Supporting partial isolation in flat transactions

Abstract

This paper describes a new adaptable correctness criterion, called ACCtr, that allows correctness requirements to be attached to each individual transaction. Correctness requirements may vary both between transactions and within a single transaction. ACCtr allows partial isolation, where isolation may be required for some parts of a transaction T, while interleaving is allowed for other parts of T. To support ACCtr, we describe a concurrency control mechanism, called Two-dimensional locking. Through early release of locks, Two-dimensional locking allows controlled access to a partially updated database. This is offered as a new type of isolation level which is available for both read and update transactions. The paper briefly considers the recovery mechanism needed to support Two-dimensional locking. Here we suggest using a combination of compensation and rollback to undo fiat transactions.