(C) 2009 J. M. Garrido1 Object Oriented Simulation with Java

(C) 2009 J. M. Garrido 1

Object Oriented Simulation with Java


Entities and Objects

The major components in a model are entities or objects, entity types are implemented as Java classes

The active objects have a life of their own, and are called processes

The passive objects are the other objects in a model: user-defined objects, resources and queues, etc.


Objects in a Simulation Model

There are several objects in a simulation model

The activate objects are instances of the classes that inherit the library class Process

The passive objects are instances of any other class, including some of the library classes (e.g., queue classes)


Single-Server Models

There is only one server object that provides service to customer objects

Arriving customer objects join a queue to wait for service

The random events are customer arrivals and service completions


Simple Queue A waiting line for arriving customers (or

other objects). It is a passive entity in the model, used as ‘global’ shared resource.

An arriving customer can only enter the queue at the tail

A customer can only leave if it is at the head of the queue

The usual order of customers is FCFS.


A Simple Queue


The Car-Wash Model

Cars arrive for complete wash (service) Arriving cars join a line (queue) to wait for

service There is only one car-wash machine that can

service one car at a time After the service is complete for a car, it

leaves the system


The Carwash Model Components A set of customer processes, the cars One server process, the wash-machine One conceptual process that models

arrivals (this represents the environment) One global shared resource, the queue. Events:

customer arrival start of service end of service and departure of a customer


Single Server System Model


The Conceptual Model

The model of the simple carwash system consists of the following processes:

The Car process, which define the customer objects

The Arrivals process, which represent the environment

The Wash-machine process, which represents the server.


Modeling Diagrams

The UML diagram shows the main entities in the model

The UML static modeling diagrams show the basic structure of the classes and their relationships

The dynamic modeling diagrams show all the process interactions and the use of resources for every model.


UML class diagrams for Car and Wash-machine


UML Class Diagram for the Carwash Model


UML Collaboration Diagram for the Carwash Model


UML Sequence Diagram for the Carwash Model


UML State Diagram for a Car Object


Results of a Simulation Run

The trace, which is a sequence of all relevant events with time of occurrence

The summary statistics with the values of the performance metrics of the model for the current run


Performance Measures in the Carwash System

The average number of customer in the system The average number of customers in the queue(s)

(i.e., that are waiting) The average time that a customer spends in the

system The average time that a customer spends in the

queue(s) The server (wash-machine) utilization.


Performance and Workload Characterization

The usual objective for determining the performance measures in a queuing system is to achieve the following criteria:

Reduce the customer waiting periods Improve the server utilization Maximize throughput (the number of

customers served) for a given workload.


Workload Parameters

The performance of the system depends on the workload submitted, and for this queuing model it consists of the following parameters: The average customer arrival rate, The average customer service rate, The resource demand for the customers


System Parameters

The queue sizeThe resource capacity of the system.


Performance Measures Depend on Workload

The performance metrics computed in the single-server model, depend on the workload submitted, and on the system parameters.

Modifying the workload on a model and/or the system parameters changes the behavior of the model.


Examples of Performance and Workload

To compare two models with different servers, is equivalent to changing the workload by providing two different values for the average service time demands of the customers.

If the system workload increases, the server utilization will also increase


Bottleneck

The bottleneck of the system at capacity will be localized in the server or resource with a utilization of 1, while the other servers or resources each have utilization significantly below 1.

The bottleneck can be localized at the server, the queue, or at the resources.


Studying System Behavior

All the changes that occur in the system are analyzed

In the model of the simple car-wash system, the system changes state when a customer arrives, when a customer starts service, when a customer completes service, etc.

These changes are instantaneous occurrences and are called events.


The Car-wash Stochastic Model

These models include entities with attributes that change value in a non-deterministic manner

The occurrence of the random events follow a probabilistic distribution


Random Variables in the Model

In the Car-wash model, the following random variables are defined:

The inter-arrival period for the car objects The service period for each car object.


Generation of Random Variables

To facilitate the simulation of random events, PsimJ provides random number generators using several probability distributions

Some of the distributions return an integer (long) value, others return a real (double) value


Probability Distributions in PsimJ

Uniform Exponential Poisson Normal Erlang Geometric Hyper-exponential


Random Events in the Carwash Model

Two random events: customer arrivals end of a customer service

Two random variables are modeled: inter-arrival periods service periods

Both random variables follow an exponential distribution.


Documentation of PsimJ2

The sample models Reference manual Using PsimJ2 with Eclipse

The Psim Web page

http://science.kennesaw.edu/~jgarrido/psim.html


Using Psim with Java

All processes (active objects) are created from a user-defined class that inherits the library class Process.

All non-process objects (passive objects) are created directly from the corresponding library class (e.g., class Squeue), or other classes.


The Structure of a Model in Java

Define all classes for the model’s processes, and inherit from the library class Process.

Define one class (main class) that includes method main.

In method Main_body of the main class, start the simulation by invoking method start_sim, then calculate summary statistics.


Method main

In method main: Define a simulation object of the library class Simulation; give a name to the model.

Create all the objects used in the model. Start the active objects.


Using GUI with Models

Java facilitates the implementation of graphical interfaces for simulation models

A general GUI framework is included with PsimJ2. The package name is gui.

To run the carwash model with GUI, run the CarwashGUI class, instead of the Carwash class.


Carwash Simulation Run


Carwash Simulation Run (2)

Discrete-Event Simulation

Event approach Activity approach Process interaction approach

Psim3, and PsimJ2 use the full power of process interaction simulation.

Other simulation packages/languages have limited OO support

38(C) 2009 J. M. Garrido

The Process Interaction Approach

A simulation model using this approach consists of: A definition of the active and passive

entities in the model. A description of how different active

entities are going to interact among themselves, as time passes.

A simulation run consists of creating and starting the passive and active objects.


Advantages of the Process Style of Simulation

Compatible with the Object-Oriented approach to modeling and programming, every process is an active object.

Suitable for modeling large and complex systems.

Java threads are used to implement the active objects in the simulation models.


Process Interaction Approach to Simulation with PsimJ2

A portable library package of Java classes Object-oriented Supports the process interaction approach A good tool for courses, such as:

Object-oriented Simulation Object-oriented design and programming Operating systems Introduction to real-time systems


Applying the Process Interaction Approach to Simulation

The major components in a model are entities, these are modeled as active objects

The active entities have a life of their own, and are called processes

The passive entities are resources and queues The UML Collaboration/Communication

diagram shows the process interactions


Developing Simulation Models of Real-Time Systems

The simulation models are more complex because they need to satisfy the requirements of real-time systems.

A set of tasks are identified in developing such systems.

Identifying these tasks will simplify the

analysis and design phases of building a simulation model of real-time systems.


Developing Models of RTS

The following tasks are defined: Concurrency – Multiple simultaneous activities. Synchronous Communication – Two or more

communicate, where the sender will wait until the receiver gets the message.

Timing Constraints – Describe the time windows during which activities must occur or respond.

Communications Deadlines – Timers are used to place deadlines on how long a process can wait to communicate with another process.


Train-Gate System Example

Train-Gate System consists of a one-directional railway track crossing a road.

The gate at the crossing must be lowered or raised by a Controller system.

Sensors are used to detect entering trains and leaving trains.

The signals from the sensors are sent to a Monitor, which keeps track of the number of trains in the area.


Case Study:Train-Gate System

Train Entry Sensor Train Exit Sensor

MonitorController Gate

Open

CloseOpen/Close Signals

Gate Opened / Closed

Discrete Interarrival Times of Trains

This simulation model is available at:http:cience.kennesaw.edu/~jgarrido/train


Train-Gate System Design and Implementation

The real-time behavior of the Train-Gate System is modeled using Communicating Real-Time State Machines (CRSM). The timing constraints in CRSM are specified in the transitions between states, as shown in the diagram. The system design includes the common set of tasks identified in previous slides. Simulation of the Train-Gate system has been implemented using Psim-J, a Java based discrete-event simulation package. The implementation of the system defines the processes: Controller, Gate, Train, Monitor, and Sensors.


Developing Simulation Models

Two IDEs are currently used to edit, compile and execute programs that implement simulation models:

Eclipse jGRASP Any other you choose

The main requirement is that you have to set Psimj2 (psimj2.jar) as an external library


Summary

Real-time systems are complex; modeling with CRSM and constructing an OO simulation model enhances the understanding of these systems

Concurrency and synchronization can be analyzed more completely by studying the dynamic behavior of a RTS using simulation

Performance of a RTS can be analyzed using a simulation model


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(C) 2009 J. M. Garrido1 Object Oriented Simulation with Java