Clients can send requests
to servers, which process without any further outside assistance (at least from the point of
view of the requesters). The job of the client (the requester) is to preprocess and prepare a
task and send a request to the server for service. This architecture is very useful for two- or
three-tier business applications. However, there is frequently a performance and reliability
bottleneck when requests run at a peak.
With the evolution of network infrastructures and the enhancement of the computational
power of personal computers, Peer-to-Peer (P2P) computing provides another model of
distributed computing, in which the computing devices (computers, servers, or all other
devices) can link to each other easily and directly. Each device is called a ???peer???, and communication
or sharing occurs between two peers. A peer can play the role of both client
and server, unlike in server-centric computing (Microsoft Glossary, 2004). This is termed
an asymmetric client-server system (Foster & Kesselman, 2003). P2P computing is typically
used for connecting hosts and sharing resources (particularly, various types of files).
The main purpose of P2P is that resources can be provided and consumed by every peer.
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