DESpy 0.1.15

Support for DES Modeling using Event Graphs

DESpy

DESpy is a Python implementation of Discrete Event Simulation (DES) methodology based on Schruben's Event Graphs (see Simulation Modeling with Event Graphs). It also suports the LEGO component framework (see Building Complex Models with LEGOs and Component Based Simulation Modeling with Simkit).

Installation

• Python 3.7 or greater installed
• pip install DESpy

Running Example Scenarios

The sample components are in the examples package, with the sample scenarios in the examples.run package

• SimpleServer
• FiniteCapacityServer
• EntityServer
• ServerWithReneges
• TransferLine
• Confidence Interval Example: terminating case
• Confidence Interval Example: steady-state case

Defining an Event Graph Component

Each Event Graph component corresponds to a specific element in a DESpy model

Event Graph	DESpy
Component	Subclass of SimEntityBase
Parameter	attribute passed in to __init()__
State variable	attribute initialized to nan or []
Run Event	reset() method and run() method
Other Events	method of same name, first letter lower-case
State transition	assignment to state variable followed by notify_state_change()
Schedule event	call schedule(<event name>, <delay>, [<optional parameters>])
Cancel event	call cancel(<event name>, [<optional arguments>]

Executing Model

Action	EventList call
Run verbose mode	EventList.verbose=True
Run for xxx simtime units	EventList.stop_at_time(xxx)
Run for n Foo events	EventList.stop_on_event(n, 'Foo')
Prepare for running model	EventList.reset()
Run Model	EventList.start_simulation()

Example: SimpleServer

The SimpleServer component is the most basic implementation of a multiple server queue. Its state representation consists of integers representing the number of customers in queue (number_in_queue) and the number of available servers (number_available_servers). It is not a stand-alone model, but must be set up to "listen" to another component that periodically schedules an Arrival event. The most basic such component is the ArrivalProcess.

# Instantiate ArrivalProcess component with interarrival times Exponential(1.7)
interarrival_time_generator = RandomVariate.instance('Exponential', mean=1.7)
arrival_process = ArrivalProcess(interarrival_time_generator)

# Instantiate SimpleServer component with 2 servers and service times Gamma(1.7, 1.8)
number_servers = 2;
service_time_generator = RandomVariate.instance('Gamma', alpha=1.7, beta=1.8)
simple_server = SimpleServer(number_servers, service_time_generator)

# Add the SimpleServer instance to the ArrivalProcess instance as a
# SimEventListener
arrival_process.add_sim_event_listener(simple_server)

# These statistics objects will collect the time-varying number_in_queue
# and number_available_servers of the SimpleServer instance
number_in_queue_stat = SimpleStatsTimeVarying('number_in_queue')
number_available_servers_stat = SimpleStatsTimeVarying('number_available_servers')

# Add the statistics objects as StateChangeListeners
simple_server.add_state_change_listener(number_in_queue_stat)
simple_server.add_state_change_listener(number_available_servers_stat)

# Execute the model for 100,000 time units
stopTime = 100000;
EventList.stop_at_time(stopTime)

# Initialize the EventList and put all Run events on the EventList
EventList.reset()

# Execute the simulation
EventList.start_simulation()

Running Multiple Replications

The most straightforward way to estimate confidence intervals is by running multiple independent replications. To run multiple replications, wrap the reset() and start_simulation() calls in a for loop. Collecting statistics, however, needs to be different for the "inner" statistics objects and "outer" ones.

Statistics objects are StateChangeListeners that implement the stateChange() method to update their counters. The two main types are "tally" and "time-varying." They are typically used in tow different ways: "inner" and "outer."

An Inner statistics object uses state trajectories from a single replication to produce a value - typically a mean - for that replication. Since simulation data are tyically auto-correlated, estimates of the variance can be extremely biased. Thus, the usual expression for a confidence interval cannot be applied. It is important to clear() each inner statistics object before each replication in order to ensure independence between replications.

An Outer statistics object is typically used to collect data from the inner statistics objects. After each replication, a value from an inner statistics object (often the mean) is passed to the outer object.

In this manner, regardless of the value passed, the outer statistics object can then (with sufficient quantity of replications) produce a confidence interval for the value in question (with all the "usual" assumptions about the central limit theorem).

Parameters vs State Variables

Parameters

Parameters are variables in a component that do not change during a given replication of the simulation. These are inputs to the simulation and, as such, must be passed in via the __init()__ method. Parameters may be scalars, such as the total number of servers, or RandomVariates which generate different values on each call, such as the service time generator. In such cases, while the generated values may be different, the distribution itself remains the same.

State Variables

State variables do change within a given replication of a model. The full definition of a state variable must include its initial value, since that is set in the reset() method of each component. Only event methods are permitted to change the value of a state variable, since events are identified with state transitions. Thus, the value of a given state at any point in simulated time is completely determinded by its initial value and the subsequent state transitions.

Every state transition must be accompanied by a notify_state_change() call, which notifies StateChangeListsners that the given state has changed. This allows components to be written to the dynamics of the model only and not be concerned with collecting statistics, since that can be done with the appropriate statistical objects, which are StateChanegListsners.

Defining Events

An Event is defined in a subclass of SimEntityBase as simply an ordinary method. Within an event method, there should only be (in order):

1. State transitions (followed by state change notifications)
2. Canceling events (if needed) by a call to self.cancel()
3. Scheduling events (if needed) by a call to self.sechedule()

RandomVariate Instantiation

By convention, a RandomVariate class specifies its parameters as named ones in the constructor.

There are several ways to instantiate a RandomVariate.

• Direct instantiation, e.g. Exponential(mean=2.3)
• Using the RandomVariate factory method with keywords: RandomVariate.instance('Exponential', mean=2.3)
• Using the RandomVariate factory method with a dictionary (using the params keyword):

params_map={mean:2.3}
RandomVariate.instance('Exponential', params=params_map)