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Orphan detection and termination in distributed systems is a well
researched field for which many solutions exist. These solutions
exploit well defined parent-child relationships given in distributed
systems. But they are not applicable in mobile agent systems, since
no similar natural relationship between agents exist. Thus new
protocols have to be developed. In this paper one such protocol for
orphan detection and agent termination is presented. First we
present two approaches, the energy concept and the path concept. The
energy concept is a passive termination protocol, and the path
concept is a protocol for finding agents, that can be used to
implement active termination. The energy' approach is based on
the idea that an agent is provided with a limited amount of energy,
which can be spent in exchange for the resources used by the agent.
From time to time the agent has to request additional energy from
its creator. The agent is terminated as soon as the energy falls to
0. This approach to agent termination implicitly implements orphan
detection, i.e. if the creator has terminated, the dependent agents
are killed as soon as they have no energy left. The path'
approach uses a chain of proxies. As soon as an agent leaves a
location, a proxy is created that points to the new location. By
following the chain of proxies, the path, one can find any agent,
and consequently terminate it. Both approaches have disadvantages.
By merging them on different levels we create a protocol that
combines the advantages of both approaches, and at the same time
minimizes the disadvantages. The shadow' approach uses the
idea of a placeholder (shadow) which is assigned by the agent system
to each new agent. The shadow records the location of all dependent
agents. Removing the root shadow implies that all dependent shadows
and agents are terminated recursively. We demonstrate that the
shadow approach can be used for termination of groups of agents even
if the exact location of each single agent is not known. We show
that shadow protocol is less fault-sensitive than the path approach
and needs less communication cost than the energy approach
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