XPeer : A Self-Organizing XML P2P Database System

Abstract

This paper describes XPeer, a zero-administration system for sharing and querying XML data. The system allows users to share XML data without significant human intervention, and to pose XQuery FLWR queries against them.The proposed system can be used in any application field, being a general purpose XML p2p DBMS, even though its main application is the management of resource descriptions in GRID environments. This work was partly funded by the FIRB GRID.IT project. Our Contribution This paper describes a zero-administration p2p system for sharing and querying XML data (XPeer). The system allows users to share XML data and to pose XQuery FLWR queries against them without any significant human intervention (the user still has to write her own queries). The system, based on a hybrid p2p architecture, self-organizes its superpeer network, and allows for arbitrary changes in the network topology. Paper Outline The paper is organized as follows. Section 2 describes some important issues that emerge in the management of p2p XML databases. Section 3, then, presents an overview of the system, while Section 4 illustrates the system architecture in more detail. Section 5, next, outlines the techniques used in XPeer for processing queries. Section 6, then, discusses some related works. In Section 7, finally, we draw our conclusions and describe some future work. 2 Issues in P2P XML Data Management The problem of managing p2p XML databases is quite complex. The source of most issues is the dynamic nature of these systems, where both data and topology may suddenly change. Hence, a closer look at these aspects is necessary. Changing topology Peer-to-peer systems are usually described as open-ended networks of peers willing to share resources. Peers are autonomous, in the sense that they are free to choose the data to contribute to the system, to manage local data without external constraints, and to connect and disconnect at any time. As a consequence, the system is formed by a collection of nodes S = {p 1 ,. .. , p n } that can evolve over time. Topology changes mostly affect the indexing structures used for routing queries. For instance, if a node p i containing data (let's say a set of XML nodes s) relevant for a query q suddenly becomes unreachable, then any index entry associating p i to s should be updated to avoid unnecessary messages, or, in the worst case, run-time problems. Local updates Peer autonomy implies that peers have the right to update their data, even if shared, at any time. In particular, peers can perform both value and schema changing updates (unlike in relational databases, the loose structure of XML data blurs the distinction between value and schema updates). Value and schema updates influence query mediation and query routing since sudden data changes may invalidate existing query plans or routing structures, hence imposing potentially expensive updates of distributed index structures. Moreover, most schema-driven data management approaches (see [1]) are severely affected by local updates, hence requi...

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“…Further work needs to be done to investigate the performance of applications based on these networks in relation to other peer-to-peer information retrieval applications (e.g. [36]), and the consequences of adjusting properties of the networks used on application performance.…”

A New Document Sharing System Based on a Semantic Hierarchical Peer-to-Peer Network

“…The XPeer system [11] is a hybrid architecture for sharing XML data. Each Peer exports data description to share in the form of a tree.…”

An Internet-Scale Service for Publishing and Locating XML Documents

“…Most of the previous work use a form of path summarization (e.g., DataGuide summarization [4], root-to-node paths [3], bloom-filters [11], path hashes [6]) to index XML documents and locate them in a P2P network. Therefore, a complex XPath expression is first decomposed into simple paths and each path is processed separately by issuing separate lookups into the P2P network.…”

“…Recently there has been a rising interest for P2P systems that adopt XML as their data model [3] [4] [5] [6][7] [8]. The ability to model the underlying heterogeneity of data sources and expressiveness of query languages such as XPath and XQuery make XML a suitable choice.…”