MLR: a recovery method for multi-level systems

To achieve high concurrency in a database system has meant building a system that copes well with important special cases. Recent work on multi-level systems suggest a systematic path to high concurrency. A multi-level system using locks permits restrictive low level locks of a subtransaction to be replaced with less restrictive high level locks when sub-transactions commit, enhancing concurrency. This is possible because sub-transactions can be undone via high level compensation actions rather than by restoring a prior lower level state. We describe a recovery scheme, called Multi-Level Recovery (MLR) that logs this high level undo operation with the commit record for the subtransaction that it compensates, posting log records to only a single log. A variant of the method copes with nested transactions, and both nested and multi-level transactions can be treated in a unified fashion.