Agent-based modeling and simulation tutorial - EASSS 2009 - Giuseppe Vizzari

Agent-Based Modeling for the Simulation of Complex Systems Giuseppe Vizzari Department of Informatics, Systems and Communication University of Milan-Bicocca

Agenda Simulation: a definition and motivations Agent Based Simulation Differences from other approaches Peculiarities, advantages and risks A Case Study: Crowd Modeling Some reflections From reality, to models, to a simulation The role of the environment and indirect interaction models Frameworks and tools supporting Agent-Based modeling and simulation A brief tutorial on Repast Conclusions EASSS 2009 - Torino – 3-4/9/2009

Complex Systems? Several (more ore less formal) definitions: A complex system is a highly structured system, which shows structure with variations A complex system is one whose evolution is very sensitive to initial conditions or to small perturbations , one in which the number of independent interacting components is large, or one in which there are multiple pathways by which the system can evolve A complex system is one that by design or function or both is difficult to understand and verify A complex system is one in which there are multiple interactions between many different components Complex systems are systems in process that constantly evolve and unfold over time … Features of complex systems: Composed by several interacting elements Nonlinearity Networked structure Hierarchical structure Positive and negative feedbacks Possibility to evolve and adapt Robustness and plasticity … Complex systems research is a hot topic for scientists… but also for engineers! One of their main features is their internal structure and the interaction among their composing parts… that very often is studied by means of simulations EASSS 2009 - Torino – 3-4/9/2009

Simulation: a definition and motivations (Computer) Simulation represents a way to exploit a computational model to evaluate designs and plans without actually bringing them into existence in the real world to evaluate theories and models of complex systems by envisioning the effect of the modeling choices, with the aim of gaining insight of their functioning The use of “ synthetic environments” is sometimes necessary, because the simulated system cannot actually be observed Because it is actually being designed For ethical or practical reasons EASSS 2009 - Torino – 3-4/9/2009

Simulation life-cycle From the target system to its computational model and a simulator Execution of a simulation campaign Evaluation/validation of the model (and simulator) against collected data Possible usage for explaination and/or prediction Simulation execution Dynamics of Target System Modeling and design of a simulator Analysis of results + interpretation (model evaluation leading to explanation or prediction) Target System Model and simulator Data generated by the simulation(s) Collected Data EASSS 2009 - Torino – 3-4/9/2009

Agent Based Modeling and Simulation Several situations are characterized by the presence of autonomous entities whose actions and interactions determine (in a non-trivial way) the evolution of the system A growing number of disciplines are interested in considering and studying effects of decentralized decision making local-global interaction, self-organization , emergence heterogeneity in the system Agent based models are particularly suited to represent these situations, and to support this kind of study and analysis EASSS 2009 - Torino – 3-4/9/2009

Agent Based Models for simulation: peculiarities, advantages, risks The analytical unit is the individual agent , not aggregate variables This means, on one hand, that additional insight on the modeled system is required On the other hand such a model should be able to Generate the same aggregate dynamics as traditional ones Be able to represent, manage, analyze additional aspects , such as for instance spatial ones EASSS 2009 - Torino – 3-4/9/2009

A possible reference model for Agent Based models Different disciplines interested in the subject, different modeling styles Relatively young approach extremely high heterogeneity in models that legitimately claim to be agent based A reference model can be useful to present, schematize, compare different approaches, models, experiences EASSS 2009 - Torino – 3-4/9/2009

Crowds of pedestrians as Complex Systems Overall system behaviour depends on individuals’ decisions and actions … … that are generally influenced by a large number of factors … intertwined in an often unpredictable way Mixed and conflicting mechanisms Competition for the shared space… … but also cooperation (non written social norms) to prevent stall situations Emergent phenomena … EASSS 2009 - Torino – 3-4/9/2009

Crowd modeling and simulation: motivations Designer’s decision support Evacuation situations Positioning of signs Malls and shopping centres Support the study of pedestrian behaviour Envisioning of different behavioural models in realistic environments Possibility to perform ‘in-machina’ experiments EASSS 2009 - Torino – 3-4/9/2009

Possible modeling approaches Analytical May handle large simulation scenarios Entities as ‘mere’ numbers Continuous representation of space Cellular Automata based May handle a large number of entities Explicit discrete representation of the environment Entities are homogeneous (they are conceived as particular states of cells) Extensions to the basic model are often required (e.g. action-at-a-distance) Complex behaviours require a very large cell state and transition rule Multi-Agent Systems (MAS) based May handle a smaller number of entities Entities are clearly separated by the environment Entities may be heterogeneous Only a few approaches and models provide a representation of the environment EASSS 2009 - Torino – 3-4/9/2009

Analytical (physical) approach Pedestrians  particles subject to forces Goals: forces of attraction generated by points/reference point in the space Interaction among pedestrians: forces generated by particles Social forces Repulsive  tendency to stay at a distance Attractive  imitative mechanisms Lane formation ‘ Freezing by heating’ EASSS 2009 - Torino – 3-4/9/2009

Cellular Automata and crowd modelling Environment  bidimensional lattice of cells Pedestrian  specific state of a cell (e.g. occupied, empty) Movement  generated thanks to the transition rule an occupied cell becomes empty and an adjacent one, which was previously vacant, becomes occupied Choice of destination cell in a transition generally includes information which is not provided by basic CAs Benefit-Cost/ Gradient : predefined information related to cell desirability Magnetic Force /Social Force : model the effect of presence of other agents in the environment (attraction/repulsion of crowds) EASSS 2009 - Torino – 3-4/9/2009

From CA to Situated MAS Entities are reified, separated from the environment Agents, not just cell states They may have different behaviours Possibility to integrate several different action deliberation models Possibly heterogeneous system Entities interact by means of mechanisms not necessarily related to underlying cell’s adjacency Action at a distance is allowed EASSS 2009 - Torino – 3-4/9/2009

Situated MAS: action and interaction Agents are situated they perceive their context and situation their behaviour is based on their local point of view their possibility to act (move) and interact is influenced by the environment Situated Agents Interaction models Often inspired by biological systems (e.g. pheromones, computational fields) Generally provide a modification of the environment , which can be perceived by other entities But may also provide a direct communication (as for CAs interaction among neighbouring cells) EASSS 2009 - Torino – 3-4/9/2009

Situated Cellular Agents (SCA) Multi Agent model providing Explicit representation of agents’ environment Interaction model strongly related to agents’ positions in the environment Among adjacent agents (reaction) Among distant agents, through field emission-diffusion-perception mechanism Possibility to model heterogeneous agents , with different perceptive capabilities and behaviour Compare T (f  c,t) = true emit(f) react(s,a b ,s’) react(s,a c ,s’) EASSS 2009 - Torino – 3-4/9/2009

Situated MAS and crowd modelling Pedestrians  agents Environment  graph , as an abstraction of the actual environmental structure Movement  generated thanks to the field diffusion-perception-action mechanism Sources of signals (fields): objects , gateways, but also agents Agents are sensitive to these signals and can be attracted/repelled by them Possible superposition of different such effects (amplification/contrast) transport(p,q) EASSS 2009 - Torino – 3-4/9/2009

SCA Crowd Modelling Approach Definition of the MMASS spatial structure Definition of active elements of the environment and field types Definition of mobile agents (types, states, perceptive capabilities and behavioural specification) Definition of monitored parameters and specification of monitoring mechanisms Specific simulation configuration (number, type, position and initial state of mobile agents, other parameters) Abstract scenario specification Computational model for the scenario Experiment-specific parameters EASSS 2009 - Torino – 3-4/9/2009

Spatial structure of the environment Spatial structure  discrete abstraction of simulation environment Scale of discretization and adjacency  depend on specific scenario (e.g. 50cm sided cells, Von Neumann neighbourhood with some exceptions) Activity supported by software EASSS 2009 - Torino – 3-4/9/2009

Active Elements of the Environment and Field Types Movement generated thanks to field related effects (attraction/repulsion) Active elements of the environment  sources of signals, reference points objects which constraint movement objects that transmit conceptual information (e.g. exit signs or indications) EASSS 2009 - Torino – 3-4/9/2009

Mobile Agents - States Agents behaviours can be very composite Segment behavioural specification into several states  attitudes towards movement EASSS 2009 - Torino – 3-4/9/2009 W G P E Waiting : passengers on the platform waiting for a train Get Off : people on the wagon that have to get off the train Passenger : agent on the train that has no immediate urge to get off Exit : passenger that has got down the train and goes away from the station S Seated : agent seated on a seat of the wagon State Transition W G E P S

Mobile Agents – Movement Utility Movement generated thanks to field related effects ( attraction/repulsion ) When multiple fields are present in the environment Agents evaluate the utility of each possible destination site The single contributions of various fields are combined in the overall site utility, for the current agent state transport(p,q)  EASSS 2009 - Torino – 3-4/9/2009 State Exits Doors Seats Handles Presence Exit press. W - Attract (2) - - Repel (3) Repel (1) P - - Attract (1) Attract (2) Repel (3) Repel (2) G - Attract (1) - - Repel (2) - S - Attract (1) - - - - E Attract (2) - - - Repel (2) -

Case Study Preliminary results Simulation configuration 6 agents getting off 8 agents getting on “ Mixed” results Agents accomplish their goals Some ‘erratic’ phenomena Oscillations (“forth and back” movements) Semi static situations (“equivalent” groups facing each other) EASSS 2009 - Torino – 3-4/9/2009

Sample Application: Lecture hall (I) Single exit Field sources in green: e mergency lights related to corridors and exit Static obstacles in red CA models of the same scenario can be found in the literature [Kirchner, Schadschneider, et al.; Henein & White] EASSS 2009 - Torino – 3-4/9/2009

Sample Application: Lecture hall (II) Two exits EASSS 2009 - Torino – 3-4/9/2009

Additional sample applications Schematic representations to test the modeling approach in paradigmatic cases … but also in more visually complex scenarios, to test the possibility to manage larger environments, higher number of agents that are harder to manage (from a raw computing power perspective) But what if the scenario is very large or if agents’ decisions depend on non spatial factors? EASSS 2009 - Torino – 3-4/9/2009

Towards hybrid SCA agents Rather than an implicit spatial representation encapsulated in the state diagram... ... an explicit abstract representation of known places of interest (represented in the environment with field sources) In this structure it is possible to autonomously select sequences of intermediate places leading from the current position to the desired destination EASSS 2009 - Torino – 3-4/9/2009 Gallery entrance Scala entrance Bank Square centre Palazzo Marino’s entrance

A qualitative spatial representation Essentially an annotated relational structure nodes are points of interest, associated to fields in the ‘real’ agents’ environment they can be annotated with labels they are generally associated to a portion of space, a set of SCA sites arcs denote the possibility to move from the the area associated to a point of interest to another A commonsense spatial representation A Exit B C D E F G Hall Lab. A Lab. A Lect. Hall T014 Corr. S Corr. N EASSS 2009 - Torino – 3-4/9/2009

Thinking before moving… Agents can reflect on this representation… they essentially perform a path-finding operation searching for a node annotated with their goal label At birth agents are not provided with a ‘plan’ of movement Their first operation is to inspect their knowledge ( think ) about the environment, and construct a sequence of points of interest to be visited A Exit B C D E F G Hall Lab. A Lab. A Lect. Hall T014 Corr. S Corr. N EASSS 2009 - Torino – 3-4/9/2009

Hybrid agents at work… 200 agents are created and situated in the (dense!) environment They are able to perceive the fields generated by their final goal If this is not perceived, they reflect on their knowledge about the environment and define a set of intermediate steps Actuation is carried out by the original SCA agent ‘body’ EASSS 2009 - Torino – 3-4/9/2009

Why not use simple queues? One could model such a system with one (or more) simple queues ... ... but the model would not be able to ‘answer’ some of the questions we can pose to the previous models What if t is unknown ? Only aggregate quantities are managed No heterogeneity Moreover, it would be very difficult to manage more complex situations, in terms of: Environmental structure Possible behaviours for the pedestrians Pedestrian in the lecture hall Exit (one pedestrian every t ms) Exit (one pedestrian every t ms) EASSS 2009 - Torino – 3-4/9/2009

So, no “best” modeling approach? The choice of the abstract/computational model depends on several factors: Available knowledge on the simulated phenomenon/ situation/reality Available data on actual scenarios, for sake of calibration, verification and validation Goals of the simulation activity Possible tension between these elements! EASSS 2009 - Torino – 3-4/9/2009

What about Agent Based Modeling and Simulation methodologies? It is very difficult to define a methodology that is both general and actually useful General methodologies tend to define well understood macro phases , but no useful suggestion on how actually carry them out in a specific context For instance, in different domains it is not even clear what is the most proper modeling granularity level In crowd modeling, a pedestrian is modeled as an agent But in biological systems, what should be represented as an agent? An organism ? An organ ? A cell ? A molecule ? Specific useful (more or less) formalized modeling approaches and methodologies only in specific contexts EASSS 2009 - Torino – 3-4/9/2009 ?

A reflection: from reality, to models, to a simulation The overall simulation project involves several phase , roles , types of knowledge and competences The frequent passages (translation, encoding, decoding, interpretation...) between different levels of abstraction can lead to several problems Non documented assumptions Unrealistic/unfeasible simplifications Simulation projects are difficult R. Shannon, “Introduction to the Art and Science of Simulation” (1998) ... But some even talk of “ dark arts” [J.P. Marney and H. Tarbert, “Why do simulation? Towards a working epistemology for practitioners of the dark arts” (2000)] Reality Subsystem Abstract model Computational model Simulator EASSS 2009 - Torino – 3-4/9/2009

Some remarks about validation and documentation… The ‘new’ context does not necessary call for new methods and methodologies… But the peculiarity of the approach call for particular attention and care: The basic elements of the agent based approach ‘meta-model’ look particularly expressive and cognitively effective in supporting rich and informative models… … but validity of models so rich of parameters is difficult to assure… See: Franziska Klügl: A validation methodology for agent-based simulations. SAC 2008: 39-43 Cornelia Triebig and Franziska Klügl: Elements of a Documentation Framework for Agent-Based Simulation Models. Cybernetics and Systems 40(5):441–474, 2009 EASSS 2009 - Torino – 3-4/9/2009

The role of the environment Most agent definitions include the term environment, but in some cases it is conceived as a mere communication infrastructure In most simulation scenarios the environment plays a more relevant role , since Spatial features of the environment influence agents behaviour Relevant simulation results are often spatially related The environment is used as a ‘channel’ for (indirect) agent interaction Some laws and properties must be enacted and preserved: the environment is the best ‘place’ to do so Communication infrastructure Agent Agent Agent Comprehensive environment Agent Agent Agent EASSS 2009 - Torino – 3-4/9/2009

Agent Based Modeling and Simulation Frameworks They provide tools facilitating the development of agent-based simulations in terms of abstractions and mechanisms in terms of generic functionalities (e.g. monitoring and visualization, scheduling and control of the simulation, data export and analysis) Three main categories General purpose frameworks based on ‘ad-hoc languages’ (e.g. Logo dialects) General purpose frameworks based on general purpose languages (e.g. Java) Model specific frameworks (SimSesam) EASSS 2009 - Torino – 3-4/9/2009

Brief tutorial on Repast… (code will be available soon)

Agent Based Simulation Industrial Applications? Several specific applications in logistics Danny Weyns, Kurt Schelfthout, Tom Holvoet, Tom Lefever: Decentralized control of E'GV transportation systems. AAMAS Industrial Applications 2005: 67-74 Cornelia Triebig, Tanja Credner, Peter Fischer, Titus Leskien, Andreas Deppisch, Stefan Landvogt: Agent-Based Simulation for Testing Control Software of High Bay Warehouses. MATES 2005: 229-234 … Pedestrian modeling and simulation Most commercial tools adopt an individual based approach (not necessarily agent-based) Several applications to support decision/policy makers: Electronic market: Isabel Praça, Carlos Ramos, Zita A. Vale, Manuel Cordeiro: Mascem: A Multiagent System That Simulates Competitive Electricity Markets. IEEE Intelligent Systems 18(6): 54-60 (2003) … Urban planning: Saarloos, D.J.M., Arentze, T.A., Borgers, A.W.J. and Timmermans, H.J.P. 2005. "A multi-agent model for alternative plan generation." Environment and Planning B: Planning and Design. vol. 32. 505-522 … EASSS 2009 - Torino – 3-4/9/2009

Conclusions Agent Based models represent a useful kind of instrument for the study of complex systems to realize simulators support decision making activities (system design, configuration, operation) These models have different requirements and ‘performances’ from traditional approaches, but there is no silver bullet ... Simulation is a complex activity , with several possible pitfalls Benefits of being aware of the approaches, techniques and advances of different disciplines in this topic EASSS 2009 - Torino – 3-4/9/2009

Giuseppe Vizzari Department of Informatics, Systems and Communication University of Milan-Bicocca [email_address] Thank you! EASSS 2009 - Torino – 3-4/9/2009

References (very partial...) General resources Growing Artificial Societies: Social Science From the Bottom Up , Joshua M. Epstein and Robert L. Axtell, MIT Press 1996 MABS workshops serie (Lecture Notes in Computer Science vol. 1534, 1979, 2581, 2927, 3415, 3891, Springer-Verlag ) (http://www.pcs.usp.br/~mabs/) Agent Based Modeling and Simulation Symposium, at EMCSR (www.lintar.disco.unimib.it/ABModSim) EASSS 2009 - Torino – 3-4/9/2009

References (very partial...) Pedestrian modeling Environment and planning B, vol. 28 no. 3, Theme issue: Pedestrian modeling, Editor: Michael Batty In general Environment and planning is a good source of papers adopting agent based models in the architecture and urban planning area Social simulation Simulation for the Social Scientist, Nigel Gilbert, Klaus G. Troitzsch, Open University Press Journal of Artificial Societies and Social Simulation (http://jasss.soc.surrey.ac.uk/JASSS.html) Biomedical context MAS*BIOMED workshop serie EASSS 2009 - Torino – 3-4/9/2009

References (very partial...) Some additional links SWARM Development Group Wiki (http://www.swarm.org): “ A resource for agent- and individual-based modelers and the home of Swarm” Individual-Based Models (http://www.red3d.com/cwr/ibm.html): an annotated list of links by Craig Reynolds (the author of the Boids model) EASSS 2009 - Torino – 3-4/9/2009

Quotes... “ What is MABS? It could be: Entertainment – a sort of intellectual computer game where one sets up an artificial system with lots of agents and then play with it to see what sort of effects one can get; Art – MAS designed and/or constructed for others to admire and enjoy; Illustration – multi-agent systems designed to animate or otherwise illustrate some sociological, philosophical or mathematical principle, in other words, a sophisticated pedagogic tool; Mathematics – using simulation as a stand-in for symbolic deduction in distributed systems where such deduction is impractical; Communication – multi-agent systems as an interactive medium for social exploration, negotiation and communication; or Science – multi-agent systems as a tool for understanding observed systems. All of the above are legitimate uses of multi-agent systems. Each has different goals. Each has different roles in society. Each has different criteria for success. It is not obvious that academics who attend MABS workshops have decided what MABS is. Indeed, it is not immediately obvious that there is a need to decide – these different activities can have much to contribute to each other.” [Bruce Edmonds, The Use of Models - Making MABS More Informative, MABS 2000] EASSS 2009 - Torino – 3-4/9/2009

Quotes... “ New approaches based on modeling individual objects, agents, and particles take a very different view of probability than that used in more traditional transport and traffic models. We now have sufficient knowledge of the limits to prediction in human systems to know that the search for total predictability is a myth.” [Mike Batty, Editorial, Environment and Planning B: Planning and Design 2001, volume 28, no.3] EASSS 2009 - Torino – 3-4/9/2009

Agent-based modeling and simulation tutorial - EASSS 2009 - Giuseppe Vizzari

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