical ProcessSMP=Symmetric Multi-Processor

Simulation Application• state variables• code modeling system behavior• I/O and user interface software

Simulation Executive• event list management• managing advances in simulation time

calls toscheduleevents

calls to eventhandlers

Discrete Event Simulation System

model of the physical system

independent

of the simulation

application

Object Oriented Simulation• Models defined in terms of objects and interactions.• Object-orientation includes:

– Encapsulation• Attributes of an object are entirely contained within the object

scope• E.g. Fuel attribute & land() method of an Aircraft object

– Inheritance• Attributes of an object are copied into inheriting class• E.g. Fuel attribute & land() method of an Aircraft object are

copied into a Boeing object that inherits from Aircraft

– Polymorphism• Method bodies can be changed in inheriting class• The correct, class-specific method gets invoked at runtime• E.g. The land() method of an Aircraft object is changed in Boeing class to model Boeing-specific landing behavior

Object-Oriented Vs. Object-Based

• Object-oriented includes:– Encapsulation– Inheritance– Polymorphism

• Object-based includes:– Encapsulation– Limited inheritance– No polymorphism

Simulation Class Hierarchies

• Objects organized in class hierarchies• Both the simulator as well as the application can be

object-oriented• Simulator and application share the hierarchies

– Event hierarchy– Process hierarchy– Is-A and Has-A hierarchies

Event: Is-A HierarchyEvent

InternalEvent

ROEvent

TSOEvent

AppEvent

RO = Receive-orderedTSO = Timestamp-ordered

AppEvent

… TimerEvent

RetractEvent

… Add/DelEvent

App = ApplicationAdd/Del = Add or Delete a ProcessResume = Resume a suspended process

ResumeEvent

Commonly used in real-time simulations

Focus on this in rest of the class

Logical Process: Is-A HierarchyLogical Process

Event-OrientedProcess

ThreadedProcess

…

Event-oriented Process: Object with simple event handlerPeriodic Process: Object with a simple, periodically repeating actionThreaded Process: Object with full process-oriented behavior using a stack

PeriodicProcess

… …

Simulator: Has-A HierarchySimulator

Process ListGroup

List

LogicalProcesses

Process List: Set of currently active logical processesLogical Processes: Mixture of event-oriented, periodic and process-oriented processesPending Event List: Priority queue of unprocessed eventsTSO Events: Timestamp-ordered eventsGroup List: Table of “event interest” groupsSubscribed Groups: Event interest groups some processes are subscribed toPublishing Groups: Event interest groups some processes publish to

PendingEvent List

TSOEvents

SubscribedGroups

PublishingGroups

Clock

Object Interactions

Object interaction modeled with– Direct method invocation– Direct event exchange– Indirect event exchange

Object Interaction – Method InvocationInteraction via the usual method invocation:• One object invokes another object’s method by

reference, synchronously• Simulation time remains constant during method

execution• The source object’s method could be an event

handler or a process• Typically, the destination object’s method is not a

processObject A:Method(){ … B.Method() …}

Object B:Method(){ … …}

Object Interaction – Direct EventsInteraction via direct event exchange:• One object invokes another object’s method by

sending it an event• Simulation time can advance before interaction• The source specifies the destination object by

reference/identifier• Either of source and destination methods could be an

event handler or a process

Object A:Method(){ … Send(E) …}

Object B:Method(){ … …}

Event Eat time T

Object Interaction – Indirect EventsInteraction via indirect event exchange:• One object invokes another object’s method by

“posting” an event to an “interest group”• Simulation executive routes the event to a set of zero

or more objects

Object A:Method(){ … Send(G,E) …}

Object C:Method(){ … …}

Event Eat time T

Object B:Method(){ … …}

Group G

Interest Group Communication• Events exchanged via a “group” intermediary• Analogous to Internet newsgroups (bulletin boards)• Objects send events to a specific group instead of

to a specific object• Simulator executive forwards event to “interested”

objects• Example: State updates in Virtual Environments

– Interaction among geographically distributed entities– Entities periodically send their position updates– Updates are sent to groups corresponding to

geographical grid regions– Only entities subscribed to those regions receive the

update events

Interest Group CommunicationPrimitives:

– Create(G)• Analogous to creation of a newsgroup G

– Subscribe(G)• Analogous to subscribing to a newsgroup G

– Publish(G)• Analogous to obtaining permission to post to newsgroup G

– Update(G,E)• Analogous to posting a message E to newsgroup G

– Reflect(G,E)• Analogous to receiving/reading a message E on newsgroup G

Group Communication Advantages

• Loose coupling among objects– Leads to easier model development and maintenance– Aids in easy extensibility

• Simulation executive can perform filtering– Leads to better performance due to reduced

communication, compared to broadcast

Event Retraction• Retract previously scheduled events

– “Cancel”

• Air traffic example– Airport closing because of bad weather– Schedule a “closing” event at an airport with a time

stamp indicating when the airport closes

• Event retraction mechanism– “Un-schedule” departure event– Reschedule departure events based on re-opening

time of airport

• Or ignore the invalid departure events without event retraction mechanism

• What about arrival events?

Summary

• Object-oriented simulation– Object-oriented vs. Object-Based– Is-A and Has-A class hierarchies– Object Interactions

• Additional simulation concepts– Indirect event exchange via group communication– Event retraction