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.

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Overview of multidatabase transaction management

This book provides comprehensive coverage of fundamentals of database management system. It contains a detailed description on Relational Database Management System Concepts. There are a variety of solved examples and review questions with solutions. This book is for those who require a better understanding of relational data modeling, its purpose, its nature, and the standards used in creating relational data model.

Fundamentals of relational database management systems

A remote backup database system tracks the state of a primary system, taking over transaction processing when disaster hits the primary site. The primary and backup sites are physically isolated so that failures at one site are unlikely to propogate to the other. For correctness, the execution schedule at the backup must be equivalent to that at the primary. When the primary and backup sites contain a single processor, it is easy to achieve this property. However, this is harder to do when each site contains multiple processors and sites are connected via multiple communication lines. We present an efficient transaction processing mechanism for multiprocessor systems that guarantees this and other important properties. We also present a database initialization algorithm that copies the database to a backup site while transactions are being processed.

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Management of a remote backup copy for disaster recovery

Three ordering properties are characterized, namely, single-source, multiple-source, and multiple-group ordering, and their solutions are discussed. The multiple-group ordering property, which guarantees that two messages destined to two processes are delivered in the same relative order, even if they originate at different sources and are addressed to different multicast groups, is concentrated on. A protocol that solves the multiple-group ordering problem is presented. The issues of performance and reliability are addressed by providing comparisons with other techniques for ordering multicasts. In many cases this new algorithm solves the problem with greater efficiency than previous solutions without sacrificing reliability. It is pointed out that one disadvantage of the technique is that sometimes sites are required to handle messages which they do not need to deliver locally. These so-called extra nodes, however, do not occur frequently according to the experiments presented. >

Message ordering in a multicast environment

The technology for distributed computing is available. However, it is argued that decentralized systems will always require more careful design, planning, and management than their centralized counterparts. The rationale for and against decentralization is given, and a technical approach to decentralized systems is sketched. This approach contrasts with the popular concept of a distributed integrated database which transparently provides remote IO against single system image. Rather, it proposes that functions be distributed as `servers' which abstract data as high-level operations on objects and communicate with `requestors' via a standard message protocol. This requestor-server approach has the advantages of modularity and performance.

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An approach to decentralized computer systems

Distributed computer applications built from off-the-shelf hard-ware and software are increasingly common. This paper examines four such distributed systems with contrasting degrees of decentralized hardware, control, and redundancy. The first is a one-site system, the second is a node-replicated system at a remote site for disaster backup, the third is a multi-site system with central control, and the fourth is a multi-site system with node autonomy. The application, design rationale, and experience of each of these systems are briefly sketched.

J.N. Gray

Papers

Distributed computer systems: Four case studies