A multidatabase system (MDBS) is a facility that allows users access to data located in multiple autonomous database management systems (DBMSs). 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 DBMSs. 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 any of the transactions executing at its site. Typically, no design or internal DBMS structure changes are allowed in order to accommodate the MDBS. Furthermore, the local DBMSs 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 article is to provide a brief review of the most current work in the area of multidatabase transaction management. We first define the problem and argue that the multidatabase research will become increasingly important in the coming years. We then outline basic research issues in multidatabase transaction management and review recent results in the area. We conclude with a discussion of open problems and practical implications of this research.

/pdf/overview-of-multidatabase-transaction-management-3w3j81jofr.pdf

Overview of multidatabase transaction management

Ensuring relaxed atomicity for flexible transactions in multidatabase systems

Global transaction management requires cooperation from local sites to ensure the consistent and reliable execution of global transactions in a distributed database system. In a heterogeneous distributed database (or multidatabase) environment, various local sites make conflicting assertions of autonomy over the execution of global transactions. A flexible transaction model for the specification of global transactions makes it possible to deal robustly with these conflicting requirements. This paper presents an approach that preserves the semi-atomicity (a weaker form of atomicity) of flexible transactions, allowing local sites to autonomously maintain serializability and recoverability. We offer a fundamental characterization of the flexible transaction model and precisely define the semi-atomicity. We investigate the commit dependencies among the subtransactions of a flexible transaction. These dependencies are used to control the commitment order of the subtransactions. We next identify those restrictions that must be placed upon a flexible transaction to ensure the maintenance of its semi-atomicity. As atomicity is a restrictive criterion, semi-atomicity enhances the class of executable global transactions.

/pdf/ensuring-relaxed-atomicity-for-flexible-transactions-in-suu437d7kb.pdf

Failure recovery in a multidatabase environment is addressed. It is shown that local autonomy considerations force the designer of a multidatabase system to trade off certain desirable properties to achieve reliability for transaction management. Representative techniques in the research literature are contrasted and compared. The author's approach to the problem is described. >

Failure-resilient transaction management in multidatabase

A multidatabase system (MDBS) is a facility that allows users access to data located in multiple autonomous database management systems (DBMSs). 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 DBMSs. 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 any of the transactions executing at its site. Typically, no design or internal DBMS structure changes are allowed in order to accommodate the MDBS. Furthermore, the local DBMSs 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 brief review of the most current work in the area of multidatabase transaction management. We first define the problem and argue that the multidatabase research will become increasingly important in the coming years. We then outline basic research issues in multidatabase transaction management and review recent results in the area. We conclude the paper with a discussion of open problems and practical implications of this research.

http://delab.csd.auth.gr/~dimitris/courses/mpc_fall05/papers/transactions/multi_db_transaction_processing_I.pdf

In this paper we show how global serializability and atomic commit can be attained in a Heterogeneous Distributed Database Management System in which each local DBMS is assumed to be an off-the-shelf, binarylicensed commercial product providing the IBM SAA Common Programming Interface ([SAA88]). Our HYDRO system achieves global serializability using a set of objects based on the Request Order Linked List or ROLL object developed in [PER89] and [PER91 ]. ROLL is based on the general Serialization Graph Test methodology ([BER87]), and provides freedom from idle-wait, deadJock, livelock and restart. Atomic commitment is based on Two-Phase Commit. Two options are offered to achieve the PREPARED state locally. HYDRO-I achieves the PREPARED state by protecting writes during the uncertainty period. HYDRO-H provides more concurrency, but raises the commitment overhead in the absence of a visible PREPARED state offered by the local DBMS.

HYDRO: a heterogeneous distributed database system

Focuses on the transaction management and atomic commitment features of HYDRO, a distributed multidatabase system. HYDRO provides global serializability, deadlock and livelock freedom, multiple concurrent global transactions at any site, and a high level of concurrency control parallelism. Two varieties of local HYDRO concurrency control objects are discussed, and the paper concludes with a brief presentation of the HYDRO atomic commitment scheme. >

Transaction management in HYDRO: a multidatabase system

Eficient join acceleration methods illat combine hybrid Imslring, join indices and domain vectors wirh semi-join techniqires to accelerate distribirted joins ore developed An analyric performance model wliich nllows for poralle1 execirrion of [lie methods nt each join site ir; dadoped to compare the toro1 task dirration time of the methods The methods are compared over a wide rnnge of memory sizes, semi-join selectiviiies nnd network transmission speeds The distribiiied join index method performs well if memory size is large and selectiviry is low The distributed hybrid liosli meiliod oluwys lios [lie advontoge ilioi it reqiiires no residiiol dnin IO be siored between riins, howeve< performonce is poor OI low nerwork tronsmission rotes dire to high pre-senitjoin tronsmission reqirirements The distribiited doinnin vector liasli meiliod performs well in higli selcctiviiy conditions

Prabhu Ram

Papers

HYDRO: a heterogeneous distributed database system

Transaction management in HYDRO: a multidatabase system

Distributed Join Processing Performance Evaluation