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Support for Peer-to-Peer Interactions in Web Brokering Systems



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Support for Peer to Peer Interactions in Web Brokering Systems Geoffrey Fox and Shrideep Pallickara Community Grid Labs Department of Computer Science Indiana University The peer to peer abbreviated as P2P style interaction model is a very powerful one and is one in which clients can interact directly with each other The traditional boundaries between clients and servers are blurred with clients initiating sophisticated requests and those requests being serviced by other clients The P2P style model facilitates sophisticated resource sharing environments Driven entirely on demand a resource may be replicated several times a process that is decentralized and one over which the original peer that advertised the resource has sometimes little control over The greater the demand the greater the system initiated replications for the resulting in fewer bottlenecks and faster accesses to that resource Peers usually do their interactions using XML this ensures that peers could be written in any language and be compiled for any platform and thus interactions between peers could be cross platform and cross language The p2p request response semantics differ from those prevalent in traditional systems where the request response model is fixed one response for a request with the client initiating the request having no ambiguity on how the request would be interpreted and also being aware of what the response would be like In contrast in p2p systems not every request goes through and even if it does there could be zero or more valid responses and peers anticipate neither the template that the responses would conform to nor on the order in which these responses would be received Furthermore responses are not identical with each responding peer processing the request based on the resources at its disposal and its interpretation of the request Requesting peers and those peers that bounce the requests responses are thus made aware of different capabilities that exist at other peers This discovery of services offered by other peers constitutes dynamic real time knowledge propagation Peer requests are sometimes satisfied through cached responses and peers generally have a choice on whether to accept these cached responses or not Its up to the peer to discard responses that it deems is not right P2P interactions are self attenuating with requests dying out after a certain number of hops These attenuations in tandem with traces of the peers that the interactions have passed through eliminate the continuous echoing problem that result from loops in peer connectivity However attenuation of interactions sometimes prevents clients from discovering certain services that are being offered Peers hosting these services could not respond because the message attenuated before it could have reached the hosting peer This results in p2p interactions being very localized Of course if the peer at the edge of the attenuation had cached the response for a similar request that it made or the resource itself that particular request would be available to the requesting peer Resources in web brokering systems are generally within the purview of the broker network P2P systems comprise peers exposing the resources they share Unlike clients in web brokering systems that interact via the broker network peers in p2p systems interact directly with each other and sometimes use other peers as intermediaries in interactions Specialized peers are sometimes deployed to enhance routing characteristics Nevertheless sophisticated routing schemes are seldom in place and interactions are primarily through simple forwarding of requests with the propagation range being determined by the attenuation indicated in the message This is where distributed web brokering systems could come in place Having a single broker solution would lead to bottlenecks where a lot of p2p interactions are being funneled through the broker The associated queuing delays scaling issues and the single point of failure that such a scheme constitutes are among the reasons why a distributed model should be in place Distributed brokering systems could be used to optimize the request response discovery and advertisement interactions Furthermore these systems could be used to connect islands of peers together Peers that are not directly connected through the peer network could be indirectly connected through the broker network Integration strategies need to ensure that minimal changes need to be made to the brokering core zero changes to the peers and no straitjacketing of any interactions that peers had prior to integration This would be done via proxies that provide an interconnection bridge between the two systems The proxies are part routing peers and part clients of the web brokering system Peers would interact with the proxies as it would with any other peer while the proxy also inherits guarantees accorded to clients of the brokering system Thus no peers need to be changed neither would this entail any major support within the existing distributed broker network The broker network would be used primarily as a delivery engine and a pretty efficient one at that while locating peers and propagating interactions to relevant peers Broker networks are also best suited to react to changes in peer requests concentrations and resource availability Brokers links can be dynamically instantiated or purged to compensate such changing conditions Peers may be implemented in different languages with interoperability being achieved through XML based data interchange XML s data description and encapsulation properties allow for ease of accessing specific elements of data that is then used to achieve best possible routing characteristics Similarly some resources are best managed by the system rather than being left to the discretion of peers who may or may not be present at any given time An understanding of the network topology and an ability to pin point the existence of peers interested in that resource are paramount to efficient replications of a resource The distributed broker network best handles this In supporting p2p interactions the broker network itself should not be flooded with the processing of duplicate messages as resulting from message propagations from multiple peers as a result of loops in peer connectivity Time expended on processing these messages if they were not duplicate detected would significantly add to the delay in handling new requests The problem is


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