Abstract

The concept of nested transactions offers more decomposable execution units and finer grained control over concurrency and recovery than `flat' transactions. Furthermore, it supports the decomposition of a `unit of work' into subtasks and their appropriate distribution in a computer system as a prerequisite of intra-transaction parallelism. However, to exploit its full potential, suitable granules of concurrency control as well as access modes for shared data are necessary. In this paper, we investigate various issues of concurrency control for nested transactions. First of all, the mechanisms for cooperation and communication within nested transactions should not impede parallel execution of transactions, neither among parent and children nor among siblings. Therefore,a model for nested transactions is proposed allowing for effective exploitation of intra-transaction parallelism. Starting with a set of basic locking rules, we introduce the concept of `downward inheritance of locks' to make data manipulated by a parent available to its children. To support supervised and restricted access, this concept is refined to `controlled downward inheritance'. The initial concurrency control scheme was based upon S-X locks for `flat', non-overlapping data objects. In order to adjust this scheme to the needs of practical applications, a set of concurrency control rules is derived for generalized lock modes described by a compatibility matrix. Moreover, these rules are combined with a hierarchical locking scheme to improve selective access to data granules of varying sizes. After having tied together both types of hierarchies (transaction and object hierarchies) it could be shown how `controlled downward inheritance' for hierarchical objects may be achieved in nested transactions...