The role of exception mechanisms in software systems design

Exception handling is a crucial aspect of practical programming, particularly in systems allowing logical concurrency such as multi-process distributed systems. First, a survey of existing exception handling mechanisms in operating systems is performed, which shows a diversity of implementations, depending on the process model and the method of inter-process communication. The thesis then develops a model for designing software which exploits the different mechanisms for handling normal and exceptional events. The model is applicable in many multi-process programming environments, and not only preserves modularity, but also enhances efficiency and reliability, while often increasing concurrency. To derive such a model, exceptions in multi-process software are classified primarily according to the program level at which they are detected and handled. Server-to-client exceptions are of particular interest because of their ubiquity; these are exceptions detected by a server and handled by a client. The model treats systems programs as event driven, and proposes dividing the events into normal or exceptional, according to the cost and mechanisms for handling them. Techniques are described for designing software according to three criteria: minimising the average run-time, minimising the exception processing time, and incrementally increasing the program's functionality. Many examples are given which illustrate the use of the general model. Program paradigms in several languages and in several systems are introduced to model features which are system dependent, through illustrative examples for asynchronous i/o multiplexing, and for exception notification from a server to its client or clients. Finally, some programs which have been implemented according to the rules of the model are described and compared with their more conventional counterparts. These programs illustrate the practicality and usefulness of the model for diverse systems and concurrent environments.Science, Faculty ofComputer Science, Department ofGraduat