# Lesson 1 - Unit 13 Overview

"I try to get a vision of the future, and then I try to figure out where the discontinuities are."

- Nolan Bushnell

When tangible things move through or change within a system, the dynamics can be conceptualized in two different ways: continuously or discretely. Things that move continuously can be thought of as flowing.  An example of this is the movement of water.  Other things move or happen discretely (e.g., such that they must be tracked individually).  Examples of this include financial transactions or the movement of items through a factory.

Everything we have discussed up to this point in this Course has dealt with continuous dynamics, as GoldSim is most commonly used to model dynamics that are primarily continuous. It is important to understand, however, that GoldSim provides powerful capabilities for representing discrete dynamics as well.  In fact, most real-world systems are best described using a combination of continuous and discrete dynamics (i.e., hybrid systems).

For example, just because something is considered to flow (like water), it does not mean that all of the dynamics for that system can be treated as being continuous.  For example, if water is flowing from one pond to another (via a pump), and the pump suddenly breaks down, that is a “discrete event”, and must be treated in an appropriate manner.

This Unit focuses on GoldSim’s capabilities for representing discrete dynamics.

Note: There is an entire class of highly specialized simulation tools referred to as discrete event simulators, and you should not confuse GoldSim with these. Discrete event simulators are designed for simulating processes such as call centers, assembly lines, various types of queues, and shipping facilities. Although GoldSim can model some of these kinds of systems, it was not specifically designed to do so (i.e., its design is optimized for tackling uncertain continuous and/or hybrid systems).  As a result, GoldSim would generally not be the most appropriate tool for modeling large quantities (e.g., millions) of items with multiple attributes moving through systems such as assembly lines and call centers. If the system you are trying to model resembles something like that, a pure discrete event simulator would typically be a more appropriate tool than GoldSim.

In this Unit, we will discuss the following:

• Basic discrete event modeling concepts;
• Generating a Timed Event;
• Understanding event triggering;
• Triggering multiple elements;
• Responding to an event - causing a Discrete Change to a quantity;
• Complex triggering;
• Tracking the state of a system using Status and Milestone elements;
• Using Status elements for feedback control;
• Using feedback control and allocation to realistically represent outflows;
• Delaying events and simulating processes and queues;
• Routing events; and
• Interrupting a simulation.

This Unit also includes three Exercises (in addition to several Examples that we will explore together). This Unit has a total of 14 Lessons (including this overview and a summary at the end).