At this step in the development process, the set of organizations composing the system, their roles and the associated communications have been identified and specified. The architectures of the various agents are also specified. The Holarchy Design activity is the last activity of the Agent Society design phase and aims at providing a global synthesis where previous activities work products are combined and summarized in a single work-product describing the overall structure of the system and the rules that will govern the dynamics of this structure.
In order to properly define the discussed aspects of each holon, the Holarchy Design activity is decomposed in four main tasks:
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Holonification task
In a few words, this task aims at mapping the previously identified hierarchy of organizations to an holarchy. This mapping is based on the association between holons composing the holarchy with the set of roles defined in the organization hierarchy they have to play. To build holarchies, organisations that composed the system are instantiated in form of groups. A set of previously identified agents compose the lowest level of the holarchies. A set of holons is then created at each upper level, each playing one or several roles in one or several groups in the level of interest. Composition relationships among super- and sub-holons are then specified according to the contributions required by the organizations (OID and ODD work products). In this activity, two points of view on the system are used to conceive the final system holarchy, each of these viewpoints corresponds to one dimension the holon concept :
- Horizontal: This step consists for each level of abstraction of the system in instantiating organizations of the same level in terms of groups. Then, for each of them a set of holons will be created, and associated to the roles it plays according to the results of the ODD and RCI activities.
- Vertical: This step aims at specifying the composition relationship between holons. It specifies how a group of holons of level n will contribute to the behavior of role played by a holon of level n + 1. This association is done according the contributions of the roles located at different levels of abstraction that have been specified in the ODD activity through the use of
capacity and service concepts. </ul>
Holon Government Model Definition task
The second task focuses on newly composed holons and aims at identifying a government type for each of them. The objective consists in describing the various rules used to take decisions inside each super-holon. Defining the holon government type essentially means defining the holon decision making process. For instance when an external holon is requesting its admission as a member, the decision to accept or refuse it should be taken according to a specific decision making mechanism that has to be defined (for instance, a voting mechanism may be used).
Two aspects of the decision making process should be analyzed:
- who is in charge of taking the decisions and how this happens (head, vote, etc);
- how the requesting process could be started (who is to be contacted by the external holon that wants to enter the super-holon or that is requesting a service).
The decision process for the admission of a new member is an example of decision process that fits most of the cases and for this reason we will mainly refer to that without loosing in generality. The decision can be done according to several different internal policies representing different levels of involvement of the holon member community: federation is positioned at one side of the spectrum, dictatorship on the opposite one. In the federation configuration, all members are equal when a decision has to be taken. Opposite to that, in dictatorship, heads are omnipotent; a decision taken by one of them does not have to be validated by any other member. In this government form, members loose most of their autonomy having to request the head permission for providing a service or requesting a collective action. Another possibility consists in establishing a voting mechanism for each functionality. Specific and interesting configurations can arise from the number of voters and the percentage of heads and peers involved in the decision-making process, because of their relevance it is worth to analyze them in details:
- Monarchy : the command is centralized and a Head is in charge. Monarchy, here, doesn’t refer to the process of Head’s nomination/election. The nomination process is a different issue from the decision-making process. Monarchy here describes the situation where only one head controls the entire decision-making process.
- Oligarchy : A little group of heads share the command without referring to the other (peer) members.
- Polyarchy : A little group of heads share the command but they have to refer to the Peer for some decisions.
- Apanarchy : command is completely shared between all members of the super-holon. Everyone takes part to the decision-making process.
Holarchy Definition task
All the previously described elements are merged in order to obtain the complete set of holons (composed or not) involved in the solution. In this way, the complete holarchy of the system is described. The results of this task are summarized in an organizational cheese-board, that is an extension of the cheese-board diagrams introduced in [1] integrating the holonic perspective. This diagram is then associated to a set of documents that describes the government of each holon and the rules governing their dynamics.
Holon self-Organization Mechanisms Definition task
The description obtained with the previous tasks is just the initial structure of the system, the last objective is now to specify holons’ self-organization mechanisms (creation, new member integration, scheduling policies for roles) in order to support a dynamic evolution of the system holarchy.
Because of space concerns, only the most common and important rules governing
holon dynamics are discussed in this part, mainly those dealing with
members recruitment and holon creation.
Once a super-holon has been created, new members may request to join it or
the super-holon itself may recruit new members to achieve its own goals (the
new member admission process is called Merging). In order to support the integration
of new members, a "standard" interface should be provided to external
holons to request their admission. A specific organization with two roles, StandAlone
(played by the candidate), and Representative (played by at least one
of the representative of holons members), has been designed to manage this
recruitment process.
As regards with the holon creation mechanism, it is important to study the motivations for the birth of a new holon; these can in fact either depend:
- on the need to satisfy in a collective way a requirement that cannot be accomplished by a single entity alone, or
- on the need to improve the internal structure of an existing holon that is becoming too big and whose tasks are too complex to be managed.
It is therefore possible to distinguish two different mechanisms:
- A top-down mechanism, the sub-division: a super-holon whose tasks are too complex, decides to create a set of internal organizations that are able to execute these tasks thus distributing the computational cost and breaking down the organization complexity. This case could be reduced to a specific case of the initial creation process, because newly created holons are configured to satisfy integration constraints with the super-holon.
- A bottom-up mechanism, the fusion (merging process): a set of holons decides to merge and to create a super-holon to satisfy a common goal. In this case, all rules that will govern the life of the new super-holon have to be defined.
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Goal
This activity aims at refining the solution architecture and it is composed of three main tasks: firstly the agentification task consists in identifying all the required holons and associating them with the set of roles they can play. This allows the identification of the set of capacities required by each holon thus giving precise indications on the architecture that should adopted for it. Indeed the holon architecture is at least defined by the set of roles that the holon should play, the minimal set of services that implement capacities required by its role.
The second task focuses on composed holons and aims at identifying a government
type for each of them. The objective consists in describing the various
rules used to take decisions inside each super-holon. At this point, all the previously
described elements are merged in order to obtain the complete set of holons
(composed or not) involved in the solution. In this way, the complete holarchy of
the system at the instance level is described.
The description obtained with the previous tasks is just the initial structure of
the system, the last objective is now to specify rules that govern holons’ dynamics
in the system (creation, new member integration, specific self-organization mechanisms,
scheduling policies for roles) in order to support a dynamic evolution of
the system holarchy.
Input
This activity is the final one of the agent society design phase. It aims at summarizing all the informations resulting from this phase in a single work product describing the initial structure of the system. The major input of this activity are the work products of organizations dependencies description and role constraints identification phases. These results will allow to determine for each holon its set of roles and the associated set of required services.
Output
The output of this phase describes the initial structure of the complete designed system. Each level of the holarchy and its set of groups have to be described. Each holon and agent with its associated set of roles is also reported on it.
MAS Meta-Model Elements
Define(Agent), Define(Holon), Relate(Agent, AgentRole), Relate(Holon, AgentRole), Relate(Holon, Holonic Organization), Define( Holonic Role), Define (Government rules).
References
- [1] From Agents to Organizations: an Organizational View of Multi-Agent Systems
- J. Ferber, O. Gutknecht, and F. Michel.
In AOSE-IV@AAMAS03, volume 2935 of LNCS, pages 214–230. Springer Verlag, march, 2004.
