Multilevel atomicity

‘l’hc IISMI IllOddS for distributed dak?basc~ [RSIJG] arc ba>ed on a set of “entities” distributed amuunL the nodes of a network. ‘l‘hcse entities arc xcwcd by USCI’S OF the database through “trcuxactions”, which are ccr& SCquCnccS of steps (“actions”) involviiig the jndi\idllal entities. ‘L‘hC Steps arc grouped into trxwctions for twu disdnct purpuscs. i.jr,q a tramaction is t6cd as a unit of rccovcry: tither all of the steps of a transaction should bc carricci out, or none of them should; tl1llS, jf a transaction cannot bc complctcd. its initial steps must bc “undone” in some way. Second, a transaction is used to define atomicity: all of the !Xci)s of a transaction furm a logical atomic unit in the sense that it should appear to users of the database that all of thcsc steps are carried 014 consccutivcly, without any intcrvcning steps of other lransactions. This rcquircmcnt that transactions appear to be atomic is called “scriali/nbility” in the hlcralure [IGLT. RSI., IIG], and has been widely acccptcd as an important correctness criterion for distributed