A survey on self‐organized semantic storage <s> Self-organized semantic storage <s> RDF-based P2P networks have a number of advantages compared to simpler P2P networks such as Napster, Gnutella or to approaches based on distributed indices on binary keys such as CAN and CHORD. RDF-based P2P networks allow complex and extendable descriptions of resources instead of fixed and limited ones, and they provide complex query facilities against these metadata instead of simple keyword-based searches. In this paper we will discuss RDF-based P2P networks like Edutella as a specific example of a new type of P2P networks - schema-based P2P networks - and describe the use of super-peer based topologies for these networks. Super-peer based networks can provide better scalability than broadcast based networks, and provide support for inhomogeneous schema-based networks, with different metadata schemas and ontologies (crucial for the Semantic Web). Based on (dynamic) metadata routing indices, stated in RDF, the superpeer network supports sophisticated routing and distribution strategies, as well as preparing the ground for advanced mediation and clustering functionalities. <s> BIB001 </s> A survey on self‐organized semantic storage <s> Self-organized semantic storage <s> Purpose – In any distributed application, the communication between the distributed processes/nodes of the distributed systems is essential for both reliability and efficiency matters. The purpose of this paper is to address this issue for distributed applications based on JXTA protocols aiming at extending and evaluating the protocols of the JXTA library for reliable P2P computing.Design/methodology/approach – After a careful examination of the current version of JXTA protocols, the need was observed for improving the original JXTA protocols such as pipe services to ensure reliable communication between nodes of the grid platform and the discovery and presence service to increase the performance of the applications. Using a mixed P2P network based on broker peers and client peers architecture, which served as a basis to extend the JXTA protocols, was the basis of the approach.Findings – The original JXTA protocols are extented/re‐implemented to support the development of reliable P2P distributed applicat... <s> BIB002
If a request for an unknown object hash arrives at a node, the request is forwarded to the neighbor node with the longest common prefix. Nodes publishing objects do so by resolving the node within the network responsible for the object's hash, and then store a pointer to themselves on that node. This way, if a request for a certain hash arrives at the responsible node, it can forward the request to the node where the object is actually stored. This scheme is shown in Figure 2 : several nodes S1 through S6 are organized in a tree overlay network structure. Each node is responsible for a certain prefix, for example all key prefixed with 1.1 are routed to the node S3. Requests can be sent to any node, which will then use the overlay network to route for the longest matching prefix. While being a very elegant object location scheme, the various indirections incur a high number of messages that have to be exchanged in order to locate an object. Also, if the distributed objects are not equally popular, some nodes in the network can be easily overloaded while other sit idle. Changes of the network structure always require the costly re-organization of node responsibilities in a considerable part of the network. Edutella uses the Tapestry model to allow peers to register the queries they have results for as Tapestry objects. Nodes trying to handle the user's queries can now find the nodes storing these objects, retrieve data from them, and thus answer the query. For example, a node could publish the fact that it stores RDF triples for a particular predicate URI, or even a predicate URI with a specific literal value. Edutella's development predates the standardization of the query language SPARQL, thus a custom query language family "RDF-QEL-i" with various levels of expressiveness is defined, however, due to very similar semantics, SPARQL support is feasible. Even though Edutella completely abstracts the underlying network organization through its use of JXTA, it is still subject to the ability of the Tapestry concept to handle the unique properties of RDF data. While this can be beneficial for the design, if -for example -JXTA is extended to become more reliable BIB002 , it also implies the limitations of the underlying protocols onto Edutella's performance. As it has been noted previously, the RDF predicates rdfs:label or rdf:type are both occurring and requested frequently in many RDF data sets. If no further precautions are taken, every node that is part of an Edutella network would now publish the fact that it has data containing these predicates stored. The all-to common queries for these predicates would now result in a request to every node, which would inhibit scalability. As a refinement of the initial concept, Edutella has been extended to support two groups of nodes BIB001 . One group of "super nodes" acts as global query brokers with a complex overlay structure limiting the number of messages between them. A second group of simple "storage nodes" stores the actual data and is only communicating with super nodes. While certainly achieving the goals set by its authors, this network setup is no longer self-organized, and simply moves the problems described in the previous paragraph to another layer.