Group communication provides communication primitives with various semantics and their use greatly simplifies the development of highly available services. However, despite tremendous advances in research and numerous prototypes, group communication stays confined to small niches and academic prototypes. In contrast, message-oriented middleware such as the Java message service (JMS) is widely used, and has become a de-facto standard. We believe that the lack of a well-defined and easily understandable standard is the reason that hinders the deployment of group communication systems. Since JMS is a well-established technology, an interesting solution is to extend JMS adding group communication primitives to it. Foremost, this requires extending the traditional semantics of group communication in order to take into account various features of JMS, e.g., durable/nondurable subscriptions and persistent/non-persistent messages. The resulting new group communication specification, together with the corresponding API, defines group communication primitives compatible with JMS.

Fault tolerance is fundamental to the further development of mobile agent applications. In the context of mobile agents, fault-tolerance prevents a partial or complete loss of the agent, i.e., it ensures that the agent arrives at its destination. We present FATOMAS, a Java-based fault-tolerant mobile agent system based on an algorithm presented in an earlier paper (2000). Contrary to the standard "place-dependent" architectural approach, FATOMAS uses the novel "agent-dependent" approach. In this approach, the protocol that provides fault tolerance travels with the agent. This has the important advantage to allow fault-tolerant mobile agent execution without the need to modify the underlying mobile agent platform (in our case ObjectSpace's Voyager). In our performance evaluation, we show the costs of our approach relative to the single, non-replicated agent execution. Pipelined mode and optimized agent forwarding are two optimizations that reduce the overhead of a fault-tolerant mobile agent execution.

Fault-tolerance is fundamental to the further development of mobile agent applications. In the context of mobile agents, fault-tolerance prevents a partial or complete loss of the agent, i.e., ensures that the agent arrives at its destination. Simple approaches such as checkpointing are prone to blocking. Replication can in principle improve solutions based on checkpointing. However, existing solutions in this context either assume a perfect failure detection mechanism (which is not realistic in an environment such as the Inter-net), or rely on complex solutions based on leader election and distributed transactions, where only a subset of solutions prevents blocking. This paper proposes a novel approach to fault-tolerant mobile agent execution, which is based on modeling agent execution as a sequence of agreement problems. Each agreement problem is one instance of the well-understood consensus problem. Our solution does not require a perfect failure detection mechanism, while preventing blocking and ensuring that the agent is executed exactly once.