A Multi-Agent System for Information Semantic Sharing

Abstract

AOIS is a multi-agent system that supports the sharing of information among a community of users connected through the Internet. In respect to Web search engines, this system enhances the search through domain ontologies, avoids the burden of publishing the information on the Web and guaranties a controlled and dynamic access to the information. The use of agent technologies has made the realization of three of the main features of the system straightforward: i) filtering of information coming from different users, on the basis of the previous experience of the local user, ii) pushing of new information that can be of interest for a user, and iii) delegation of access capabilities, on the basis of a reputation network, built by the agents of the system on the community of its users.

1 Introduction

Nowadays, the Web is the most powerful means for getting information about any kind of topic. However, the Web assigns a passive role to the large part of its users. Therefore, when Internet must be used to allow the active sharing of information among the members of a community, the use of a peer-to-peer solution may provide several advantages [ 19 ].

This paper presents a system, called AOIS (Agents and Ontology based Information Sharing), trying to couple the features of peer-to-peer and multi-agent systems. The next section introduces related work on multi-agent systems for information retrieval. Section three describes the main features and the behaviour of the AOIS system. Section four describes how this system has been designed and implemented by using some well-known technologies and software tools. Section five briefly discusses the testing of the system. Finally, section six reports some concluding remarks, gives a short introduction about the first experimentation of the system and presents our future research directions. Multi-agent systems have always been considered one of the most important ingredients for the development of distributed information management systems and for proving the different services needed in such systems [ 12 ]. In particular, several interesting works demonstrate: how multi-agent systems are a suitable means for the management of information in a community of users, how they can take advantage of a peer-to-peer network for performing a distributed search of information and how the use of ontologies and user profile allows an improvement of the quality of their work.

DIAMS is a multi-agent system that provides services for users to access, manage, share and learn information collaboratively on the Web [ 5 ]. DIAMS can be considered one of the most complete multi-agent infrastructures for the management and retrieval of information in a community of users. In fact, it supports the searching and retrieval of the information from local and/or remote repositories and it encourages the collaboration among its users by supporting the sharing and exchange of information among them.

ACP2P (Agent Community based Peer-to-Peer) is an information retrieval system that uses agent communities to manage and search information of interest to users [ 16 ]. In the ACP2P system, an agent works as a delegate of its user and searches for information that the user wants by coupling the typical propagation of the query on the peer-to-peer infrastructure. It supports the community with the identification of the agents that may have such information through the use of the experience gained in its previous interactions. The experimental results of the use of the ACP2P system demonstrated that the use of the agent experience provides a higher accuracy in retrieving information.

CinemaScreen is a recommender system, which combines collaborative filtering and content-based filtering [ 22 ]. The first method requires matching a user with other users with similar behaviours and interests. The second method requires matching the items on the basis of their characteristics (CinemaScreen, in particular, deals with genres, actors, directors etc.). While both mechanisms exhibit weaknesses in particular situations, their combination allows better performances since the very beginning of the system activity. The system is built in the form of an intelligent agent, but apparently it is modelled as an essentially centralized system.

On the other hand, pSearch is a decentralized information retrieval system [ 26 ]. In this system, which is P2P but non-flooding, document indices are distributed through the network according to a classification of document content. The document semantics is generated and managed through a technique called Latent Semantic Indexing [ 29 ]. The resulting system is proven to be efficient in the number of nodes to contact to perform a search.

In [ 10 ] a social resource sharing system is presented. In this case, it uses a form of lightweight knowledge representation, called folksonomy, is used. In fact, the conceptual structures of ‘taxonomy’ are created bottom-up by ‘folks’, thus creating an emergent semantics, instead of using the more rigid approach of the traditional Semantic Web

Sanchez and his colleague proposed an integrated agent-based ontology-driven multi-agent system that automatically retrieves Web pages that contain data relevant to the main concepts of a specific domain [ 23 ]. The multi-agent system is based on the use a Web-based ontology learning method able to automatically build ontologies for any domain [ 17 ], and then on a set of agents that use such ontologies for the retrieval, filtering and classification of information.

3 AOIS

AOIS (Agents and Ontology based Information Sharing) is a multi-agent system composed of different agent platforms connected through the internet that supports the sharing of information among a community of users. Each agent platform acts as a “peer” of the system and is based on five agents: a personal assistant, a repository manager, an information finder, an information pusher, and a directory facilitator.

A personal assistant (PA) is an agent that allows the interaction between the AOIS system and the user. This agent receives the user’s queries, forwards them to the available information finders and presents the results to the user. Moreover, a PA allows the user to be informed about the new information that other users made available and that may be of her/his interest. Finally, a PA maintains the information that a user may share allowing her/him to add and remove information in a repository where information is partitioned on the basis of the topics of interest of the user.

A repository manager (RM) is an agent that builds and maintains both the indexes for searching information and the ontologies describing the topics of interest of its user. Each time the user adds or removes some information, the OM updates the corresponding index and ontology.

An information finder (IF) is an agent that searches information on the repository contained into the computer where it lives and provides this information both to its user and to other users of the AOIS system. An IF receives users’ queries, finds appropriate results, on the basis of both the queries and the topic ontology, and filters them on the basis of its user’s policies (e.g., the results from non-public folders are not sent to other users).

An information pusher (IP) is an agent that monitors the changes in the local repository and pushes the new information to the PA of the users whose previous queries match such information.

Finally, the directory facilitator (DF) is responsible to register the agent platform in the AOIS network. The DF is also responsible to inform the agents of its platform about the address of the agents that live in the other platforms available on the AOIS network (e.g., a PA can ask about the address of the active IF agents).

The exchange of information among the users of an AOIS system is driven by the creation of both a search index and an ontology for each topic. The search index allows the ranking of information on the basis of the terms contained in a query. The ontology allows to identify additional information on the basis of the terms contained in the ontology that have some semantic relationships (i.e., synonyms, hyponyms, hypernyms, meronyms and holonyms) with the terms contained in the query. Both the search index and the ontology are automatically built by the RM on the basis of the information stored in the topic repository.

The following subsections describe the behaviour of the AOIS system through five practical scenarios and introduce a detailed description of how security and privacy are managed to show how AOIS copes with the problems of working in a real open community.

3.1 Information Searching Scenario

The first scenario describes how a user can take advantage of the agents of the AOIS system for searching information. This scenario can be divided in the following five phases (see also figure 1): 1) a user requests her/his PA to search information on the basis of a topic, a set of keywords. The PA asks the DF for the addresses of available IF agents and sends the topic and the keywords to such agents (information search request phase); 2) each IF checks if the querying user has the access to at least a part of the information about the topic stored in the corresponding topic repository, and, if it happens, searches the information on the basis of both the received query and a set of additional queries obtained by replacing each keyword of the received query with the possible substitutes contained in the topic ontology. Moreover, the IF sends the received query to the local IP and RM agents: the IP adds the query to the profile of the corresponding remote user and the RM add the query keywords to the list of the keywords for updating the repository ontology (information search execution phase), 3) each IF filters the searching results on the basis of the querying user access permissions of the querying user sends the filtered list of results to the querying PA (information filtering and sending phase); 4) the querying PA orders the various results as soon as it receives them, omitting duplicate results and presents them to its user (information presentation phase); 5) after the examination of the results list, the user can ask her/his PA for retrieving the information corresponding to an element of the list. Therefore, the PA forwards the request to the appropriate IF, waits for its answer and presents the information to the user (information retrieval phase).

3.2 Information Pushing Scenario

The second scenario illustrates how a user can take advantage of the AOIS system to be aware about the availability of new information of her/his interest. This scenario can be divided in the following five phases (see also figure 2): 1) a user requests her/his PA to add some information in a specific topic repository and the PA propagates the request to the RM (information addition request phase); 2) the RM adds the information in the repository, updates the indexes for the searching of information and then informs the IP about the new information (information addition phase); 3) the IP checks if such new information satisfy some queries maintained in the profiles of the remote users and, when happens, then the IP either sends such information to the PA of the remote user (if the corresponding AOIS platforms are alive), or maintains such an information until such a platform becomes alive again. (information pushing phase); 4) Of course, when a PA receives a list of pushing results, it presents them to its user (information presentation phase); 5) after the examination of the results list, the user can ask her/his PA for retrieving the information corresponding to an element of the list. Therefore, the PA forwards the request to the appropriate IF, waits for its answer and presents the information to the user (information retrieval phase).

3.3 Repository Creation Scenario

The third scenario illustrates how a user can take advantage of the AOIS system for the creation of a repository for maintaining the information about a specific topic. This scenario can be divided in the following four phases: 1) a user requests her/his PA to create a repository for a specific topic indicating the set of terms (named ontology top terms) that describe such a topic and listing a set of information to store in the repository. The PA propagates the request to the RM (repository creation request phase); 2) the RM creates the repository, builds the topic ontology finding the semantic relationships (i.e., synonyms, hyponyms, hypernyms, meronyms and holonyms) among the top terms, adds the set of information, builds the search indexes and then informs the PA about the creation (repository creation phase); 3) the PA asks its user if she/he wants to populate the ontology with terms extracted from the information stored in the repository and the maximum permitted semantic distance between a new and a top term (ontology population request phase); 4) If the user enables the operation, the PA asks the RM to analyse the repository search indexes for finding the terms that are in direct or indirect relations with the top terms of the ontology. Of course, in the case of indirect relationship, each new term is only added if it satisfies the maximum semantic distance constraint (ontology population phase).

3.4 Repository Updating Scenarios

The forth and fifth scenarios illustrate how a user can take advantage of the AOIS system for updating both the search indexex of a repository and the related ontology.

The forth scenario is driven by the user that wants to add some information to a repository. This scenario can be divided in the following four phases: 1) the user requests her/his PA to add some information to a repository and the PA propagates the request to the RM (information addition request phase); 2) the RM adds the information in the repository, updates the indexes for the searching of information and then informs the PA about the new information (information addition phase); 3) the PA asks its user if she/he wants to populate the ontology with terms extracted from the new information added in the repository and the maximum permitted semantic distance between a new and a top term (ontology updating request phase); 4) If the user enables the operation, the PA asks the RM to analyse the repository search indexes for finding the terms that are in direct or indirect relations with the top terms of the ontology. Of course, in the case of indirect relationship, each new term is only added if it satisfies the maximum semantic distance constraint (ontology population phase).

The fifth scenario starts when the user logs to the system and her/his RM has some new keywords coming from last remote users queries. This scenario can be divided in the following two phases: 1) when the user logs the system, the PA gets the new keywords from the RM and asks its user if she/he likes to add some of them as top terms of the ontology (keywords selection request phase); 2) if the user selected some of the keywords to populate the ontology, the PA asks the RM to update the ontology finding the semantic relationships (i.e., synonyms, hyponyms, hypernyms, meronyms and holonyms) among the new and the old top terms (ontology updating phase); 3) Then the PA asks her/his user if she/he wants to populate the ontology with terms extracted from the information stored in the repository and the maximum permitted semantic distance between a new and a top term (ontology population request phase); 4) If the user enables the operation, the PA asks the RM to analyse the repository search index for finding the terms that are in direct or indirect relations with such new top terms of the ontology. Of course, in the case of indirect relationship, each term is only added if it satisfies the maximum semantic distance constraint (ontology population phase).

3.5 Security and Privacy Management

The information stored into the different repositories of a AOIS network is not accessible to all the users of the system in the same way. In fact, it’s important to avoid the access to private documents and personal files, but also to files reserved to a restricted group of users (e.g.: the participants of a project). The AOIS system takes care of users’ privacy allowing the access to the information on the basis of the identity, the roles and the attributes of the querying user, as defined into a local knowledge base of trusted users. In this case, it is the user that defines who and in which way can access to her/his information, but. Moreover, the user can also allow grant the access to unknown users by enabling a certificate based delegation, built on a network of the users registered into the AOIS community. In this sense, the system completely adheres to the principles of trust management. For instance, if the user Ui enables the delegation and grants to the user Uj the access to its repository with capabilities C0 to the user Uj, and Uj in turn grants to the user Uk the access to its the repository with the same capabilities C0 to the user Uk, then Uk can access Ui‘s repository with the same capabilities of Uj.

The definition of roles and attributes is made in a local namespace, and the whole system is, in this regard, completely distributed. Local names are distinguished by prefixing them with the principal defining them, i.e. a hash of the public key associated with the local runtime. Links among different local namespace, again, can be explicitly defined by issuing appropriate certificates. In this sense, local names are the distributed counterpart of roles in role based access control frameworks [ 13 ]. This model is centred on a set of roles. Each role can be granted a set of permissions, and each user can be assigned to one or more roles. A many to many relationship binds principals and the roles they’re assigned to. In the same way, a many to many relationship binds permissions and the roles they’re granted to, thus creating a level of indirection between a principal and his access rights. Like roles, local names can be used as a level of indirection between principals and permissions. Both local names and roles represent at the same time a set of principals and a set of permissions granted to those principals. But, while roles are usually defined in a centralized fashion by a system administrator, local names, instead, are fully decentralized. This way, they better scale to Internet-wide, peer-to-peer applications, without loosening in any way the principles of trust management.

In AOIS, the user can not only provide the permission to access his own files, but can also assign the permission to upload a new version of one or more existing files. In this case the PA informs his/her user about the updated files the first time he/she logs in. This functionality, together with the trust delegation, can be useful for the members of a workgroup involved in common projects or activities

4 Implementation

The AOIS system has been designed and implemented taking advantage of agent, peer-to-peer, information retrieval and security management technologies and, in particular, of five main components: JADE [ 3 ], JXTA [ 8 ], Nutch [ 1 ], WordNet [ 15 ] and JAWS [ 24 ].

AOIS agent platforms have been realized by using JADE [ 3,4,27 ]. JADE is probably the most known agent development environment enabling the integration of agents and both knowledge and Internet-oriented technologies. Currently, JADE is considered the reference implementation of the FIPA (Foundation for Intelligent Physical Agents) specifications [ 7 ]. In fact, it is available under an LPGL open source license, it has a large user group, involving more than two thousands active members, it has been used to realize real systems in different application sectors, and its development is guided by a governing board involving some important industrial companies.

The JADE development environment does not provide any support for the realization of real peer-to-peer systems because it only provides the possibility of federating different platforms through a hierarchical organization of the platform directory facilitators on the basis of a priori knowledge of the agent platforms addresses. Therefore, we extended the JADE directory facilitator to realize real peerto-peer agent platforms networks thanks to the JXTA technology and thanks to two FIPA specifications for the Agent Discovery Service and for the JXTA Discovery Middleware.

JXTA technology [ 8 ] is a set of open, general-purpose protocols that allow any connected device on the network (from cell phones to laptops and servers) to communicate and collaborate in a peer-to-peer fashion. The project was originally started by Sun Microsystems, but its development was kept open from the very beginning. JXTA comprises six protocols allowing the discovery, organization, monitoring and communication between peers. These protocols are all implemented on the basis of an underlying messaging layer, which binds the JXTA protocols to different network transports.

FIPA has acknowledged the growing importance of the JXTA protocols, and it has released some specifications for the interoperability of FIPA platforms connected to peer-to-peer networks. In particular, in the “FIPA JXTA Discovery Middleware Specification” a Generic Discovery Service (GDS) is described, to discover agents and services deployed on FIPA platforms working together in a peer-to-peer network. AOIS integrates a JXTA-based Agent Discovery Service (ADS), which has been developed in the respect of relevant FIPA specifications to implement a GDS. This way, each AOIS platform connects to the Agent Peer Group, as well as to other system-specific peer groups. The Generic Discovery Protocol is finally used to advertise and discover df-agent-descriptions, wrapped in Generic Discovery Advertisements, in order to implement a DF service, which in the background is spanned over a whole peer group.

Even if there are some specific tools and software libraries for searching information in a local repository (see, for example, Beagle [ 2 ] and Google Desktop Search [ 9 ]), we adapted Nutch [ 1 ], an open source web-search software, for searching the local repository. It has been done because it is very easy to develop Nutch plugins for extending its capabilities (we used this feature for using its term extraction module for building the topic ontologies) and because is available a Nutch plugin, that extends keywords based search through the use of OWL ontologies [ 28 ]. Figure 3 shows a graphical description of the work done by the Nutch core software and by its two plugins for indexing and building the topic ontologies and for using them for searching information.

As introduced above, topic ontologies are built by a Nutch plugin. This plugin receives the terms extracted from the information to be indexed by the Nutch software. Then, accessing the WordNet lexical database [ 15,20 ] though the use of the JAWS Java software library [ 24 ], for each term it identifies the top terms of the ontology and the other terms extracted from the information that have some semantic relationships (i.e., synonyms, hyponyms, hypernyms, meronyms and holonyms). At the end of this process, all the terms that have a semantic distance greater than the one fixed by the user are removed and then the WordNet ontology is saved as an OWL file.

As introduces before, authentication and authorization are performed on the basis of the local knowledge base of trusted users, though they can be delegated to external entities through an explicit, certificate based delegation. In this sense, the system completely adheres to the principles of trust management. The definition of roles and attributes is also made in a local namespace, and the whole system is, in this regard, completely distributed. Local names are distinguished by prefixing them with the principal defining them, i.e., an hash of the public key associated with the local agent platform. Links among different local namespace, again, can be explicitly defined by issuing appropriate certificates. The theory of AOIS delegation certificates is founded on SPKI/SDSI specifications [ 6 ], though the certificate encoding is different. As in SPKI, principals are identified by their public keys, or by a cryptographic hash of their public keys. Instead of s-expressions, AOIS uses XML signed documents, in the form of SAML assertions [ 18 ], to convey identity, role and property assignments. As in SPKI, delegation is possible if the delegating principal issues a certificate whose subject is a name defined by another, trusted, principal. The latter can successively issue other certificates to assign other principals (public keys) to its local name. In this sense, local names act as distributed roles [ 13 ].

Finally, the extraction of a digest for each search result is required to avoid the presentation of duplicate results to the user. This feature is provided by a Java implementation of a hash function [ 20 ]. 5

Testing

Practical tests on the first prototype of the AOIS system were done installing the system in different labs and offices of our department asking some students and colleagues to use it for sharing and exchanging information. Moreover, we tested the system setting some computers of a Lab with different access policies and distributing information on their repositories providing, in some cases, different copies of the same information on different computers. The tests covered with success all the system features and the searching and pushing of information satisfied our expectations.

Moreover, a part of the experimentation was oriented to compare the results of the searching and pushing operations based on the use of topic ontologies with the ones based only on the use of keywords and what happened is that: i) the use of topic ontologies increases the number of results, but very few were of no interest for the users if, in particular, the users chose a good set of top terms.

Up to now, we do not perform a numeric analysis of the results, but only a qualitative analysis derived from a discussion with the people that used the system. The main result is that usually the quality of search and pushing operations mainly depends on an appropriate set of top keywords. Therefore, the goodness of an ontology usually does not depends on the keywords extracted from the information of the repository, but mainly depends on an appropriate initial set of top keywords and then by the introduction of the other appropriate keywords coming from the queries of remote users.

6 Conclusions

In this paper, we presented a peer-to-peer multi-agent system, called AOIS (Agents and Ontology based Information Sharing), that supports the sharing of information among a community of users connected through the Internet. AOIS is an evolution of a previous system [ 14 ], called RAIS (Remote Assistant for Information Sharing), that performed a similar task, but was implemented by using a different search technology (i.e., Google Desktop Search) and did not take advantage of topic ontologies for the search of information.

AOIS derives a large part of its features from the systems for information sharing described in the related work section. However, it offers a new feature that seems to improve the quality of search and pushing operations: the creation of a topic ontology through the use of a set of initial terms (i.e., the top terms), its automatic extension through the information maintained by the user, the possibility of controlling the semantic distance from the top terms and the terms automatically added, and, in particular, the possibility of using the terms contained in the queries of the other users for refining the ontology, allow the construction of high quality ontologies. Then, a topic ontology can be customized by each user, but taking into account of the implicit suggestions of the other users of the community, Moreover, its implementation based on some well-known software tools guarantees good performance and reliability.

The successful experimentation encouraged us in the further development of the system and we are currently working on extending the system with new features.

A current activity is oriented to improve the pushing operations. The current implementation of the system maintains in the remote user profiles all the queries she/he did. Often the information retrieved through some old queries might be not yet of interest for the remote user. Therefore, we are working on a more sophisticated technique for managing remote user profiles: all the queries are stored together with the time they were executed; every day the IP checks the remote user profiles and for all the queries that are older than a fixed duration (e.g., a week), it asks the relative PA about the interest of its user in maintaining such queries and refreshes the execution time for all the queries for which it receives a positive acknowledge.

The creation of topic ontologies may be a difficult activity because it requires the identification of an appropriate set of top terms and its completion through the use of an appropriate set of information. Therefore, the possibility of using the topic ontologies built by other users may be an important feature of the system. In fact, we are working to the possibility that PA agents can require some topic ontologies to other PA agents and then either directly use them for driving the search or build new topic ontologies by merging them with the local topic ontologies.

Beyond the definition of the top terms and of the maximum semantic distance between terms, users have not the possibility of managing the topic ontologies. But this would be a very important feature in the future, when the system will allow the use of topic ontologies defined by other users and the merging among different topic ontologies. In the current version of the system the topic ontologies are also saved as OWL files because the search ontology Nutch plugin requires an OWL file for proving an ontology based search. Therefore, users may manipulate topic ontologies by using one of the available tools for manipulating OWL ontologies (e.g., Protégé [ 20 ]). However, in the OWL view of the topic ontologies there is not information about the top terms. Therefore, we are developing a very simple graphical tools (based on the use of the Jung software library [ 11 ]) that: i) shows the graph defining an ontology, ii) distinguishes top terms from the other terms, iii) distinguishes the different kinds of semantic relationships among terms, iv) allows the introduction of new terms and the deletion of existing terms, v) allows the introduction and the deletion of the “top” attribute to any term, and vi) allow the modification of the maximum semantic distance (when such a distance is reduced, the tool removes all the terms that do not satisfy the new constraint).

Acknowledgments

This work is partially supported by the Italian

dell’Istruzione, dell’Università e della Ricerca).

Ministry

MIUR (Ministero

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