A general mechanism to support nested atomic transactions

The concept of nested transaction extends the traditional idea of transaction in that the nested transaction can contain any number of atomic (sub)transactions (nested in their turn, if more nesting levels are supported). However, the nested transaction as a whole must remain atomic, i.e indivisibility and all-or-nothing properties must be guaranteed. In our opinion, among the well-known advantages offered by nesting, the modularity property, i.e. the capability of freely composing previously existing transaction modules within an enclosing parent transaction, is the most attractive feature. We suggest that nesting should be supported by exploiting a general synchronization mechanism based on the two-phase lock protocol. We reject the usual rule which maintains objects locked until transaction completion and allow the early release of locks immediately after the termination of the growing phase. Although this mechanism requires heavier book-keeping and implies cascading aborts, on the other hand, it enhances potential concurrency between running transactions. Current literature dealing with concurrency control and nested transactions is summarily reviewed and compared with our approach.