Peer-to-Peer

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Peer-to-Peer
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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

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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.

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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., DAMLOIL 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.

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Information Integration (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

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Integration Architecture (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.

User Query
View
Mediator
Mediator
Wrapper
Wrapper
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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.

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Verschiedene Peer-to-Peer Architekturen
9
Verbreitung von Peer-to-Peer Systemen?
The 3 laws of technology Moores LawFormulated
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) Gilders LawProposed 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 Gilders Law. Metcalfes
LawAttributed 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.