Bruce Hammer, Steve Wallis, Raymond Ho Distributed Systems Concepts and Design Chapter 10: Peer-to-Peer Systems Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Client-Server relationships are more transaction-based than defined as permanent roles. Peer-to-Peer elevates clients to servers – on an asynchronous basis Subordinate hosts need to be aware that they are participating in the peer-to-peer system Objective – to build a reliable resource sharing layer over an unreliable and untrusted collection of computers and networks Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Eliminating any requirement for separately-managed servers and their associated infrastructure – Can use peer-to-peer within one company, but this relies on each subordinate host to maintain its own infrastructure The professor has mentioned the SETI@home project, Search for Extra-Terrestrial Intelligence, which partitions a stream of digitzed radio telescope data into 107-second work units, and parcels them out to personal computers on the network. This is controlled by one single server. SETI does not involve any communication or coordination between computers while they are processing the work units. Single message to central server. Design aims to deliver a service that is fully decentralized and self-organizing, dynamically balancing the storage and processing loades between all the participaating computers as computers join and leave the service Increasingly attractive as the performance difference narrows between host servers and personal computers Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Napster legal problems need to add anonymity Not needed in corporate peer-to-peer systems Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Steve will demonstrate BitTorrent Napster and Peer-to-Peer middleware are the next two presentations Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Globally Unique Identifiers, usually derived as a secure hash – from some or all of the resource’s state. (metioned in Chapter 7) The use of a secure hash makes a resource ‘self certifying’ The use of p2p that demand a high level of availability for the objects stored requires careful application design to avoid situations in all of the replicas are unavailable. The use of randomly-distributed GUIDs assists by distributing the object replicas to randomly-located nodes in the underlying network. Spans many organizations across the globe minimizes the risk. Bruce Hammer, Steve Wallis, Raymond Ho

10.2: Napster and its Legacy Bruce Hammer, Steve Wallis, Raymond Ho

10.2: Napster and Its Legacy Bruce Hammer, Steve Wallis, Raymond Ho

10.3: Peer-to-Peer Middleware Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Routing Overlay is a distributed algorithm Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 From Section 9.1.1. pure names are simply uninterpreted bit patterns. Non-pure names contain information about the object that the name, e.g., the location. Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Bruce Hammer, Steve Wallis, Raymond Ho

10.5: Overlay Case Studies: Pastry, Tapestry Bruce Hammer, Steve Wallis, Raymond Ho

10.6: Application Case Studies: Squirrel, OceanStore, Ivy Squirrel Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 10.7: Summary Napster – immutable data, unsophisticated routing Current – mutable data, routing overlays, sophisticated algorithms Internet or company intranet support Distributed Computing (SETI) For the professor, Peer-to-Peer systems seem to be an evolving technology. Have there been significant changes since the book was published? Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Bruce Hammer, Steve Wallis, Raymond Ho

Bruce Hammer, Steve Wallis, Raymond Ho 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 Bruce Hammer, Steve Wallis, Raymond Ho

10: Peer-to-Peer Systems Questions???? Comments?? For the professor, This fields seems to be evolving rapidly, do you have interesting updates for us? Bruce Hammer, Steve Wallis, Raymond Ho