tailieunhanh - Advances in Database Technology- P2

Tham khảo tài liệu 'advances in database technology- p2', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 32 G. Koloniari and E. Pitoura Query Routing A given query may be matched by documents at various nodes. Thus central to a P2P system is a mechanism for locating nodes with matching documents. In this regard there are two types of P2P systems. In structured P2P systems documents or indexes of documents are placed at specific nodes usually based on distributed hashing such as in CAN 21 and Chord 20 . With distributed hashing each document is associated with a key and each node is assigned a range of keys and thus documents. Although structured P2P systems provide very efficient searching they compromise node autonomy and in addition require sophisticated load balancing procedures. In unstructured P2P systems resources are located at random points. Unstructured P2P systems can be further distinguished between systems that use indexes and those that are based on flooding and its variations. With flooding such as in Gnutella 22 a node searching for a document contacts its neighbor nodes which in turn contact their own neighbors until a matching node is reached. Flooding incurs large network overheads. In the case of indexes these can be either centralized as in Napster 8 or distributed among the nodes as in routing indexes 19 providing for each node a partial view of the system. Our approach is based on unstructured P2P systems with distributed indexes. We propose maintaining as indexes specialized data structures called filters to facilitate propagating the query only to those nodes that may contain relevant information. In particular each node maintains one filter that summarizes all documents that exist locally in the node. This is called a local filter. Besides its local filter each node also maintains one or more filter called merged filters summarizing the documents of a set of its neighbors. When a query reaches a node the node first checks its local filter and uses the merged filters to direct the query only to those nodes whose filters match the query. .