A single car, a driver, and red traffic light in the middle of the night.

Stanhope Forbes A Ferryman at Flushing

Red Light, Matthias Ripp (CC BY 2.0)

Let’s start with a very simple model. The example demonstrates the main mode of operation, the core API and the component process model implemented in kalasim. We want to build a simulation where a single car is driving around for a some time before stopping in front of a red traffic light.

import org.kalasim.*
import kotlin.time.Duration.Companion.hours
import kotlin.time.Duration.Companion.minutes

class Driver : Resource()
class TrafficLight : State<String>("red")

class Car : Component() {

    val trafficLight = get<TrafficLight>()
    val driver = get<Driver>()

    override fun process() = sequence {
        request(driver) {
            hold(30.minutes, description = "driving")

            wait(trafficLight, "green")

createSimulation {

    dependency { TrafficLight() }
    dependency { Driver() }


For each (active) component we (can) define a type such as:

class Car : Component()

The class inherits from org.kalasim.Component.

Our car depends on a state TrafficLight and resource Driver for operation. To implement that, we first declare these dependencies with dependency{} in the main body of the simulation, and secondly inject them into our car with get<T>. Note, we could also directly inject states and resources with dependency {State("red")} without sub-classing.

Although it is possible to define other processes within a class, the standard way is to define a generator function called process in the class. A generator is a function that returns Sequence<Component>. Within these process definitions we use suspendable interaction function calls as a signal to give control to the centralized event loop.

In this example,


suspends execution control and comes back after 1 time unit (referred to as tick). Apart from hold, kalasim supports a rich vocabulary of interaction methods including passivate, request, wait and component.

The main body of every kalasim model usually starts with:

Here, we enable event logging of state changes to see the status of simulation on the console. After declaring our dependencies, we instantiate a single car with Car(). It automatically is assigned the name Car.0.

As there is a generator function called process in Car, this process description will be activated (by default at time now, which is 0 by default at the beginning of a simulation). It is possible to start a process later, but this is by far the most common way to start a process.



we start the simulation and get back control after 5 ticks. A component called main is defined under the hood to get access to the main process.

When we run this program, we get the following output (displayed as table for convenience):

time   current  receiver  action                             info               
------ -------- --------- ---------------------------------- -------------------
.00             main      Created
.00    main
.00             Driver.1  Created                             capacity=1
.00             Car.1     Created
.00                       activate                           scheduled for .00
.00             main      run +5.00                          scheduled for 5.00
.00    Car.1    Car.1
.00                       Requesting 1.0 from Driver.1 
.00                       Claimed 1.0 from 'Car.1'
.00                       Request honor Driver.1             scheduled for .00
.00                       hold +1.00                         scheduled for 1.00
1.00                      entering waiters of TrafficLight.1
1.00                      wait                               scheduled for <inf>
5.00   main     main
Process finished with exit code 0

There are plenty of other more advanced (that is more fun!) examples listed in examples chapter.