Object mobility in distributed computer systems
Ghasem S. Alijani · 1990
The design of an integrated distributed system that is capable of functioning in a dynamic environment and with independent processing units requires a significant effort in specification, design and implementation. This research work presents a new methodology and formalism for the specification and design of an adaptive object-oriented model for the dynamic file assignment problem. Such a model provides the means to explore the dynamic behavior of the system and the effectiveness of the adaptive policies in an unpredictable environment. Our observation on previous research work in distributed computing systems, specifically the dynamic file assignment problem, indicates that most of the existing models have been constructed based on a centralized file assigner that is functioning under a set of predefined policies. Furthermore, there is a significant lack of interaction between the file assigner and the other system application programs. For a model, to be adaptable to the dynamic behavior of the system and suitable for real-time applications, it must be designed based upon local and global dynamic policies. In addition, to have an effective decision making mechanism, the states of the objects that are affected by external events must be mapped into the history profile. Formulating the problem with respect to the cited objectives requires a new methodology and formalism that is able to specify the functionality and interaction between the logical components of the system. Under the scope of our methodology and formalism, a framework is established concerning the physical and the logical components of the system. Through the process of specifying the physical components, the structure of the system is specified from the node to the system level. At each level, the relationships between nodes and subsystems are defined. The logical components are established based upon the concepts of distributed processes and modularity. At the node level, the relationships between these logical components and their functionality are defined. A new formalism is developed based on the definition of active and passive objects. The view of the active object is extended to a structured form as the generic structure which specifies the functionality and the interaction between the logical components of the system. This formalism is applied in designing an object assigner for the dynamic file assignment problem. in this context, a new method for deleting, transmitting and converting public files based upon voting and threshold schemes is established. To investigate the performance of the model under the dynamic behavior of the system a simulation model is developed. Finally, the effectiveness of the object assigner in managing private and public files is evaluated and the results are presented.