Revising 1-Copy Equivalence in Replicated Databases with Snapshot Isolation
OTM '09 Proceedings of the Confederated International Conferences, CoopIS, DOA, IS, and ODBASE 2009 on On the Move to Meaningful Internet Systems: Part I
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Whereas the Strict Snapshot Isolation (SSI) level is nowadays offered by most of the centralized DBMSs, replicated databases do not usually provide it, since the traditionally considered approach, the pessimistic management, introduces enormous performance penalties that preclude it from production applications. Instead, distributed databases usually offer Generalized Snapshot Isolation (GSI), a more relaxed level, in which a transaction may get a snapshot older than the one that was applied on the database by the time the transaction started.This paper takes advantage of our MADIS middleware and one of its implemented Snapshot Isolation protocols (SIRC) to design, implement and evaluate the performance of an extended version of SIRC (called gB-SIRC). This protocol is able to concurrently execute Generalized Read Committed (GRC), GSI, g-Bound --a non-standard SI level limiting the outdatedness of transactions wanting to commit-- and optimistic SSI transactions on top of a cluster of centralized DBMSs offering RC and SSI. This work is the first implementation and evaluation of an optimistic SSI level. Although the abort rate of g-Bounded transactions is significantly higher than the GSI ones, the performance results show that introducing transactions at more restrictive levels is not detrimental to the completion time or to the abort rate of the transactions using GSI.