Agent Cooperation

Protocols with a large number of states can become crucial for agent communication because of possible computation problems as well as due its decreased flexibility for agents [Chaib-draa et al., 2006].

1.2.4.2 Agent Cooperation

Especially in environments with a lot of cooperation between participants agent tech-nology can map emerging requirements and patterns because of their ability to cooper-ate with themselves [Kargl et al., 1999]. This cooperation between entities can be the largest context of interaction [Dumke et al., 2000]. Mentionable aspects of this activ-ity are the coordination of actions, the degree of parallelism, the sharing of resources, system robustness, the non-redundancy of actions as well as the non-persistence of con-flicts. Cooperation indicators are increasing individual and group survival capacity, performance improvement and conflict resolution. Therefore a usual definition of coop-eration is given as [Ferber, 1999]:

Definition 10 Cooperationis collaboration, coordination of actions and the resolution of conflicts.

Cooperation is mainly implemented due to several desires. That includes the re-duction of communication costs that are associated with a central problem solver, the improvement of performance through parallelism, increased reactivity because of not needed consultancy of a central problem solver and the improved robustness by reduced dependencies [Hayzelden and Bigham, 1999a]. Therefore the addition of new agents should lead to an increased performance of the group and their performed actions should solve or avoid actual or potential conflicts [Ferber, 1999].

Cooperation methods are classifiable into six categories [Ferber, 1999]. Correlating problems, techniques and objectives are visualised in figure 1.18.

28 1 Introduction

Figure 1.18:Characteristics of cooperation in agent-based organisations (cp. [Ferber, 1999])

◦ Grouping and multiplication: Grouping is a natural phenomenon that describes a more or less homogeneous unit that emerges from physical closeness or the existence of a communication network. It is the basis for specialisation and supports learn-ing. Multiplication comprises several advantages in situations that benefit from pure quantity of individuals, resources or skills. Overall performance and reliability can be increased without increase of individual productivity.

◦ Communication:This aspect is the base of every other cooperation. It connects the individuals of the agent society either by explicit messages or signals in the environ-ment.

◦ Specialisation:Specialisation is a process of adaptation towards specific tasks. This special performance increase has as a trade-off the decreased ability to perform other

for an overall task.

◦ Collaborating by sharing tasks and resources: Collaboration is one of the inten-tions of communication. It requires a general goal to be achieved. To keep it, a dis-tributed allocation of tasks, information and resources is needed [Dumke et al., 2000].

◦ Coordination of actions: Coordination in MAS is needed due to several reasons.

That includes prevention from confusion, the meeting of global constraints, special-ity of agents and depending sub-actions [Hayzelden and Bigham, 1999a]. Mainly the reasons evolve from the fact of a missing global view on the complete problem. They need further information and services to get their local problem solutions that are intended to subsume to the global solution. That needs to be arranged in a reason-able way. Coordination can be achieved by synchronisation, planning, reaction and regulation.

◦ Conflict resolution by arbitration and negotiation: These to approaches are used to minimize decrease of system performance due to conflicts between individual agents. Arbitrations lead to behavioural rules whose concern is to restrict conflicts and preserve the society of agents.

In the following we want to briefly define the main parts of cooperation after Ferber.

Collaboration Agents collaborate, when they are working together. Collaboration techniques are those that distribute tasks, information and resources among agents in the advancement of a common labour. Such a distribution can be centralised by coordina-tion agents or decentralised by offering supplies and demands. Distributed approaches itself may base on the market principle or on mutual representations of the agents’ ca-pacities [Ferber, 1999].

Definition 11 Collaboration is the collective solution of a problem or the collection processing of a task by a society of agents.

The adressed advantages of agent collaboration like increased processing speed and robustness are ’paid’ by trade-offs related to overheads in terms of team formation and collaboration, agent communication and team maintenance [Wilsker, 1996]. Some exemplary multi agent collaboration strategies are:

◦ Joint Intentions model of Cohen and Levesque [Cohen and Levesque, 1990], [Cohen and Levesque, 1991]

◦ SharedPlan model of collaboration [Grosz and Sidner, 1990]

◦ Planned Team Activity by Kinny [Kinny et al., 1994]

◦ Commitment based on agents’ mental states and relationships by Castelfranchi [Castelfranchi, 1995]

◦ Responsibility delegation by Matsubayashi [Matsubayashi and Tokoro, 1993]

◦ Team formation after Tidhar [Tidhar et al., 1992]

30 1 Introduction

Coordination Coordination is the next part of interaction following Ferber’s widely accepted definition. Main research and definition approaches base, among others, on [Malone and Crowston, 1994], [Wegner, 1996] and [Gelernter and Carriero, 1992].

Definition 12 Coordination is the management of interaction and dependencies be-tween certain agents [Omicini et al., 2001].

Coordination techniques can be classified [Nwana et al., 1997] as:

◦ Organisational structuringby defining an interaction framework with roles, com-munication paths and authority relationships.

◦ Contractingby using manager agents for problem decomposition and task assign-ment.

◦ Multi-Agent planningby a centralised or distributed planning of interaction to avoid conflicting actions.

◦ Negotiationby interaction to reach a mutually accepted agreement.

Within coordination techniques and strategies, agents may serve as coordination components [Papadopoulos, 2001]. A corresponding approach is a facilitator/mediator where the agent provides services and thereby satisfies requests of other agents. Bro-ker agents also satisfy request, but by provding third-party services. A special look-up service (yellow pages) is provided by matchmaker agents. Repository agents managing requests for other agents follow the blackboard approach. The management and conduc-tion of communicaconduc-tion for other agents in a well defined area leads to a job descripconduc-tion of a local area coordinator agent. Cooperation domain servers are agents providing facilities to access shared information and to subscribe, exchange messages.

Conflict Resolution Classically conflicts are seen as disturbances within a MAS [Tessier et al., 2001]. On a conceptual level exist resource conflicts and knowledge conflicts. The first type can occur when resource, like processing time, is involved.

Knowledge conflicts arise when the agents’ information differ. Contradiction between propositions is the one that is most dealt with.

Definition 13 Aconflictis a subset of all propositional attitude(s) (e.g. beliefs, desires, intentions, hopes, etc.) of the agent that must be reduced by removing a propositional attitude [Tessier et al., 2001].

In other words - it is a situation with incompatible or exclusive attitudes. Appropriate approaches either try to anticipate, solve or avoid them, otherwise conflicts remain unsolved and change agents’ behaviours or enrich agents’ knowledge. Following [Aïmeur, 2001] the three modes for conflict resolution are

◦ Negotiation: as a discussion procedure to reach a common agreement between the involved parties,

◦ Mediation: as a negotiation with a neutral part that facilitates the solution research,

◦ Arbitration: as the decision of a solution by a neutral part.

the transition from a situation with conflicting agent attitudes to a situation with less or no conflicting agent attitudes.