Polyvalues: A tool for implementing atomic updates to distributed data

• Authors: Warren A. Montgomery
• Venue: SOSP '79 Proceedings of the seventh ACM symposium on Operating systems principles
• Year: 1979

The coordination of atomic updates to distributed data is a difficult problem in the design of a distributed information system. A common goal for solutions to this problem is that the failure of a site should not prevent any processing that does not require the data stored at that site. While this goal has been shown to be impossible to achieve all of the time, several approaches have been developed that perform atomic updates such that most site failures do not affect processing at other sites. This paper presents another such approach, one that provides a mechanism by which processing can proceed even if a failure occurs during a critical moment in an atomic update. The solution presented is based on the notion of maintaining several potential current values (a polyvalue) for each database item whose exact value is not known, due to failures interrupting atomic updates. A polyvalue represents the possible set of values that an item could have, depending on the outcome of transactions that have been delayed by failures. Transactions may operate on polyvalues, and in many cases a polyvalue may provide sufficient information to allow the results of a transaction to be computed, even though the polyvalue does not specify an exact value. An analysis and simulation of the polyvalue mechanism shows that the mechanism is suitable for databases with reasonable failure rates and recovery times. The polyvalue mechanism is most useful where prompt processing is essential, but the results that must be produced promptly depend only loosely on the database state. Many applications, such as electronic funds transfer, reservations, and process control, have these characteristics.