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Mobile agents are autonomous objects that can migrate from node to
node of a computer network. Mobile agent technology has been
proposed for various application areas, including electronic
commerce, systems management and active messaging. Many of these
applications - especially those for electronic commerce - require
agents to be performed "exactly once", independent of
communication and node failures. In other words, once a mobile agent
has been launched, it must never be lost before its execution is
finished. Moreover, each "portion" of the agent performed
at the visited nodes is performed exactly once.
Due to the autonomy of mobile agents, there is no
"natural" instance that monitors the progress of an
agent's execution. As a result of that agents may be blocked
due to node crashes or network partitioning even if there are other
nodes available that could continue processing. In this paper, we
will describe a protocol that ensures the exactly once property of
agents and additionally reduces the blocking probability of agents
by introducing so-called observer nodes for monitoring the progress
of agents. This protocol is based on conventional transactional
technology, such as defined by X/Open DTP or CORBA OTS. It is
implemented in the Mole, a mobile agent system developed at
Stuttgart University.
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