Robustness in a distributed storage server for engineering design data with versions

Graduation date: 1987Traditional database management systems have been deemed unsuitable for use in the\ud Computer Aided Engineering environment Object-oriented data models with project\ud management features have been proposed as an alternative. The Distributed Hypothetical\ud Storage System is intended to serve as the underlying storage mechanism for an object-oriented\ud data model. The storage system provides features that are essential to project\ud management such as version histories, alternate data versions, and optimistic concurrency\ud control in a distributed environment\ud This thesis presents a robustness mechanism for DHSS. Three classes of system failure\ud are considered: site failure, communication failure, and media failure. The protocols\ud described here make the system robust in two senses: first, users are provided with continued\ud read access to their data; and second, users are provided with continued write access to their\ud data. The traditional approach of data replication is used to support continued read access.\ud However, traditional systems support no write access or highly restricted write access in the\ud event of a failure. The DHSS robustness mechanism supports continued write access for all\ud visible data by allowing the execution of possibly conflicting write requests and attempting to\ud merge or resolve the results at a later time if they are indeed conflicting. The resolution\ud protocol requires one exchange of information among the communicating sites followed by\ud unilateral calculation to resolve all conflicts existing among that set of sites. It is essential that\ud the resolution be mutually consistent among the set of communicating sites. It is shown that\ud the robustness protocols produce a mutually consistent resolution of conflicting requests