Peer-to-Peer. Peer-to-Peer (P2P) Definitions P2P is an architectural principle based on decentralization and resource sharing It is replacing the current.

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1 Peer-to-Peer

2 Peer-to-Peer (P2P) Definitions P2P is an architectural principle based on decentralization and resource sharing It is replacing the current paradigm of client-server computing The contribution of database community to P2P is the introduction of schemas Most important problem of P2P is finding the distributed resources P2P System Features: –local control of data –dynamic addition and removal of peers –only local knowledge of available data and schemas –self-organization and optimization

3 Super-Peer Networks Super-peer networks, where a small subset of peers takes over –peer aggregation, –query routing and –possibly mediation Super-peers routes queries first in the super-peer backbone, and then distributes them to the peers connected to the super-peers.

4 Mediation Typical file-sharing systems have a single global schema for describing their data P2P networks have to consider heterogeneous schemas in the network and have to rely on local transformation mechanisms and rules. Mediation TRIPLE. As has been pointed out in [8], defining views appears to be the right means for mediation, especially in case of schemas or ontologies modeled with the help of description logics. Since on the Semantic Web current approaches for schema/ontology languages build on description logics, e.g., DAML+OIL and its W3C successor OWL [11, 34], a powerful rule language with the capability to define views seems to be a promising candidate for mediation. Especially relevant in our Edutella TRIPLE peer is its capability to define parameterized views which add the flexibility to define multi-step mappings (by nesting/sequencing such The expansion of these abstract query plans can be based on different strategies, related to the quality of clustering in the P2P network. If the data are clustered well with respect to the queries, it is most efficient to push joins in the query as near as possible to the data sources, and then take the union of the results for these joins. If the clustering does not reflect the partitions needed by the views). This peer (currently being developed as part of the ELENA project5) allows advanced querying, inferencing and mediation, and also provides reasoning services to be used in ELENA for providing personalization in the context of a smart learning space. It can also be used to express query correspondence assertions and model correspondences as flexible mechanisms to express mappings between heterogeneous schemas as discussed in [21].

5 Information Integration (Ullman, 1997)Ullman, 1997 Aerosol-relevant information arises from multiplicity of sources, each having specific evolution history, driving forces, formats etc. Data analysis, i.e. the transformation of raw data into ‘actionable’ knowledge, requires combining data from numerous sources Problems in combining data: –Legacy data systems can not be altered to support integration –Data systems use different terms or meaning of similar terms –Some data sources, do not have a schema and formal access methods

6 Integration Architecture (Ullman, 1997)Ullman, 1997 Heterogeneous sources are wrapped by software that translates between the sources local language, model and concepts and the shared global concepts Mediators obtain information from one or more components (wrappers or other mediators) and pass it on to other mediators or to external users. In a sense, a mediator is a view of the data found in one or more sources; it does not hold the data but it acts as it it did. The job of the mediator is to go to the sources and provide an answer to the query. Wrapper Mediator User QueryView

7 Peer-to-Peer Definition Es handelt sich um eine Softwarearchitektur. Die beteiligten Peers sind direkt miteinander verbunden. Diese direkte Verbundenheit bezieht sich jedoch nicht auf eine direkte physikalische Verbindung, sondern auf eine logische direkte Verbundenheit über das Internet mit Hilfe von TCP/IP Die beteiligten Peers sind strukturell ähnlich aufgebaut. Die Verrichtung von Aufgaben im P2P Netzwerk basiert auf homogener Aufgabenverteilung bezüglich Last und Art der Aufgaben. Peers übernehmen insofern temporär sowohl Client- als auch Serverfunktionalitäten. Für die Verrichtung von Aufgaben werden dezentrale Ressourcen geteilt. Diese Ressourcen können z. B. Plattenspeicher, Rechenkapazität oder Inhalte sein. Eine mögliche zentrale Instanz (wie z. B. ein Indexserver) ist nur mit den nötigsten Funktionalitäten ausgestattet. Die grundsätzliche Kommunikation verläuft von Peer zu Peer, so dass das Netzwerk einen hohen Grad an Dezentralität aufweist.

8 Verschiedene Peer-to-Peer Architekturen BrokerPeers

9 Verbreitung von Peer-to-Peer Systemen? The 3 laws of technology Moore’s Law: Formulated by Gordon Moore of Intel in the early 1970s: The processing power of a microchip doubles every 18 months. Corollary: computers become faster - and the price of a given level of computing power halves - every 18 months. Disk space Every 12 Month (Franklin) Gilder’s Law: Proposed by George Gilder, prolific author and prophet of the new technology age: The total bandwidth of communication systems triples every 12 months. New developments seem to confirm that bandwidth availability will continue to expand at a rate that supports Gilder’s Law. Metcalfe’s Law: Attributed to Robert Metcalfe, originator of Ethernet and founder of 3COM: The value of a network is proportional to the square of the number of nodes; so, as a network grows, the value of being connected to it grows exponentially, while the cost per user remains the same or even reduces.