Διαχείριση ερωτημάτων σε δίκτυα ομοτίμων

Abstract

We are interested in query processing problems that occur in peer-to-peer (hereafter P2P) overlays. We consider both structured and unstructured overlays that handle structured or unstructured data. While structured frameworks provide reliable and efficient solutions, in many realistic scenarios the P2P topology cannot be controlled and, thus, structured overlays are not applicable. Since P2P file sharing is so popular in successful commercial applications and P2P data management systems are a new promising trend, we investigate competitive frameworks in object management. We study the heterogeneity and the data coordination problem that arise in unstructured overlays that consist of peer databases. We present GrouPeer, an adaptive, automated approach for the problem of sharing structured data in a P2P network in the absence of a standard mediated schema. By allowing peers to individually choose which rewritten version of a query to answer and evaluate the received answers, information-rich sources left hidden otherwise are discovered. Gradually, the overlay is restructured as semantically similar peers are clustered together. Extending the capabilities of GrouPeer, we propose a scheme that automates the process of creating schema synopses from semantic clusters of peers which own autonomous relational databases. The resulting mediated schemas can be used as global interfaces for relevant queries. For the data coordination problem in peer databases we describe distributed triggers. We present an SQL3 compatible trigger language for the P2P setting. We also extend the SQL3 processing mechanism to this setting. Our trigger processing mechanism consists of an execution semantics, a set of termination protocols to deal with peer transiency, and a set of protocols for managing peer acquaintances in presence of distributed triggers. Moreover, we discuss problems that arise in unstructured fast evolving overlays both in content and connectivity. We investigate the issues related to the above problem and propose a P2P and Mobile Agent architecture based on Active Database technology. Finally, we consider the problem of handling spatial data in distributed environments, focusing on applications of structured P2P overlays. We perform a general study on the construction of a framework for autonomous sites that are bound to spatial information. We investigate the parameters of such a distributed environment in order to perform search guided by locality and directionality in space. Furthermore, we present SpatialP2P, a totally decentralized indexing and routing technique that is suitable for P2P applications, in which spatial information of various sizes can be dynamically inserted or deleted, and peers can join or leave. The proposed technique preserves well locality and directionality of space.