Multi - Node Multi - Level Transactions 1

Multi-Level Transactions (MLT) o er a useful framework for increasing transaction concurrency in database systems. In this paper, we develop various approaches to implementing MLT in a shared disk multi-node system. By combining correctness and implementation speci cations, we present concrete insights into the resulting tradeo s between the degree of concurrency, the complexity of recovery, and runtime overheads in multi-node MLT. The main challenges include e ciently supporting subtransaction inverses on a multi-node system when coupled with performance enhancing mechanisms, such as ne-granularity locking, coarse-grain cache coherency, and coarse-grain persistence mechanisms. For example, in a two level implementation of multi-node MLT, serializability is achieved with respect to objects, and cache coherency is achieved with respect to pages. The migration of a page containing uncommitted objects may violate subtransaction atomicity and cause inter-transaction abort dependencies. These problems can be overcome by adjusting the object-to-page mapping policies, and by enforcing speci c stability properties (whether an object or operation is logged, is stable logged, or is persistent) at page migration. These and other tradeo s are investigated in a logical fashion by methodically studying (1) mechanisms invoked in response to signi cant events (such as Commit and Abort), (2) policies for object-to-page mapping, and (3) stability policies. The resulting implementation options are formally characterized by tailoring the ACTA formalism to include speci cations of these implementation oriented considerations, in addition to (the traditionally speci ed) transaction model requirements. 1 Partially supported by the National Science Foundation under grant IRI-9109210