Mobility in distributed object systems is useful as it can provide such properties as load balancing, code to data movement, fault tolerance, migration to stable storage, and autonomous semantics. In a widely distributed system, these properties are important as they can help alleviate latency issues and increase performance within the system. Additionally, they provid more flexibility in the programming of distributed systems by relaxing static location restrictions. Location transparency removes the need for client objects to explicitly know or define the location of a server object when communicating. If a server object is capable of migration, relocation transparency maintains reference validity throughout the migration. Several models for providing relocation transparency exist, including the home location, forwarding location, and broadcast models. This thesis proposes a model that uses a distributed registry system and dynamic reference updating to provide location and relocation transparency. A registry system is used to provide location independence by resolving a location independent name to a reference that can be used by a client. A naming system is used to provide correct binding and production of names within the naming restrictions of the system. The thesis proposes that the choice of naming system within a distributed or mobile object system has a large effect on the system's ability to support efficient transparent object relocation. This thesis proposes that a formal analysis of naming systems enables the selection of an appropriate naming system for a distributed or mobile object system given the object system's naming, distribution and transparency requirements. This thesis presents a new classification scheme for naming systems, based on analysis of a broad spectrum of naming systems. A classification of existing mobile and distributed object systems with respect to existing naming models is provided. It is shown that the current models need to be refined and extended to completely and correctly classify the example systems. This thesis proposes extensions and refinements that enable correct and complete classification of mobile and distributed object systems with a need for transparency. The extended naming model is then used to describe a naming system that is capable of implementing any naming system classifiable by the extended model. A classification of a naming system to support the proposed model of location and relocation transparency is presented. A distributed ORB system is designed and implemented to support the distributed namespace and generic naming system implementation. The distributed ORB system is hierarchically structured and is capable of adapting in response to node failure. This ORB system is used to support client and server object integration in the DISCWorld metacomputing environment. The ORB system is used to provide migration, replication and cloning services to the DISCWorld metacomputing environment. A qualitative analysis of the generic naming system and the DISCWorld ORB system is performed. A comparison between the proposed model for location and relocation transparency and existing models is also presented. This comparison shows that the proposed model exhibits better location and relocation performance within the DISCWorld environment. The distributed nature of the ORB system and namespace provides a scalable nature in terms of namespace size, the number of objects within the system, and the frequency of location and relocation requests. / Thesis (Ph.D.)--Engineering (Department of Computer Science), 2000.

Em aplicações distribuídas baseadas em agentes móveis, a coordenação das ações dos agentes móveis é uma tarefa complexa. A maior dificuldade é devido ao fato que agentes móveis podem mudar de endereço dinamicamente. Nesta dissertação, apresentamos o projeto e a implementação de um mecanismo de coordenação de agentes móveis que contorna este problema. Este mecanismo, que chamamos de Canal de Broadcast, está baseado na difusão de mensagens e possibilita que os membros de um grupo de agentes móveis interajam entre si, independentemente de suas localizações correntes. Modelos de coordenação existentes oferecem formas de interação entre agentes móveis, mas todos eles impõem alguma exigência. Ou os agentes móveis devem conhecer a localização de outros agentes, ou devem estar localizados no mesmo lugar ou devem migrar para um lugar específico. A principal vantagem deste mecanismo de coordenação está na total transparência de localização: as mensagens podem ser endereçadas a um conjunto de agentes móveis independentemente de sua localização corrente. Este mecanismo foi implementado no ASDK (Aglets Software Development Kit) da IBM e a sua utilidade foi demonstrada usando dois problemas típicos de coordenação em Sistemas Distribuídos: a Exclusão Mútua e o protocolo Manager-Workers. Testamos o desempenho do mecanismo e identificamos que o custo do Canal de Broadcast não é tão alto comparado aos benefícios que proporciona. Através deste mecanismo, os agentes móveis poderão executar as suas tarefas e interagir entre sí com o propósito da coordenação sem as exigências impostas por outros modelos de coordenação. / In distributed applications based on mobile agents, the coordination of mobile agents actions is a difficult task. The main difficulty comes from the fact that mobile agents can change their address dynamically. In this dissertation, we present the project and implementation of a mechanism for coordinating mobile agents that overcomes this problem. This mechanism, which we called Broadcast Channel, is based on the difusion of messages and enables the members of a mobile agent group to interact with each other independently of their current locations. Existing coodination models offer some means of interaction between mobile agents, but all of them impose some requirements. Either the mobile agents must know about each other's location, or must be located at the same place or must move to a specific place. The main advantage of our coordination mechanism is the full location transparency: the messages can be sent to a set of mobile agents independently of their current locations. This mechanism was implemented using IBM's ASDK (Aglets Software Development Kit) and its use was demonstrated using two typical coordination problems in Distributed Systems: the Mutual Exclusion and the Manager-Workers protocol. We tested the performance of the mechanism and identified that the cost of the Broadcast Channel is not too high, compared to the benefit it provides. Through this mechanism, the mobile agents can do their tasks and interact with each other for the purpose of coordination without the requirements imposed by other models.

Em aplicações distribuídas baseadas em agentes móveis, a coordenação das ações dos agentes móveis é uma tarefa complexa. A maior dificuldade é devido ao fato que agentes móveis podem mudar de endereço dinamicamente. Nesta dissertação, apresentamos o projeto e a implementação de um mecanismo de coordenação de agentes móveis que contorna este problema. Este mecanismo, que chamamos de Canal de Broadcast, está baseado na difusão de mensagens e possibilita que os membros de um grupo de agentes móveis interajam entre si, independentemente de suas localizações correntes. Modelos de coordenação existentes oferecem formas de interação entre agentes móveis, mas todos eles impõem alguma exigência. Ou os agentes móveis devem conhecer a localização de outros agentes, ou devem estar localizados no mesmo lugar ou devem migrar para um lugar específico. A principal vantagem deste mecanismo de coordenação está na total transparência de localização: as mensagens podem ser endereçadas a um conjunto de agentes móveis independentemente de sua localização corrente. Este mecanismo foi implementado no ASDK (Aglets Software Development Kit) da IBM e a sua utilidade foi demonstrada usando dois problemas típicos de coordenação em Sistemas Distribuídos: a Exclusão Mútua e o protocolo Manager-Workers. Testamos o desempenho do mecanismo e identificamos que o custo do Canal de Broadcast não é tão alto comparado aos benefícios que proporciona. Através deste mecanismo, os agentes móveis poderão executar as suas tarefas e interagir entre sí com o propósito da coordenação sem as exigências impostas por outros modelos de coordenação. / In distributed applications based on mobile agents, the coordination of mobile agents actions is a difficult task. The main difficulty comes from the fact that mobile agents can change their address dynamically. In this dissertation, we present the project and implementation of a mechanism for coordinating mobile agents that overcomes this problem. This mechanism, which we called Broadcast Channel, is based on the difusion of messages and enables the members of a mobile agent group to interact with each other independently of their current locations. Existing coodination models offer some means of interaction between mobile agents, but all of them impose some requirements. Either the mobile agents must know about each other's location, or must be located at the same place or must move to a specific place. The main advantage of our coordination mechanism is the full location transparency: the messages can be sent to a set of mobile agents independently of their current locations. This mechanism was implemented using IBM's ASDK (Aglets Software Development Kit) and its use was demonstrated using two typical coordination problems in Distributed Systems: the Mutual Exclusion and the Manager-Workers protocol. We tested the performance of the mechanism and identified that the cost of the Broadcast Channel is not too high, compared to the benefit it provides. Through this mechanism, the mobile agents can do their tasks and interact with each other for the purpose of coordination without the requirements imposed by other models.