Distributed Systems Concepts and Design Chapter 10: Peer-to-Peer Systems
Bruce Hammer, Steve Wallis, Raymond Ho
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10.1: Introduction Peer-to-Peer Systems
Where data and computational resources are contributed by many hosts
Objective to balance network traffic and reduce the load on the primary host
Management requires knowledge of all hosts, their accessibility, (distance in number of hops), availability and performance.
They exploit existing naming, routing, data replication and security techniques in new ways
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10.1: Introduction Goal of Peer-to-Peer Systems
Sharing data and resources on a very large scale ‘Applications that exploit resources available at
the edges of the Internet – storage, cycles, content, human presence’ (Shirky 2000)
Uses data and computing resources available in the personal computers and workstations
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10.1: Introduction Characteristics of Peer-to-Peer Systems
Each computer contributes resources All the nodes have the same functional
capabilities and responsibilities No centrally-administered system Offers a limited degree of anonymity Algorithm for placing and accessing the data
Balance workload, ensure availability Without adding undue overhead
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10.1: Introduction Evolution of Peer-to-Peer Systems
Napster – download music, return address Freenet, Gnutella, Kazaa and BitTorrent
More sophisticated – greater scalability, anonymity and fault tolerance
Pastry, Tapestry, CAN, Chord, Kademlia Peer-to-peer middleware
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10.1: Introduction Evolution (Continued)
Immutable Files, (music, video) GUIDs (Globally Unique Identifiers) Middleware to provide better routing algorithms,
react to outages Evolve to mutable files Application within one company’s intranet
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10.2: Napster and its Legacy
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10.2: Napster and Its Legacy
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10.3: Peer-to-Peer Middleware
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10.4: Routing Overlays Routing Overlays
Sub-systems, APIs, within the peer-to-peer middleware Responsible for locating nodes and objects Implements a routing mechanism in the application layer
Separate from any other routing mechanisms such as IP routing
Ensures that any node can access any object by routing each request thru a sequence of nodes Exploits knowledge at each node to locate the destination
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10.4: Routing Overlays GUIDs
‘pure’ names or opaque identifiers Reveal nothing about the locations of the objects Building blocks for routing overlays
Computed from all or part of the state of the object using a function that deliver a value that is very likely to be unique. Uniqueness is then checked against all other GUIDs
Not human readable
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10.4: Routing Overlays Tasks of a routing overlay
Client submits a request including the object GUID, routing overlay routes the request to a node at which a replica of the object resides
A node introduces a new object by computing its GUID and announces it to the routing overlay
Clients can remove an object Nodes may join and leave the service
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10.4: Routing Overlays Types of Routing Overlays
DHT – Distributed Hash Tables DOLR – Distributed Object Location and
Routing DOLR is a layer over the DHT that maps GUIDs
and address of nodes DHT – GUIDs are stored based on the hash value DOLR – GUIDs host address is notified using the
Publish() operation
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10.5: Overlay Case Studies: Pastry, Tapestry
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10.6: Application Case Studies: Squirrel, OceanStore, Ivy Squirrel
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10.7: Summary Napster – immutable data, unsophisticated
routing Current – mutable data, routing overlays,
sophisticated algorithms Internet or company intranet support Distributed Computing (SETI)
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10.7: Summary Benefits of Peer-to-Peer Systems
Ability to exploit unused resources (storage, processing) in the host computers
Scalability to support large numbers of clients and hosts with load balancing of network links and host computer resources
Self-organizing properties of the middleware platforms reduces costs
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10.7: Summary Weaknesses of Peer-to-Peer Systems
Costly for the storage of mutable data compared to trusted, centralized service
Can not yet guarantee anonymity to hosts
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10: Peer-to-Peer Systems Questions????
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