Multidatabase Transaction Research Articles

On Concurrency Control in Multidatabase Systems with an Extended Transaction Model

A major concurrency control problem that we have to cope in multidatabase systems is the global deadlock detection and resolution problem. This detection must take into account the autonomy of local systems, which make impossible the visibility of the state of local transactions. A well-known approach to detect such deadlocks, called potential global deadlocks, is one based on the potential conflict graph (PCG) appropriate for the multidatabase transaction model with a global commit protocol. This classical transaction model is very constraining for applications manipulating great volumes of information, and where subtransaction terminations (commit or abort) of global transactions are not totally dependant. In this paper we present an effective potential global deadlock characterization, and an efficient potential global deadlock detection algorithm, in multidatabase systems with an extended transaction model more suited for such applications.

Achieving high degree of concurrency in multidatabase transaction scheduling: MTOS

Abstract A multidatabase system (MDBS), which is also called a heterogeneous distributed database system (HDDBS) or federated database system (FDBS), is a facility that allows users or applications to access data located in multiple local database systems (LDBSs), each of which is autonomously operated. Several practical MDBS concurrency control schemes have been proposed whilst maintaining global consistency without compromising local autonomy. However, they could degrade either local concurrency of local transactions or global concurrency of global transactions for the purpose of ensuring the global serializability. In this paper, we propose a new ticket-based global concurrency control scheme that employs multiple tickets at each site. The basic idea is that, with proper consideration of the potential cyclic global serialization orders, subtransactions with the possibility of the local indirect conflicts must access the same ticket at their sites, otherwise they are allowed to access the different tickets. This scheme alleviates the blockage on local resources, and, as a result, increases both global concurrency and local concurrency, whilst preserving the local autonomy.

Concurrency control in hierarchical multidatabase systems

Over the past decade, significant research has been done towards developing transaction management algorithms for multidatabase systems. Most of this work assumes a monolithic architecture of the multidatabase system with a single software module that follows a single transaction management algorithm to ensure the consistency of data stored in the local databases. This monolithic architecture is not appropriate in a multidatabase environment where the system spans multiple different organizations that are distributed over various geographically distant locations. In this paper, we propose an alternative multidatabase transaction management architecture, where the system is hierarchical in nature. Hierarchical architecture has consequences on the design of transaction management algorithms. An implication of the architecture is that the transaction management algorithms followed by a multidatabase system must be composable– that is, it must be possible to incorporate individual multidatabase systems as elements in a larger multidatabase system. We present a hierarchical architecture for a multidatabase environment and develop techniques for concurrency control in such systems.

Multidatabase transaction management scheme supporting multiple subtransactions of a global transaction at a site

The purpose of multidatabase systems is to allow users to access and manipulate data from existing databases in the distributed environment without modifying the existing database management systems (DBMSs). In the multidatabase environment, determining if a schedule for a set of global transactions is globally serializable is difficult because each local DBMS operates autonomously. Thus, we adopt the approach that relaxes the isolation and atomicity requirements of transactions. Moreover, in some applications, multiple subtransactions of a global transaction need to be submitted to a local site. To support this new transaction model, we propose a new correctness criterion for global schedules that is based on the semantic compatibility of global transactions. The conditions that global transactions should satisfy to ensure the correct executions are investigated, and the concept of well-formed global transaction is defined. We also present a concurrency control scheme that ensures the proposed correctness criterion. Our scheme uses both compensation and retrial techniques to eliminate the requirement of local prepare-to-commit operations, and hence it allows the global subtransactions to be committed unilaterally at local sites before the decision of global commit.

Semantics-based transaction management for multidatabase systems

We propose a multidatabase transaction management scheme using the Flexible transaction model. In our approach, global subtransactions can be committed unilaterally at local sites before the global commit. Since such executions may not guarantee the traditional atomicity requirement of global transactions, a new correctness criterion for schedules is proposed. We also present a semantics-based concurrency control scheme and a commit protocol which ensure the proposed correctness criterion without requiring local DBMSs to provide prepare-to-commit operations.

A reliable global atomic commitment protocol for distributed multidatabase systems

Many works on multidatabase transaction processing have been made in the past with the assumption of the absence of failures. This is because it has been shown that correct executions of multidatabase transactions cannot be achieved in the presence of failures without certain tradeoffs. In this paper, we first review the problems of preserving the global atomicity of multidatabase transactions in the presence of failures, while preserving local autonomy. Then, we propose a reliable global atomic commitment protocol that solves those problems. We show that the proposed protocol is correct and guarantees the global atomicity. We also present a practical approach that allows easy integration of existing local database systems into our multidatabase system environment. The most elegant feature of our protocol is that it guarantees global atomicity, even in the presence of failures, while the protocol preserves maximal local autonomy, and does not place any restriction on the multidatabase transactions that are often assumed in the literature.

A reliable global atomic commitment protocol for distributed multidatabase systems

Many works on multidatabase transaction processing have been made in the past with the assumption of the absence of failures. This is because it has been shown that correct executions of multidatabase transactions cannot be achieved in the presence of failures without certain tradeoffs. In this paper, we first review the problems of preserving the global atomicity of multidatabase transactions in the presence of failures, while preserving local autonomy. Then, we propose a reliable global atomic commitment protocol that solves those problems. We show that the proposed protocol is correct and guarantees global atomicity. We also present a practical approach that allows easy integration of existing local data base systems into our multidatabase system environment. The most elegant feature of our protocol is that it guarantees global atomicity even in the presence of failures while the protocol preserves maximal local autonomy, and does not place any restriction on the multidatabase transactions that are often assumed in the literature.

Using tickets to enforce the serializability of multidatabase transactions

To enforce global serializability in a multidatabase environment the multidatabase transaction manager must take into account the indirect (transitive) conflicts between multidatabase transactions caused by local transactions. Such conflicts are difficult to resolve because the behavior or even the existence of local transactions is not known to the multidatabase system. To overcome these difficulties, we propose to incorporate additional data manipulation operations in the subtransactions of each multidatabase transaction. We show that if these operations create direct conflicts between subtransactions at each participating local database system, indirect conflicts can be resolved even if the multidatabase system is not aware of their existence. Based on this approach, we introduce optimistic and conservative multidatabase transaction management methods that require the local database systems to ensure only local serializability. The proposed methods do not violate the autonomy of the local database systems and guarantee global serializability by preventing multidatabase transactions from being serialized in different ways at the participating database systems. Refinements of these methods are also proposed for multidatabase environments where the participating database systems allow schedules that are cascadeless or transactions have analogous execution and serialization orders. In particular, we show that forced local conflicts can be eliminated in rigorous local systems, local cascadelessness simplifies the design of a global scheduler, and that local strictness offers no significant advantages over cascadelessness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

TOWARDS A MODEL FOR MULTIDATABASE TRANSACTIONS

In many application areas the information that may be of interest to a user is stored under the control of multiple, autonomous database systems. To support global transactions in a multidatabase environment, we must coordinate the activities of multiple Database Management Systems that were designed for independent, stand-alone operation. The autonomy and heterogeneity of these systems present a major impediment to the direct adaptation of transaction management mechanisms developed for distributed databases. In this paper we introduce a transaction model designed for a multidatabase environment. A multidatabase transaction is defined by providing a set of (local) sub-transactions, together with their precedence and dataflow requirements. Additionally, the transaction designer may specify failure atomicity and execution atomicity requirements of the multidatabase transaction. These high-level specifications are then used by the scheduler of a multidatabase transaction to assure that its execution satisfies the constraints imposed by the semantics of the application. Uncontrolled interleaving of multidatabase transactions may lead to the violation of interdatabase integrity constraints. We discuss the issues involved in a concurrent execution of multidatabase transactions and propose a new concurrency control correctness criterion that is less restrictive than global serializability. We also show how the multidatabase SQL can be extended to allow the user to specify multidatabase transactions in a nonprocedural way.

Overview of multidatabase transaction management

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.

Critical issues in multidatabase systems

For many applications, a multidatabase approach constitutes an attractive alternative to a single, integrated database. In this paper we describe our experiences gained in the course of the development of experimental multidatabase systems: OMNIBASE, at the University of Houston and InterBase at Purdue University. We concentrate on the problems of query processing and multidatabase transaction management and discuss solutions that were developed within both projects. We identify the main issues related to supporting updates spanning across multiple autonomous databases and define an extended transaction model suitable for heterogeneous computing environments. Then we review the major outstanding research problems and discuss the directions of further research in the area of multidatabase systems.

On rigorous transaction scheduling

The class of transaction scheduling mechanisms in which the transaction serialization order can be determined by controlling their commitment order, is defined. This class of transaction management mechanisms is important, because it simplifies transaction management in a multidatabase system environment. The notion of analogous execution and serialization orders of transactions is defined and the concept of strongly recoverable and rigorous execution schedules is introduced. It is then proven that rigorous schedulers always produce analogous execution and serialization orders. It is shown that the systems using the rigorous scheduling can be naturally incorporated in hierarchical transaction management mechanisms. It is proven that several previously proposed multidatabase transaction management mechanisms guarantee global serializability only if all participating databases systems produce rigorous schedules.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>