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This paper addresses the problem of forming groups in peer-to-peer
(P2P) systems and examines what dependability means in decentralized
distributed systems. Much of the literature in this field assumes
that the participants form a local picture of global state, yet
little research has been done discussing how this state remains
stable as nodes enter and leave the system. We assume that nodes
remain in the system long enough to benefit from retaining state,
but not sufficiently long that the dynamic nature of the problem
can be ignored. We look at the components that describe a system's
dependability and argue that next-generation decentralized systems
must explicitly delineate the information dispersal mechanisms
(e.g., probe, event-driven, broadcast), the capabilities assumed
about constituent nodes (bandwidth, uptime, re-entry distributions),
and distribution of information demands (needles in a haystack vs.
hay in a haystack. We evaluate two systems based on
these criteria: Chord and a heterogeneous-node
hierarchical grouping scheme. The former gives
a greater than 1 failed request rate under normal P2P conditions
and a prototype of the latter a similar rate under more strenuous
conditions with an order of magnitude more organizational messages.
This analysis suggests several methods to greatly improve the
prototype.
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