Navigation stability: A new isolation level in ORDBMSs

In order to enhance the performance, many database management systems (DBMS s) execute transactions at isolation level 2 rather than at isolation level 3, the strict two phase locking, even if it sacrifices consistency to a ce rtain degree. Cursor stability, a variant of isolation level 2 in relationa l DBMs (RDBMSs), has been widely used as a useful technique for obtaining c oncurrency achievable at level 2 without much sacrificing consistency. Howe ver, cursor stability is much less usable in object-relational DBMSs (ORDBM Ss) because navigational applications in ORDBMSs can suffer from critical i nconsistency problems such as dangling pointers, lost updates, and reading inconsistent complex objects. In this paper, we propose a new isolation lev el, navigation stability, that prevents the inconsistency problems of curso r stability for navigational applications, while avoiding significant degra dation of the concurrency of level 3. First, we analyze the inconsistency p roblems of cursor stability for navigational applications. Second, we defin e navigation stability as an extension of cursor stability and show that it solves those inconsistency problems of cursor stability in ORDBMSs. Third, through extensive simulation, we show that navigation stability significan tly enhances the performance compared with level 3. For workloads consistin g of transactions of long duration, compared with level 3, the throughput o f navigation stability is enhanced by up to 200%; the average response time reduced by as much as 55%; and the abort ratio reduced by as much as 77%. From these results, we conclude that navigation stability is a useful isola tion level in ORDBMSs that can be used in place of isolation level 3 to imp rove the performance and concurrency without significant sacrifice of consi stency.