Sébastien Picault, SMAC Research Team, Lille 1 University (sebastien.picault at univ-lille1.fr)

IODA: A Hands-On Tutorial - Steps in the Model Design
Social Simulation Conference, Barcelona, 2014

Table of Contents

The slides describing the main ideas underlying the Interaction-Oriented Approach to Agent-Based Simulation can be found here.

1 Setup: Starting with the IODA NetLogo extension

Structure of the directory tree for a "standard" installation of the IODA extension within the NetLogo installation directory

-- NetLogo5	[i.e. your own NetLogo installation directory]
   |-- NetLogo.jar
   |-- ...
   |-- extensions
        |-- ioda
            |-- ioda.jar
            |-- IODA_2_2.nls   
	    |-- tutorials
	    |   |-- 0-template
	    |   |   |-- interaction.txt
	    |   |   |-- matrix.txt
	    |   |   |-- model.nlogo
	    |	|
	    |   |-- ...
	    |-- docs

The IODA extension comes with a tutorial directory, which contains a large number of code examples. Especially, the 0-template directory provides 3 files that can be used for starting your own model:

  • model.nlogo : a NetLogo file (see below)
  • interactions.txt : a text file which describes all available interactions
  • matrix.txt : a text file in CSV format which describes what interactions are assigned to source and target agent families, with the corresponding priority, limit distance and target selection policy.

For this tutorial we have packed a template.zip file which you just need to download and unpack.

1.1 Include and extension

The IODA NetLogo extension is composed of 2 parts:

  • the ioda directory, containing the file ioda.jar, can be either installed within the extensions directory of your NetLogo installation directory, or at the same level of the models that you develop
  • the file IODA_2_2.nls, which is a source file written in NetLogo, can be put anywhere but you have to specify the full path of the file to access it; a quite simple solution is to put a copy of this file in the directory where you have your own models.

Thus the template model begins with the following lines:

__includes ["IODA_2_2.nls"]  ; here the path of the nls file
extensions [ioda]

which ensure that both the include source code and the Java extension are loaded.

The template model contains the two main procedures required to initialize and run any model: setup and go (attached to the corresponding buttons in the NetLogo graphical interface).

to setup
  ioda:load-interactions "interactions.txt"
  ioda:load-matrices "matrix.txt" " \t"

to go

The setup procedure is in charge of loading the definition of interactions in the specified text file, then loading the assignation of interactions to specific agents through an interaction and an update matrix, then the IODA engine initializes the model. You may use your own filenames (provided that you change them in the setup procedure).

The go procedure delegates the scheduling of the IODA agents to a dedicated procedure, ioda:go, which is defined in the include file.

In the next steps, you should have very few things to add inside those procedures.

1.2 User Interface

If everything is installed properly, the interface view should look as shown below. If any error occurs when clicking on the setup button or on the go button, you should check again the installation procedure.


Figure 1: The NetLogo view for the template model.

2 First Steps: A Simple Predator-Prey Model

Now we are going to implement a naive predator-prey model with the classical sheep and wolves. We shall start with the following specifications:

  • sheep and wolves wander (walk randomly) through the environment
  • when two sheep meet, they can produce offspring (a delay must be introduced to prevent them to produce offspring at each tick)
  • a wolf can eat a sheep

2.1 The corresponding interaction matrix

This situation can be summarized in the form of an interaction matrix according to the IODA approach, like the matrix below. Except for the 1st column (source agents) and the 1st line (target agents), each cell contains the interactions that the source agent (agent at the left of the line) can perform on the target agent (agent at the top of the column). The column marked with ∅ represents interactions where the target agent is the source agent itself, which are called reflexive interactions.

Table 1: A IODA interaction matrix for describing what interactions can occur between agents families
Sources \ Targets sheep wolves
sheep Wander (0) Mate (10, 0.5)  
wolves Wander (0) Eat (10, 1)  

This description is to be read as follows:

  • a sheep can perform the Wander interaction alone (i.e. with no other target), with priority 0
  • same thing for a wolf
  • a sheep can perform the Mate interaction with another sheep, with priority level 10, at distance ≤ 0.5
  • a wolf can perform the Eat interaction with a sheep, with priority level 10, at distance ≤ 1.

N.B.: When an agent tries to perform interactions, it select one among the realizable interactions with the highest priority level. Thus only the order relation among interactions on the same line is relevant.

In the IODA extension, this interaction matrix is encoded as a CSV file where each line represents the affectation of an interaction to a source family, with a priority level, and possibly a target family and the corresponding limit distance.

Thus you have to add those informations to the file matrix.txt:

; This file can be used to define your own interaction
; matrix and update matrix.
; <source> <interaction> <priority>
; OR: <source> <interaction> <priority> UPDATE
; OR: <source> <interaction> <priority> <target> <distance> [<target-selection-policy>]
; NB1: this file is read as a CSV file, thus you can use any field separator char 
; since you put it in the second argument of the ioda:load-matrices command.
; NB2: any non-letter, non-digit character at the beginning
; of the line introduces a comment line. Empty lines are ignored.

sheep	Wander	0
sheep	Mate	10	sheep	0.5
wolves	Wander	0
wolves	Eat	10	sheep	1

2.2 The definition of interactions

The interactions we have here are quite simple:

  • Wander consists of a single action: the random walk
  • Mate: if both source and target are able to mate, they must produce a new agent and remember the current tick
  • Eat: if the source is hungry, if digests the target and destroys it

Thus those interactions can be expressed in the IODA formalism as follows:

; This file can be used to define your own interactions.
; [TRIGGER <tr1> ... <tr_n>]
; [...]
; [TRIGGER <tr'1> ... <tr'_n>]
; [CONDITION <cond1> ... <cond_n>]
; [...]
; [CONDITION <cond'1> ... <cond'_n>]
; [ACTIONS <act1> ... <act_n>]
; NB1: tr_i and cond_i are abstract condition primitives,
; which must be implemented by concrete reporters, while
; act_i are abstract action primitives, which must be
; implemented by concrete commands.
; NB2: any non-letter, non-digit character at the beginning
; of the line introduces a comment line. Empty lines are ignored.

  ACTIONS	choose-direction advance

  CONDITION	after-delay? target:after-delay?
  ACTIONS	reset-timer target:reset-timer produce-offspring

  TRIGGER	hungry?
  ACTIONS	digest target:die

Two important things to notice:

  1. Interactions use abstract condition and action primitives. By default, they are evaluated by the source agent (the agent that tries to perform the interaction). The special keyword target: means that the primitive is evaluated by the target agent (the partner of the interaction, the agent that undergoes the interaction).
  2. The TRIGGER clause instead of the CONDITION in interaction Eat. From an operational point of view it does not make any difference. Yet, the TRIGGER is aimed at expressing a goal (i.e. the reason why this interaction should be performed), while the CONDITION represents a prerequisite for executing the actions.

2.3 Pieces of NetLogo code

Breed declaration and initialization

The first thing to do is to declare the two turtle breeds we are going to use, with their properties (for now, the sheep just need a timer to know whether they can mate or not).

breed [sheep]
breed [wolves wolf]

sheep-own [ last-offspring ]

Then we have to write a procedure for initializing random populations. Below is an example for that. Do not forget to add the corresponding sliders in the user interface: respectively, nb-sheep and nb-wolves.

to setup
  ioda:load-interactions "interactions.txt"
  ioda:load-matrices "matrix.txt" " \t"

to init-world
  ask patches [set pcolor green]
  set-default-shape sheep "sheep"
  set-default-shape wolves "wolf"
  create-sheep nb-sheep [ setxy random-xcor random-ycor set color white]
  create-wolves nb-wolves [ setxy random-xcor random-ycor set color gray]

From abstract to concrete primitives

At this step if you click on the setup button, then on the go button, you get an error message. Indeed, the IODA engine detects agent families (NetLogo breeds) that, according to the interaction matrix, can perform interactions which make use of (yet) undefined primitives.

Now click on the show primitives to write button. In the output area, you get the following text:

to-report sheep::after-delay?

to-report wolves::hungry?

to sheep::filter-neighbors

to wolves::filter-neighbors

to sheep::reset-timer

to sheep::produce-offspring

to sheep::choose-direction

to sheep::advance

to wolves::digest

to sheep::die

to wolves::choose-direction

to wolves::advance

This is the list of all pieces of NetLogo code that you have to write so as to define the actual (operational) meaning of the primitives. Perception primitives are NetLogo reporters (to-report ... report ... end) while action primitives are NetLogo procedures (to ... end). The name of a concrete primitive is composed of the name of the breed (e.g. sheep or wolves) and the name of the abstract primitive (as it appears in the definition of the interaction), separated by ::.

  • A special primitive is required for agents that are able to act as sources on other agents (i.e. for active agent families): the filter-neighbors primitive. It defines the perception capabilities of the agent. If left empty, the agent is able to perceive all agents on which it can perform interactions (e.g. sheep would perceive all other sheep, wolves would perceive only sheep). Usually agents have a limited perception range, thus the actual neighbors, i.e. the agents that are likely to undergo interactions, result from a selection among the default list of potential targets. A simple way to do this is to use predefined IODA filtering procedures, e.g. for filtering within a given radius:
; sheep can see other agents in a circle of radius 1
to sheep::filter-neighbors
  ioda:filter-neighbors-in-radius 1

; wolves have a larger perception halo
to wolves::filter-neighbors
  ioda:filter-neighbors-in-radius 2
  • Another important thing is the way to "remove" agents from a IODA simulation. Since we use a special scheduling algorithm, you must not use the NetLogo die primitive: you have to use ioda:die instead.
  • For the same reasons, when creating a new agent, you have to notify the IODA simulation engine. This is done by calling ioda:init-agent.

Now, try to write the corresponding code and test it (setup then go).

Here is a possible solution (typical values for parameters: nb-sheep: 300, nb-wolves: 10, sheep-offspring-delay: 20).

3 Extending the model

3.1 New agents, same interactions

First, we want to add a new species in the simulation. This is quite simple, since the interactions are the abstract description of a behavior. Thus, they are designed to be re-used among different kinds of agents.

For instance, we want to add rabbits in the ecosystem. At this step, rabbits behave like sheep (they mate and walk around), and we suppose that wolves prefer eating rabbits than sheep (easier to kill ?). Also, we suppose that rabbits move a bit faster than sheep and do not have the same delay between offspring.

Table 2: The interaction matrix for describing behaviors in the extended ecosystem
Sources \ Targets sheep wolves rabbits
sheep Wander (0) Mate (10, 0.5)    
wolves Wander (0) Eat (10, 1)   Eat (20, 1)
rabbits Wander (0)     Mate (10, 0.5)

N.B.: The priority for the Eat interaction on the "wolves" row is higher for rabbits than for sheep: thus, during the interaction selection process, a wolf agent tries first to perform this interaction on a rabbit agent, if any is present and fulfils the conditions. If not possible, it tries then to perform it on a sheep agent.

  1. Add the convenient lines in the text file describing the interaction matrix.
  2. Add the new breed in the NetLogo model and initialise the population in the setup procedure.
  3. Finally, write the required primitives. The differences between sheep and rabbits (regarding how behaviors are actually carried out) are to be encoded in the primitives.

Here is a possible solution (typical values for parameters: nb-sheep: 300, nb-wolves: 10, nb-rabbits: 300, sheep-offspring-delay: 20, rabbits-offspring-delay: 10). At this step, you should also add a plot to follow the evolution of agents population.

3.2 Aging: UPDATE processes

We now want to extend the current simulation model by taking into account the following processes:

  • wolves have a limited lifetime and can die from old age
  • they also can die of starvation (e.g. when they have no food reserve)
  • to counterbalance the subsequent decrease of wolves population, we now want wolves to mate as sheep and rabbits do.

Obviously the aging process takes place at each tick, independently from interaction selection. The fact that the age increases must not prevent a wolf from eating a sheep, and vice-versa. Aging is a spontaneous state change which occurs in the agent, while Eating is a "deliberate" behavior (chosen among others, such as Wandering).

In IODA such processes are represented through an update matrix. At each tick, all interactions realizable in the update matrix are performed, then only the interaction selection step begins.

The new update and interaction matrices can be rewritten as follows:

Table 3: The update matrix
wolves Age (10)
Die (0)

N.B.: in the update matrix, all realizable interactions are performed in order of decreasing priority

Table 4: The interaction matrix
Sources \ Targets sheep wolves rabbits
sheep Wander (0) Mate (10, 0.5)    
wolves Wander (0) Eat (10, 1) Mate (30, 1) Eat (10, 1)
rabbits Wander (0)     Mate (10, 0.5)

The interaction matrix now integrates the capability of wolves to Mate.

This leads to the new matrix.txt file:

; interaction matrix
sheep	Wander	0
sheep	Mate	10	sheep	0.5
wolves	Wander	0
wolves	Eat	10	sheep	1
wolves	Eat	20	rabbits	1
wolves	Mate	30	wolves	1
rabbits	Wander	0
rabbits	Mate	10	rabbits	0.5

; update matrix
wolves	 Age	10	UPDATE
wolves	 Metabolism	10	UPDATE
wolves	 Die	0	UPDATE

Now the new interactions have to be added in the interactions.txt file:

  ACTIONS	increase-age

  ACTIONS   decrease-reserve

  TRIGGER	old-age?
  TRIGGER	starving?

An important thing to notice in the Die interaction: the two TRIGGER lines. This is the way to express a disjunction (logical OR). In that case, the Die interaction is realizable either if the source agent is old, or if it has no more food reserve. At least one must be true to allow the interaction to occur.

You also have several additional primitives to write, which themselves imply to endow the wolves with new attributes (e.g. last-offspring, food-reserve, age, lifespan):

to-report wolves::after-delay?

to-report wolves::old-age?

to-report wolves::starving?

to wolves::reset-timer

to wolves::produce-offspring

to wolves::increase-age

to wolves::decrease-reserve

to wolves::die

It is also a good idea to write a procedure wolves::init so as to initialize those new attributes and run ioda:init-agent each time a new wolf is created.

A possible solution is proposed here (typical values for parameters: nb-sheep: 200, nb-wolves: 15, nb-rabbits: 200, sheep-offspring-delay: 20, rabbits-offspring-delay: 10, wolves-offspring-delay: 15, avg-wolves-lifespan: 80, wolves-metabolism: 0.2).

3.3 Hunting and Escaping (the target selection policy)

Now we are going to refine the model so as to allow wolves to track sheep, and conversely to make sheep gather and flee the wolves. In addition, rabbits just try to flee the wolves.

Table 5: The new interaction matrix
Sources \ Targets sheep wolves rabbits
sheep Wander (0) Mate (10, 0.5)
MoveTowards (0, 1)
MoveAway (20, 1)  
wolves Wander (0) Eat (10, 1)
MoveTowards (5, 2)
Mate (30, 1) Eat (20, 1)
rabbits Wander (0)   MoveAway (20, 1) Mate (10, 0.5)

As you can see in this matrix, we need only two new interactions (MoveTowards and MoveAway) to introduce the new behaviors.

  1. Write the corresponding matrix.txt text file
  2. Write the MoveTowards and MoveAway interactions. Therefore you need to use a property called ioda:my-target, which refers to the agent which is interacting with the caller.
  3. Test your behaviors

Here is a possible implementation.

You should note that the "hunt" behavior of the wolves is not very efficient, since they choose the target randomly (even if another one is closer). IODA allows the specification of a target selection policy in the matrix file:

wolves  MoveTowards 5 sheep 2 BEST:proximity

3.4 Going further

Now you should be able to extend this model in many directions. Here are a few ideas to experiment. If you need to use more advanced features of the IODA extension, you may look at the Tutorial provided in the extension package or available on-line here, or ask me directly!

  1. In order to limit the population of rabbits, endow them with an aging process.
  2. Human Hunters arrive in the ecosystem: they hunt sheep too. But, indeed, they do not eat all the food but are able to share it.
  3. Human hunters are able to put a few collets in the environment to trap rabbits. They must memorize their position and inspect them from time to time. Of course, according to the IODA point of view, collets are agents too!
  4. Human hunters can be attacked by wolves when these are really hungry, but conversely they can kill wolves when they have few food reserves.
  5. Sometimes human hunters suceed in turning young wolves into hounds which help them to hunt. Hunters share the food with hounds, but conversely when hounds are starving they may come back to wild life.
  6. Alternatively, you can use e.g. a forum to centralize or coordinate the activity of the hunters, share information or food, and so on… Therefore, a good way to proceed is to consider the forum as an agent having a network-based perception of its neighbors (e.g. the human hunters that are related to it). To do so, you can create links between several hunters and a forum, and use the pre-defined primitive ioda:filter-neighbors-on-mylinks in the forum::filter-neighbors primitive.

Thank you for attending this tutorial. We hope you are now convinced by the advantage of separating agents from generic behaviors expressed as interaction rules. Do not hesitate to use the IODA extension for NetLogo for your own research or teaching, or to contact us (ioda at univ-lille1.fr) if you need more information or if you plan to build large-scale simulations (e.g. using our Java platform, JEDI).

© 2014 Sébastien Picault, SMAC research team, LIFL, Lille 1 University.