A Study Of Transaction Processing Strategies In Multidatabase

Recent progress in communication and database technologies has changed the user riata processing environment .The present data processing situation is characterized by a growing number of applications that require access to various preexisting local data source located in heterogeneous hardware and software environments distributed among the nodes of a network .A. multidatabase (MDBS) system is a facility that allows users access to data located in multiple distributious heterogeneous autonomous database management systems (DBMS), in such a system,global transactions are executed under the control of the MDBS. Independently, local transactions are executed under the control of the local DBMS. Each local DBMS integrated by the MDBS may employ a different transaction management scheme. In addition, each local DBMS has complete control over all transactions (global and local) executing at its site, including the ability to abort at any point of the transactions executing at its she. Typically, no design or internal DBMS structure changes are allowed in order to accommodate the MDBS. Furthermore, the local DBMS may not be aware of each other, and, as a consequence, cannot coordinate their actions. Thus, traditional techniques for ensuring transaction atomicity and consistency in homogeneous distributed database systems may not be appropriate for an MDBS environment The objective of this paper is to provide a research of transactions atomicity > consistency in mulitdatabase and a mulitdalabase prototype panorama implementation on CORBAx query process server implementatioaFirst we focus on atomic commitment of transactions in MDBS. The various solutions for homogeneous systems are not appropriate for heterogeneous systems, since they require specific commit procedures that are not provided by most of the existing transaction managers. We systematically discuss two alternative strategies, before commitment strategy and after commitment strategy, which do not require changes of the existing transaction managers. The first strategy requires additional concurrency control and recover)' components to be implemented on top of the existing database systems. Since these components cause additional runtime overhead, we developed a second, alternative strategy. If multi-level transactions are used for the heterogeneous system, our strategy reuses the concurrency control and recovery mechanisms of this transaction model in order to provide an atomic commitment for global transactions. Multi-level transactions have originally been developed to increase the degree of concurrency in centralized databases system; we reuse these mechanisms together with two alternative strategies to provide an atomic commitment for global transactions. Especially before commitment strategy together with multi-level transactions can increase the degree of concurrency of multidatabase and no additional overhead.Second, the concept of serializabilhy has been the traditionally accepted consistency criterion in database systems. However, in MDBS, ensuring global serializabilhy is a difficult task. The main difficulty in enforcing global serializability in a multidatabase environment lies in resolving indirect conflicts between multidatabasetransactions. Indirect conflicts introduced by local transactions are difficult to resolve because the behavior or even the existence of local transactions is not known to multidatabase . To overcome these problems, we propose to incorporate additional data manipulation operations in the subtranstraction of each multidatabase transaction, these operations create direct conflicts between subtransaction at each participating local database system. The method is ticket algorithms, based on the approach, which requires local database system to ensure only local serializability. The method doesn't violate the autonomy of local database systems, and guarantees global serializability. its disadvantage is low concurrency. One way to overcome the problem of low concurrency is to relax the serializability requirement...