1. Entities and Objects
The major components in a model are entities, entity types are implemented as Java classes
The active entities have a life of their own, and are called processes
The passive entities are resources and queues
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2. 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)
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3. 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
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4. 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.
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5. A Simple Queue
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6. 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
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7. 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
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8. Single Server System Model
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9. 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.
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10. 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.
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11. UML class diagrams for Car and Wash-machine
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12. UML Class Diagram for the Carwash Model
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13. UML Collaboration Diagram for the Carwash Model
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14. UML Sequence Diagram for the Carwash Model
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15. UML State Diagram for a Car Object
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16. Results of a Simulation Run
The trace, which is a sequence of all relevant events with time of occurrence
The summary results (statistics), with the values of the performance metrics of the model for the current run
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17. 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.
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18. 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.
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19. 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
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20. System Parameters The queue size The resource capacity of the system.
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21. 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.
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22. 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
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23. 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.
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24. 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.
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25. 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
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26. 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.
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27. 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
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28. Probability Distributions in PsimJ
Uniform
Exponential
Poisson
Normal
Erlang
Geometric
Hyper-exponential
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29. 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.
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30. Documentation on PsimJ2
The sample models included in the PsimJ2 Web page
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31. Using PsimJ2 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.
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32. 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.
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33. 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.
See Listing 5.2
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34. 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.
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35. Carwash Simulation Run
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36. Carwash Simulation Run (2)
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