Department of Electronic & Computer Engineering 
Intelligent Systems Laboratory                           Pdi2etP TECHNICAL UNIVERSITY OF CRETE 


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- Architecture                 - Routing                        - Security                       - Alert Features              - Super-peer Environment - Client-peer Environment
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P2P-DIET is a peer-to-peer service that unifies query and notification capabilities. The service has been implemented using the Open Source mobile agent system DIET Agent Platform developed in project DIET. Our service supports queries, subscriptions and notifications in a single unifying framework. P2P-DIET contains a fault-tolerance mechanism that guarantees connectivity, when nodes of the network fail or leave silently. The capability of location-independent addressing is supported, which enables the use of dynamic IP addresses. Because of this capability, nodes can disconnect and reconnect with a different address and at different parts of the network. The features of stored notifications and rendezvous guarantee consistency while clients disconnect. P2P-DIET has been used for the implementation of a file sharing peer-to-peer application to demonstrate its various features.

Peer-to-peer systems are currently a very popular alternative to centralized client-server systems. In its pure form, a peer-to-peer system has no servers and no functionality is centralized. All nodes of the network are equal peers. Adaptation, load-balancing, self-organization, fault-tolerance and the ability to pool together large amount of resources, are some of the benefits of peer-to-peer systems. In the last years, peer-to-peer systems have become the most popular way for users of the Internet to share huge amount of data.

Event notification systems are systems that allow a user to subscribe with a profile of interest or long-standing query so that he is notified when certain events of interest take place. In the file sharing scenario an event can be the action of a user to publish a file or subscribe with a profile. A profile is a long-standing query, that continues to produce results as time goes by and new resources are added to the system.

The main application scenario considered in recent P2P data sharing systems is that of  ad-hoc querying: a user poses a  query (e.g., ``I want MP3s by John Lennon") and the system returns a list of pointers to matching files owned by various peers in the network. Then, the user can go ahead and download files of interest. The complementary scenario of  selective dissemination of information (SDI) or  selective information push [Franklin and Zdonik,1998] has so far been considered by very few P2P systems [Carzaniga et. al., 2001; Koubarakis et. al., 2002; Pietzuch and Bacon, 2002; Gedik and Liu, 2003]. In an SDI scenario, a user posts a  profile or  continuous query to the system to receive notifications whenever certain events of interest take place (e.g., when a video-clip of John Lennon becomes available). SDI can be as useful as ad-hoc querying in many target applications of P2P networks ranging from file sharing, to more advanced applications such as alert systems for digital libraries, e-commerce networks etc.

Agent-based computing offers many desired characteristics for implementing a peer-to-peer system that supports queries, profiles and notifications. The service has been implemented using the mobile agent system DIET Agent Platform developed in project DIET. DIET stands for Decentralised Information Ecosystem Technologies. DIET is a 5th Framework project funded by the European Commission under the Future and Emerging Technologies area [European-Commission,  DIETWEB]. One of the goals of the project was the design of a multi-agent platform that is open, robust, adaptive and scalable based on an ecosystem-inspired approach. In the first year of DIET such a multi-agent platform has been developed called DIET Agent Platform [Marrow et. al., 2001; Hoile et. al., 2002; DIETAgentswebsite]. Note that the DIET Agent Platform is available as Open Source.

There is a gap currently in the peer-to-peer systems literature since there are systems with only query capabilities, i.e., Gnutella [Gnutella] and systems with only profile/notification capabilities, i.e., SIENA [Carzaniga, 1998]. We designed P2P-DIET in response to the obvious need to unify these paradigms.


  1. Franklin and Zdonik, 1998 M. J. Franklin and S. B. Zdonik. Data In Your Face'': Push Technology in Perspective. Proceedings ACM SIGMOD International Conference  on Management of Data, 1998, 516-519.

  2. Carzaniga et. al., 2001 A. Carzaniga, D.S. Rosenblum and A.L. Wolf. Design and Evaluation of a Wide-Area Event Notification Service. ACM Transactions on Computer Systems, August, 2001.

  3. Koubarakis et. al., 2002 M. Koubarakis, T. Koutris, C. Tryfonopoulos and P. Raftopoulou. Information Alert in Distributed Digital Libraries: The Models, Languages and Architecture of DIAS. Proceedings of the 6th European Conference on Research and Advanced Technology for Digital Libraries (ECDL 2002), Rome, Italy, September, 2002.

  4. Pietzuch and Bacon, 2002 P. R. Pietzuch and J. Bacon. Hermes: A Distributed Event-Based Middleware Architecture. Proceedings of the International Workshop on Distributed Event-Based systems (DEBS'02), July 2-3, 2002, Vienna, Austria.

  5. Gedik and Liu, 2003 Bugra Gedik and Ling Liu. PeerCQ: A Decentralized and Self-Configuring Peer-to-Peer Information Monitoring System. Proceedings of the 23rd IEEE International Conference on Distributed Computer Systems. 2003, May, Providence, Rhode Island, USA.

  6. European-Commission European Commission IST Future and Emerging Tecnologies. Universal Information Ecosystems Proactive Initiative, 1999,  Available at

  7. DIETWEB DIET Home page,

  8. Marrow et. al., 2001 P. Marrow, M. Koubarakis, R.H. van Lengen, F. Valverde-Albacete, E. Bonsma, J. Cid-Suerio, A.R. Figueiras-Vidal, A. Gallardo-Antolin, C. Hoile, T. Koutris, H. Molina-Bulla, A. Navia-Vazquez, P. Raftopoulou, N. Skarmeas, C. Tryfonopoulos, F. Wang and C. Xiruhaki. Agents in Decentralised Information Ecosystems: The DIET Approach. Proceedings of the AISB'01 Symposium on Information Agents for Electronic Commerce, AISB'01 Convention, University of York, United Kingdom, March, 2001.

  9. Hoile et. al., 2002 C. Hoile, F. Wang, E. Bonsma and P. Marrow. Core specification and experiments in DIET: a decentralised ecosystem-inspired mobile agent system. Proceedings of the 1st International Joint Conference on Autonomous Agents & Multiagent Systems (AAMAS 2002), 2002, Bologna, Italy.

  10. DIETAgentswebsite DIET Agents Home page,

  11. Gnutella Gnutella website,

  12. Carzaniga, 1998 A. Carzaniga. Architectures for an Event Notification Service Scalable to a Wide Area Networks. Politecnico di Milano, 1998, PhD thesis, Italy.




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2003 Intelligent Systems Laboratory, TUC