A Software Interface

Between System

Dynamics and Agent-

Based Simulations

A study by

Andreas Größler,

Myrjam Stotz,and

Nadine Schieritz

Mannheim University,

Germany

Agent-Based Simulation

Whereas in system dynamics the internal structure of a

system determines its dynamic tendencies, in the agent-

based simulation the dynamic behavior of a system arises

from the behavior of its elements, the agents, and the

interactions between them:

System Dynamics Agent-Based

Simulation

Basic building

block

Feedback loop Agent

Unit of analysis Structure Agents' rules

Level of modeling Macro Micro

Perspective Top-down Bottom-up

Scientific Problem

Agent-based simulation and system

dynamics use computer simulation to

investigate social and economic systems

characterized by non-linearity, delays and

feedback processes.

Both concentrate on understanding and

qualitative prediction of systems behavior.

An integration of both approaches

might be fruitful.

The Software Used…

A software

framework for

creating agent

based simulations

using the Java

language

A visual modeling

tool for system

dynamic models

The Vensim DLL

allows you to call

Vensim functions

from other

applications

The Technical IntegrationAt least two Java classes have to be programmed:

the simulation environment and the agents’ class…

Java class of the

simulation

environment

• Based on a class

given by RePast

• Builds and

manages the agents

• Manages the

simulation

• Provides a

graphical user

interface

Java class of

the agents

• Represents the

mental model of

the agents

• Builds a Vensim

object to

communicate with

Vensim via the

Vensim DLL

Vensim object

(provided by

Vensim)

• Calls the Vensim

DLL

• Manages the

transfer of data and

commands

between the agent-

class and Vensim

provides creates creates calls

An example from Supply Chain

Management

SupplierAgent

ManufacturerAgent1

ManufacturerAgent2

ManufacturerAgent3

Behavior of the

manufacturer

RePast Vensim DLL

Agent-based simulation System Dynamics

Behavior of the

manufacturer

Behavior of the

manufacturer

The Agents’ Behavior

InventoryAcquisition Rate Shipment

Rate

Desired

Acquisition Rate

AcquisitionAdjustment fromInventory

DesiredInventory

ExpectedOrder Rate Change in

Exp Orders

InventoryAdjustment

Time

DesiredInventoryCoverage

Time to AverageOrder Rate

OrderFulfillment

Ratio

Table for OrderFulfillment

Supply LineOrders PlacedRate

Acquisition Lag

Adjustment forSupply Line

DesiredSupply Line

Orders Placed

Supply LineAdjustment Time

Customer

Order Rate

B

OrderFulfillment

B Inventory Control

B

Supply Line

Control --

+

+

+

+

+- -

+

+

-

++

+

-

DesiredShipment

Rate

+

MaximumShipment

RateMinimum

OrderProcessing

Time

+

+-

-

+

Inventory

Coverage+ -

SafetyStock

Coverage

+

+

+

CustomerOrders

Backlog BacklogChange Rate

+ -

+

Supplies

ReceivedInit Supply Line Init Inventory

Init ExpectedOrder Rate Init Customer

Orders Backlog

+

+

In each step of a simulation the agent-based modeled manufacturers

call this System Dynamics model (modified after Sterman 2000)...

The Simulation in RePast Start and stop

a simulationGraphs represent

the results of

simulations

In a probe map

parameters of the

simulation can be set

Conclusions

The presented software solution provides a prototypical common technical platform to examine problems that suggest the integration of the two simulation concepts

More flexibility is provided in modeling and simulating dynamic systems (using in each part of the model the method fitting best)

The advantages of both methods can be used, their disadvantages can be reduced

Skills in Java are necessary

Further Research

Extend and improve the model technically, e.g. make simulations more user-friendly

Use this platform to model other problems that suggest the integration of the two simulation concepts

Use this software interface as a basis to model an integration with System Dynamics on macro-level and agent-based simulations on micro-level

Investigate effects of combined methods on validity of models

References RePast: http://repast.sourceforge.net

Venism: http://www.vensim.com

Phelan, SE. 1999. A Note on the Correspondence between Complexity and Systems Theory. Systemic Practice and Action Research 12(3): 237–246.

Schieritz, N, Größler, A. 2003. Emergent Structures in Supply Chains: A Study Integrating Agent-Based and System Dynamics Modeling. Proceedings of the 36th Hawaiian International Conference on Systems Science, Wailea.

Scholl, HJ. 2001a. Agent-based and System Dynamics Modeling: A Call for Cross Study and Joint Research. Proceedings of the 34th Hawaiian International Conference on Systems Science, Wailea.

Sterman, JD. 2000. Business Dynamics – Systems Thinking and Modeling for a Complex World, Boston.