Purpose

The Distributed Belief Revision Tested - DiBeRT is intended to study and model inherently distributed systems, with decentralised control, in which the available information is incomplete and dynamic, and where the time factor is relevant.

Methodologies

The decision of how and when to use external beliefs in an agent's knowledge base is fundamental to the characterisation of a distributed belief revision system. The DiBeRT agents act in "good faith" and exchange messages using the direct message passing mechanism, thus guaranteeing, that the information received by a recipient agent is, not only, relevant for its activity, but also, truthful from the sender's perspective. A wide range of different methodologies for the use of incoming beliefs can be adopted by the recipient agents: unconditional use, conditional use, rejection, etc.. From this spectra DiBeRT has chosen two policies for the local use of communicated beliefs:

Local evaluation of the shared propositions - the local beliefs prevail over the communicated beliefs, i.e., the use of an external belief is conditioned by the existence or absence of the belief in the agent's local knowledge base. Once the local agent perceives or infers its own local perspective about a shared proposition, the external points of view regarding the shared proposition are ignored. In the absence of locally deduced belief  the existing external beliefs will be used. A shared proposition is represented by as many nodes as there are agents with beliefs concerning the shared proposition regardless of whether the local agent considers, in the absence of its own local view, all of the external perspectives or, in the presence of its own perspective, uses exclusively its own local view;

Global evaluation of the shared propositions - every communicated belief is unconditionally used by the recipient agent. A shared proposition is represented by as many nodes as there are agents with beliefs concerning the shared proposition and every perspective is considered by the group of agents involved.

Upon accepting a set of external beliefs, an agent may find itself with conflicting belief status for the same proposition. In such circumstances which belief status to adopt? In DiBeRT three synthesis criteria were implemented guaranteeing the attribution of an unique belief status to every shared proposition:

Consensus synthesis criterion (CON) - a shared proposition is believed, if and only if, it is believed by every agent that share the proposition;

Majority synthesis criterion (MAJ) - a shared proposition is believed, if and only if, it majority of the agent that share the proposition believe in the proposition;

At least one synthesis criterion (ALO) - a shared proposition is believed as long as there is some agent where it is believed.

These synthesis criteria reflect different levels of demand: in the case of the ALO synthesis, the belief in a shared proposition by one of the involved agents is enough to make it believed by the system, in the case of the MAJ synthesis, only if the majority of the agents believed in the proposition will the shared proposition be believed by the system, while, in the case of the CON synthesis, only the consensus among the involved agents will make the shared proposition believed by the system.

Functionalities

A test bed for the evaluation and validation of different distributed consistency maintenance methodologies has to be reusable, and user friendly. In DiBeRT, the user is asked, at launch time, to select from the available set of agents the sub-set to be run, the synthesis criterion to be applied, and the level of consistency desired. One of the following six available distributed consistency modes for the shared beliefs can be selected for execution:

1 - Local evaluation and consensus belief status synthesis criterion;

2 - Local evaluation and majority belief status synthesis criterion;

3 - Local evaluation and at leat one belief status synthesis criterion;

4 - Global evaluation and consensus belief status synthesis criterion;

5 - Global evaluation and majority belief status synthesis criterion;

6 - Global evaluation and at leat one belief status synthesis criterion.

The private beliefs consistency level is unique: they are locally consistent. After launching the community of multi-agents, the interaction between DiBeRT and the User is performed by a specialised agent called User Interface Agent. The User Interface Agent architecture is identical to the remaining system agents, being the intelligent system role played by User. This interface allows, during runtime, the:

- Addition of new assumptions;

- Management of the multiple contexts;

- Assignment of specific belief status to any belief;

- Querying the system about any belief.