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
load-balancing, self-organization, fault-tolerance and the
ability to pool together large amount of resources, are some of
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.
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
The main application scenario considered in recent P2P data
sharing systems is that of ad-hoc querying: a user
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
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
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