The SIGIR Peer-to-Peer Information Retrieval Workshop
Jamie Callan
School of Computer ScienceCarnegie Mellon University
Pittsburgh, PA 15241, USAcallan@cmu.edu

Norbert Fuhr
Institute of Informatics and Interactive Systems
University of Duisburg-Essen
47048 Duisburg, Germany
fuhr@uni-duisburg.deIntroduction
Peer-to-peer (P2P) systems have emerged as a popular way to share huge volumes of data. The P2P
paradigm holds many promises: it fits naturally with the Internet, the universal knowledge andservice exchange medium; it favors scalability, by allowing the seamless plugging of data, services
and computational resources into the global system; it increases system resilience, by avoiding
unique points of failures; and it can speed up global access by distributing the indexing and query
processing tasks to multiple computing nodes. However, retrieval methods for P2P systems are stillin their infancy. P2P networks are prone to congestion when messages are not routed intelligently.
Many of the most effective routing or data placement methods developed recently rely on relatively
simple retrieval methods and homogeneous network environme nts.
This workshop focused on new methods of resource representation, resource selection, and data
fusion in peer-to-peer networks. The workshop particularly encouraged papers that addressed
heterogeneous peer-to-peer networks (e.g., a variety of data types and service providers), as well as
papers about methods that cope with partial and uncertain information. However, more broadly,
papers were solicited on any topic related to information retrieval in peer-to-peer networks. The
goals of the workshop were to discuss current retrieval methods of P2P systems, as well as theadaptation of distributed IR methods for P2P systems, and to involve both researchers from the P2Parea interested in IR methods as well as IR researchers aiming at extending their methods for P2Psystems.
The workshop was held on July 29, 2004 in Sheffield, the United Kingdom, immediately following
the SIGIR 2004 conference. Although the workshop was intended to appeal to a wide range of IRand P2P researchers, it primarily drew people with a background in IR. About 20 people
participated. The size was small enough to allow an interactive format and discussion between andduring presentations. Our impression is that most of the participants found it very productive.

Presentations
Below we provide very brief descriptions of the workshop presentations, to give a sense of therange of themes and topics covered. The complete workshop proceedings are available in electronic
form at http://p2pir.is.informatik.uni-duisburg.de/programme.html.

Christos Tryfonopoulos opened the workshop with a talk on Implementing Publish/Subscribe
Systems with Languages from Information Retrieval on Top of Structured Overlay Networks. The
goal of this project is to support both ad- hoc and publish/subscribe (information filtering, or SDI)
within a single P2P network. The network is hierarchical, and is based upon ideas from selforganizing networks and distributed hash tables. Hub nodes specialize in particular topics, and they
are where topic-specific filtering occurs. Message grouping reduces message traffic to subscribers,
and message traffic grows logarithmically. The system was evaluated initially using documentsfrom ResearchIndex.com, and synthetic filtering queries; more thorough evaluations are underway.
Paul-Alexandru Chita’s Knowing Where to Search: Personalized Search Strategies for Peers inP2P Networks described a distributed version of the popular Page Rank algorithm, and how it could
be personalized to represent an individual’s specific interests. The algorithm has been tested on
P2P networks of up to 150 hubs and 215,000 leaf nodes containing synthetic data; the retrieval
model was exact ma tch, and documents are returned until the desired number of documents isreached. The personalized Page Rank method was found to be better than alternatives such as Bestneighbor search.
Atip Asvanund ’s Content-Based Community Formation in Hybrid Peer-to-Peer Networks addressed
the problem of organizing peer-to-peer networks into virtual communities (or “clubs”, in theeconomic sense) so that nodes with similar content are grouped under the same hubs. The nodes in
his network are autonomous text-based digital libraries. The regional directory services (hubs) each
optimize their own utility functions, but in so doing the network gradually becomes more organized,
message traffic decreases, and Recall increases. The algorithm was tested on a testbed of 2,500
text-based digital libraries that is based on the TREC wt10g data.
Fabius Klemm talked about An Architecture for Peer-to-Peer Information Retrieval. His work is aformal, abstract architecture that can be operationalized to model specific P2P systems. It consistsof four layers: Transport/communication; structured overlay/network maintenance; document andcontent management; and retrieval models. The architecture is intended to help isolate and clarify
the roles and contributions of P2P and IR research in a hybrid system. The architecture has beenverified by mapping several well-known P2P systems onto it. The next steps involve greater focus
on retrieval models designed for use within P2P networks.
Sebastian Michel presented Bookmark-driven Query Routing in Peer-to-Peer Web Search. A peer’scontents are assumed to be topically similar, for example, acquired by focused crawling frombookmarks. Peers post summary information about their contents to a global directory. Individual
users can also post their bookmark lists, which are routed to peers with similar content. When a
new query is received, a query attempts to satisfy it locally; if it cannot, it acquires information fromthe centralized directory service about nodes with similar content, and then routes the query using
this information; it may also route the query to nodes that have previously published similar
bookmark lists. A prototype has been created, and the system is currently being evaluated.
Jie Lu’s Federated Search of Text-Based Digital Libraries in Hierarchical Peer-to-Peer Networks
continues her prior research on adapting text-based distributed IR techniques for P2P networks.
Each leaf node is an autonomous text-based digital library that uses best- match search. Hub nodes
are topically-organized directory services that route messages and merge ranked- lists returned byleaf nodes. Each hub node makes a local, query-specific decision about how many leaf nodes it will
forward the query to. Leaf nodes, hub nodes, and network neighborhoods are all represented byterms and term frequencies. Her research was evaluated with an extensive set of experiments on atestbed of 2,500 full-text search engines based on TREC wt10g data, 25 hub nodes, and 2 sets ofqueries. Her results suggest that good, full- text distributed IR techniques in a large P2P network
have Precision that is 15-20% lower than centralized search over the same data.
Henrik Nottelmann described A Logic-based Approach for Computing Service Execution Plans inPeer-to-Peer Networks. This research models a P2P heterogeneous network in which different
nodes offer different services (e.g., search, query expansion, query translation, …). The network isdynamic, so a dynamic method is required to form query-specific execution plans. Henrik’s
approach is to use DAML-S profiles to describe services, to use pDatalog to describe facts, and tochain together services into execution plans. Cost models can be introduced, for example
describ ing actual costs or preferences, to select optimal plans.
Mario Nascimento presented Taxonomy-based Routing Indices for Peer-to-Peer Networks. Whenthe network is based on a taxonomy, assigning documents to locations in the network, and routing
queries, become text categorization problems. Network nodes are assumed to be myopic, insofar as
the amount of detail they know about other nodes depends on the nodes’ distance in the network.
At each hop one level of detail in the taxonomy is lost; a node knows nothing beyond a horizon of 4hops (the depth of the taxonomy). Given a new query, a node rates its neighbors based on the likely
number of hops to matching documents (h-score) and an estimate of the number of documents downthat path (d-score). The query is routed along the top-scoring path. The approach was evaluatedwith synthetic data and shown to be effective.

Conclusions
One clear gap between the presentations and the workshop audience was the degree of realism in
the network designs and the evaluations. Most of the papers presented in the workshop studied
static networks, in which nodes don’t enter, leave, or change their contents over time. Most of thepapers had either no evaluation, evaluation with small networks, or evaluation with synthetic datathat is easily challenged. It is not unreasonable to want techniques that are tested on large, dynamic
peer-to-peer networks, using real data and real query streams, but these requirements are difficult
for most research groups to meet. This research area lacks a good method for modeling large
dynamic networks with reasonable hardware resources. It also lacks good datasets; although Webdata is easy to obtain, realistic networks require large query streams and relevance assessments.
One problem for the P2P IR community is that its task models are still very much undefined. The
most well-known uses of peer-to-peer networks are file-sharing networks, in which file naming
conventions are known or easily guessed, content is highly replicated across the network, and exactor partial- match retrieval is sufficient. The database research community studies an abstract versionof this task, in which the representation is a controlled-vocabulary, exact- or partial- match issufficient, and ranking isn’t much of an issue.

When asked about the advantages of peer-to-peer search for information retrieval tasks, theworkshop participants thought that text retrieval in P2P networks would be very appropriate forsmall-office environments, small family-and- friends networks, community-based search, and small,
ad-hoc networks; in other words, environments where a loosely organized group, each withmoderate information technology resources, wants to share a “small” number of documents. It isn’t
clear that these applications require research that is tested immediately on large-scale, very dynamicnetworks. There was also discussion about using peer-to-peer search for “hidden Web” typeproblems, e.g., to integrate search engines that don’t reveal their contents, or to provide morecurrent information than can be provided easily by crawling. This scenario use would clearlyrequire evaluation on large networks, but even in this setting the degree of dynamism in the network
is unclear; many large information providers have a relatively stable presence on the network.
We do not advocate one scenario over the other. We merely argue that because there is not yet
agreement on what the task models are for peer-to-peer Information Retrieval, it is difficult todefine evaluation methodologies that are widely accepted.
Perhaps because of significant research attention from the database community, peer-to-peer
research is an area where many battles from the past are being refought. There continues to be
disagreement about full- text vs. controlled- vocabulary representations; exact- match vs. best- match
retrieval; and ranking vs. not ranking results. Perhaps most jarring to an IR researcher, evaluation
on artificial datasets is common; real documents, queries, and relevance judgements aren’t required.
This workshop was a first attempt to bring together a diverse set of peer-to-peer researchers,
particularly from the IR and DB communities, to begin exchanging ideas. Although the audience
was primarily an IR audience, the papers spanned the set of issues that are currently of interest in
peer-to-peer research, thus partially achieving our goals. They indicate a few areas of agreement,
many areas of disagreement, the serious need for more clearly defined and accepted task models,
and the need for widely-accepted, reusable peer-to-peer testbeds. The workshop was a beginning,
but we hope that it is only the beginning, of a continuing dialog between the many corners of thepeer-to-peer research community.

Acknowledgements
We thank Henrik Nottelmann for his help in all aspects of preparing the workshop, including setting
up the web site, providing technical support to the PC, and compiling the workshop proceedings; itwould have been a much less professional effort without his help. We also thank the organizers of
the SIGIR conference for their support of the workshop. We especially thank them for locating theworkshop above the top floor served by the paternoster lifts; going “over the top” was an experiencethat most of the workshop participants will not forget.