Simkit

Simkit is an open-source Java library for discrete-event simulation by Arnold Buss and Kirk Stork, with a copyright running from 1997.¹ Buss wrote about it from the MOVES Institute at the Naval Postgraduate School in Monterey, California.³ In a discrete-event simulation the state of a system changes only at separate moments, called events.³ An event list holds the events still to come in time order and runs them in turn.³

Simkit models are written as event graphs, a notation from Lee Schruben.³ A model has state variables, the change each event makes to them, and scheduling edges of the form "when event A happens, if a condition holds, schedule event B after a delay".³ In code, each event becomes a method of a Java class, so an arrival event becomes doArrival().³

Larger models are put together from components through two listener patterns, a framework Buss calls Listener Event Graph Objects, or LEGO.³ With the first, one component listens for another's events and reacts to those of the same name, as a queue hears each arrival from an arrival process.³ A small bridge component renames an event, so the end of service at one queue becomes an arrival at the next.³ With the second, every change of state is announced by convention, so statistics are collected by separate objects that listen, and the model itself need contain no statistics code.³ Components are treated as black boxes and can be built from other components.³ Buss writes that this loose coupling allows much more reuse of components than traditional approaches.³ His 2002 paper, "Component based simulation modeling with Simkit" (Winter Simulation Conference, pages 243 to 249), describes the modeller working through Simkit's programming interface, with a graphical interface then in development.^3,4

Simkit needs Java 1.17 or later (as its readme puts it); the simkit.jar file goes on the classpath, and the source builds with Ant.¹ The repository carries a GNU LGPL version 3 LICENSE file and a License.txt that places Simkit under the GNU GPL, version 2 or later.² It is maintained on GitHub, with commits in 2026, the latest on 4 August.⁵

For someone modelling a place, Simkit suits systems where people or goods arrive, wait and are served, such as a clinic or a ferry terminal. Each part of such a system can be written once as a component and reused. JAMES II is another Java framework built for reuse, and AnyLogic offers discrete-event modelling with a graphical editor.