MLR : A Recovery Method forMulti

The Cambridge laboratory became operational in 1988 and is located at One Kendall Square, near MIT. CRL engages in computing research to extend the state of the computing art in areas likely to be important to Digital and its customers in future years. CRL's main focus is applications technology; that is, the creation of knowledge and tools useful for the preparation of important classes of applications. CRL Technical Reports can be ordered by electronic mail. To receive instructions, send a message to one of the following addresses, with the word help in the Subject line: This work may not be copied or reproduced for any commercial purpose. Permission to copy without payment is granted for non-profit educational and research purposes provided all such copies include a notice that such copying is by permission of the Cambridge Research Lab of Digital Equipment Corporation, an acknowledgment of the authors to the work, and all applicable portions of the copyright notice. Abstract 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 suggests a systematic way of providing high concurrency. A multi-level system using locks permits restrictive low level locks of a sub-transaction to be replaced with less restrictive high level locks when a sub-transaction commits, hence enhancing concurrency. This is possible because sub-transactions can be undone by executing 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 uniied fashion.