Concurrent Shadow Paging in the Flask Architecture

The differing requirements for concurrency models in programming languages and databases are widely diverse and often seemingly incompatible. The rigid provision of a particular concurrency control scheme in a persistent object system limits its usefulness to a particular class of application, in contrast to the generality intended by the provision of persistence. One solution is to provide a flexible system in which concurrency control schemes may be specified according to the particular task in hand, allowing the same data to be used in conjunction with different concurrency control schemes according to the needs of the application. A major difficulty in the engineering of such a system lies in the building of generic mechanisms to provide the facilities of data visibility restriction, stability, and atomicity, independently of the combination of these employed by a particular concurrency control scheme. Flask is a architecture which is designed to achieve this goal by defining a layering whereby the memory management is not tied to any one concurrency control scheme operating above. This paper outlines the Flask architecture and focuses on an implementation based on concurrent shadow paging. The architecture described is currently being used as the basis for experimentation in generic concurrency specification.