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Abstract

Abstract-Mobile agents have attracted considerable interest in recent years. In the context of mobile agents, fault tolerance is crucial to enable the integration of mobile agent technology into today's business applications. This article identifies two important properties for fault-tolerant mobile agent execution: nonblocking and exactly-once. Nonblocking ensures that the agent execution can proceed despite a single failure of the agent or the machine, for instance. Replication is the generally adopted mechanism to prevent blocking, but may lead to multiple executions of the agent (i.e., a violation of the exactly-once property), which is undesirable with operations that have side effects. Hence, we propose that fault-tolerant mobile agent execution be modeled as a sequence of agreement problems. Our approach is nonblocking and ensures exactly-once execution. FATOMAS, our prototype fault-tolerant mobile agent system, implements our approach. Its performance evaluation illustrates the overhead of the replication mechanisms.

This paper investigates the amount of information about failures needed to solve the predicate detection problem in asynchronous systems with crash failures. In particular, we show that predicate detection cannot be solved with traditional failure detectors, which are only functions of failures. In analogy to the definition of failure detectors, we define a failure detection sequencer, which can be regarded as a generalization of a failure detector. More specifically, our failure detection sequencer Σ outputs information about failures and about the final state of the crashed process. We show that Σ is necessary and sufficient to solve predicate detection. Moreover, Σ can be implemented in synchronous systems. Finally, we relate sequencers to perfect failure detectors and characterize the amount of knowledge about failures they additionally offer.

We compare the overall structure of military GIG and NCES architectures with that of the object oriented architectures (CORBA, J2EE and .NET) and of the emerging Web Services architecture. While the match is good in many ways, particularly with respect to Web Services, we also identify a series of shortcomings that could stymie attempts to implement a GIG or NCES system directly on a commercial Web Services platform. Our comparison leads to suggestions for experimental investigations of some topics, but also for more fundamental inquiry in some areas where the scientific base is inadequate. Several issues of the latter sort arise when we consider the mixture of scalability, security, robustness, and time-criticality that must be simultaneously satisfied in demanding military applications.