Serializability of Nested Transactions in Multidatabases

The correctness of nested transactions for multidatabases dif fers from that of at transactions in that for nested transactions the execution order of siblings at each related site should also be consistent In this paper we rst propose a simple but powerful theory for the se rializability of nested transactions in multidatabases and then a tech nique called Nested Tickets Method for Nested Transactions NTNT The NTNT technique provides correctness of nested transactions in mul tidatabases without violating the local autonomy of the participating DB MSs The algorithm is fully distributed in other words there is no central scheduler The correctness of the NTNT technique is proved by using the developed theory Introduction and Related Work A multidatabase system MDBS is a software that allows global applications accessing data located in multiple heterogeneous autonomous DBMSs by provid ing a single database illusion A multidatabase environment supports two types of transactions local transactions submitted directly to a single Local DBMS LDBMS and executed outside the control of MDBS and global transactions that are channeled through the MDBS interface and executed under the MDBS control The objectives of a multidatabase transaction management are to avoid inconsistent retrievals and to preserve the global consistency in the presence of updates Transaction management has always been one of the most important parts of a DBMS GR The research on transaction management for central ized DBMSs is rst extended to distributed DBMSs BHG HD HD and then to multidatabases The transaction management for at trans actions in multidatabases have received considerable attention in recent years and correctness criteria have been de ned ZE and several concurrency con trol techniques have been suggested BGS ZE GRS In GRS This work is partially being supported by the Turkish State Planning Organiza tion Project Number AFP DPT K by the Scienti c and Technical Research Council of Turkey Project Number EEEAG Yazilim by Motorola USA and by Sevgi Holding Turkey a ticket method is suggested to enforce serializability of global transactions in a MDBS environment However it has been observed that nested transactions are more suitable to distributed environments since they provide more general control structures and support reliable and distributed computing more e ec tively Nested transactions M facilitate the control of complex persistent applications by enabling both ne tuning of the scope of rollback and safe intra transaction parallelism As a result nested transactions have become integral parts of some important standards e g OMG s Common Object Services Speci cation COSS OMG s transaction service speci cation supports nested trans actions along with at transactions in a distributed heterogeneous environment based on the CORBA architecture OMG Yet to the best of our knowledge there is no technique suggested for the correctness of nested transactions in mul tidatabases although some multidatabase projects have decided to use nested transaction model in their implementations HFBK DDK Principles and realization strategies of multilevel transaction management is described in W A multi level transaction approach to federated DBMS transaction management is discussed in SWS DOM Transaction Model BOH for multidatabases allows closed nested and open nested transactions InterBase Transaction Model ELLR is based on nested transaction model and allows a combination of both compensatable and non compensatable subtransactions However the correctness theory has not yet been developed for the models mentioned above In this paper we have developed a simple neat and powerful theory for the serializability of nested transactions in multidatabases Note that the theory provided in BBG for nested transactions could have been generalized to multidatabases However the theory developed in BBG is very general in the sense that it takes semantics of transactions into account by allowing compatible transactions Thus to prove the correctness of a concurrency control technique commutativity and pruning concepts are used We are able to develop a simpler theory provided in Section by not taking the semantics of transactions into account We then present a technique called Nested Tickets Method for Nested Trans actions NTNT that provides for the correct execution of nested transactions in multidatabases It should be noted that the concurrency control techniques developed for at multidatabase transactions do not provide for the correctness of nested transactions in multidatabases because for nested transactions a con sistent order of global transactions is not enough the execution order of siblings at all levels must also be consistent at all sites The main idea of NTNT technique is to give tickets to global transactions at all levels that is both the parent and the child transactions obtain tickets Then each global sub transaction is forced into con ict with its siblings through its parent s ticket at all related sites The recursive nature of the algorithm makes it possible to handle the correctness of di erent transaction levels smoothly NTNT technique also produces correct executions for at transactions at transactions being a special case of nested transactions NTNT technique is fully distributed and does not violate the autonomy of participating LDBMSs A transaction manager using the NTNT technique is im plemented within the scope of the METU Interoperable DBMS MIND project DDK DEO MIND is based on OMG s object management architecture and is developed on top of a CORBA OMG compliant ORB namely DEC s Object Broker A generic database object is de ned through CORBA IDL and an implementation is provided for each of the participating DBMSs namely Oracle Sybase Adabas D and MOOD METU Object Oriented DBMS DAO Among these DBMSs Sybase and Adabas D support nested trans actions Therefore the restrictions of a global transaction to Sybase and Adabas D servers can be nested transactions the others are at transactions The paper is organized as follows In Section nested transaction models for centralized and multidatabase systems are given Section introduces a serial izability theory for nested transactions in multidatabases In Section NTNT technique and its correctness proof are presented We conclude with Section