The identification and resolution of semantic heterogeneity in multidatabase systems

A multidatabase system consists of several heterogeneous local database systems. Most studies of multidatabase transaction management are concerned with the centralized transaction management in which there is only one coordinator. All global transactions are submitted to the coordinator site, and thus it can be overloaded. If the coordinator site fails, the multidatabase systems cannot serve any global transactions. In decentralized transaction management, since the site to which a global transaction is submitted becomes its coordinator, the load is naturally balanced and the multidatabase system is gradually degraded even though a coordinator site fails. This paper proposes a decentralized transaction management algorithm which guarantees global serializability and local autonomy.

A heterogeneous database management system combines multiple dissimilar models of data within a single integrated system. The objective is to allow a user to access data independently of how it is actually organized. For example, a user may access a database as though it were stored relationally (i.e., in tables) [CODD70], even though it is actually stored as a CODASYL/DBTG or network database [CODA71]. In addition, different subpieces of the database may be organized under different data models. The heterogeneous database system must present these to the user as an integrated whole. The user's model of his data may be different from any of the models chosen to implement it.Rather than construct a new database system from scratch, we are interested in constructing a heterogeneous system out of existing systems. The key difficulties with this approach are: (1) the formulation of database design methods that are applicable to a variety of different data models, and (2) the development of techniques to translate programs and data between dissimilar data models. In this paper, we briefly describe how high level abstraction has been applied to these problems. The use of abstraction in database systems is related to the application of abstraction techniques in programming languages and artificial intelligence research.

Distributed data processing is becoming a reality. Businesses want to do it for many reasons, and they often must do it in order to stay competitive. While much of the infrastructure for distributed data processing is already there (e.g., modern network technology), a number of issues make distributed data processing still a complex undertaking: (1) distributed systems can become very large, involving thousands of heterogeneous sites including PCs and mainframe server machines; (2) the state of a distributed system changes rapidly because the load of sites varies over time and new sites are added to the system; (3) legacy systems need to be integrated—such legacy systems usually have not been designed for distributed data processing and now need to interact with other (modern) systems in a distributed environment. This paper presents the state of the art of query processing for distributed database and information systems. The paper presents the “textbook” architecture for distributed query processing and a series of techniques that are particularly useful for distributed database systems. These techniques include special join techniques, techniques to exploit intraquery paralleli sm, techniques to reduce communication costs, and techniques to exploit caching and replication of data. Furthermore, the paper discusses different kinds of distributed systems such as client-server, middleware (multitier), and heterogeneous database systems, and shows how query processing works in these systems.

In a previous JDM issue, one of the Associate Editors issued a challenge to database researchers to “...encourage us to think about innovation and point out areas that ... are not only important but also open to innovation.†(Pirkul, 1995, p.33). In this editorial, I address one area that Pirkul explicitly identified as a topic for innovation, namely, Heterogeneous Databases. Below I present a novel framework for understanding Heterogeneous Databases and Database Systems. This framework identifies three dimensions of heterogeneity: the “Why†?, “How†?, and “What?†of heterogeneous database systems. I claim that exploring answers to these three questions will help practitioners (1) identify heterogeneous application requirements and map them to appropriate database capabilities, (2) evaluate heterogeneous application techniques that fulfill these necessary requirements, and (3) select products that support the required techniques.

Database technology is of great variety, structure diversity, solving data integration and exchanges sharing of the heterogeneous database system have became the important problem in all fields. Because of XML is the very important data exchange technology to realize data exchange and transmission ability, this paper mainly explores the data exchange problem how to use XML in heterogeneous database system, , and .Net platform have good support to XML and heterogeneous database system, so the combination of XML and.net, can realize data exchange of heterogeneous database system.

Naturally, data processing requires three kinds of resources:

The issues and solutions for the interoperability of a class of heterogeneous databases and their database systems are expounded in two parts. Part I presented the data-sharing issues in federated databases and systems (Hsiao, 1992). The present article explores resource-consolidation issues. Interoperability in this context refers to data sharing among heterogeneous databases, and to resource consolidation of computer hardware, system software, and support personnel. Resource consolidation requires the presence of a database system architecture which supports the heterogeneous system software, thereby eliminating the need for various computer hardware and support personnel. The class of heterogeneous databases and database systems expounded herein is termed federated, meaning that they are joined in order to meet certain organizational requirements and because they require their respective application specificities, integrity constraints, and security requirements to be upheld. Federated databases and systems are new. While there are no technological solutions, there has been considerable research towards their development. This tutorial is aimed at exposing the need for such solutions. A taxonomy is introduced in our review of existing research undertakings and exploratory developments. With this taxonomy, we contrast and compare various approaches to federating databases and systems.

The issues and solutions for the interoperability of a class of heterogeneous databases and their database systems are expounded in two parts. Part I presents the data-sharing issues in federated databases and systems. Part II, which will appear in a future issue, explores resource-consolidation issues. Interoperability in this context refers to data sharing among heterogeneous databases, and to resource consolidation of computer hardware, system software, and support personnel. Resource consolidation requires the presence of a database system architecture which supports the heterogeneous system software, thereby eliminating the need for various computer hardware and support personnel. The class of heterogeneous databases and database systems expounded herein is termed federated, meaning that they are joined in order to meet certain organizational requirements and because they require their respective application specificities, integrity constraints, and security requirements to be upheld. Federated databases and systems are new. While there are no technological solutions, there has been considerable research towards their development. This tutorial is aimed at exposing the need for such solutions. A taxonomy is introduced in our review of existing research undertakings and exploratory developments. With this taxonomy, we contrast and compare various approaches to federating databases and systems.

The thesis examines issues of consistency maintenance in advanced database systems; primarily, multidatabase systems. A multidatabase system consists of a number of pre-existing local database systems. A local database system is unaware of its participation in the multidatabase system and, likewise, the multidatabase system has no knowledge of local transaction executions. Enforcing global constraints in such an environment is clearly a challenging task. A methodology for constraint enforcement is presented which utilises existing technology for the replication of data in an attempt to enforce global consistency. While it is shown to have limited applicability, it is nonetheless an interesting study and serves to qualify the limits of such a solution. An alternative method for global consistency maintenance, which relies on the existence of triggers at each of the participant local databases, is then discussed. This method is shown to be particularly suitable when the issue of local database autonomy is of concern. It is, however, only suited to systems where each of the local databases provides a capability for triggering external actions based on the occurrence of particular database events. As a result of this methodology, the requirement for identification of enforcement actions which access sites where the instigating transaction originated becomes apparent. Such enforcement actions can cause deadlock in certain circumstances when they are executed at the same site which initially triggered the global constraint. This issue is dealt with in a novel way by proposing a methodology for statically checking relations at each participant site with a view to determining their susceptibility to this form of deadlock. The method, a graphical representation of the constraint enforcement process in a distributed system, is also shown to have other desirable properties. Arising from the requirements of other work within the thesis, an algorithm for detecting all cycles in a given directed graph is presented. It is shown that, while the well-known adaptation of the Depth First Search algorithm to cycle detection in directed graphs can detect the existence of cycles, it cannot in all circumstances identify all cycles. An algorithm which performs this task is presented together with an analysis of its complexity and correctness. In a more general sense, the issue of deferred constraint enforcement is discussed. Several scenarios where deferred enforcement of constraints is required are presented, together with a method for detecting the presence of cyclic dependencies within a given database schema.

A multidatabase system (MDBS) is a database system which integrates pre-existing databases, called component local database systems (LDBSs), to support global applications accessing data at more than one LDBS. An important research issue in MDBS is query optimization. The query optimization problem in MDBS is quite different from the case of distributed database system (DDBS) since, due to schema heterogeneity and local autonomy of component LDBSs, is not possible to assume that the query optimizer has a complete information on the execution cost and database statistics. In this paper we present a distributed query optimization algorithm that works under very general assumptions for MDBSs with relational global data model. The algorithm is based on the idea of delegating the evaluation of the execution cost of the elementary steps in a query execution plan to the LDBS where the computation is performed. The optimization process is organized as a sequence of steps, in which at each step all LDBSs work in parallel to evaluate the cost of execution plans for partial queries of increasing size, and send their cost estimates to the other LDBS that need them for the next step. The computation is totally distributed, and organized in order to perform no duplicate computation, and to discard as soon as possible the execution plans that may not lead to an optimal solution.KeywordsExecution ModelExecution PlanQuery OptimizationQuery ExecutionExecution CostThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

A multidatabase system (MDBS) is a collection of databases interconnected to share data. It provides integrated access to heterogeneous and distributed databases along with a single layer to facilitate end user by a set of different database systems. One of the major tasks in building a multidatabase system, is determining and integrating the data from the different database systems into a coherent global view. The current development of multidatabase systems undergoes many obstacles, one of these obstacles is the requirement of central or distributed schema that aids in integration of logical data relationships. This paper addresses the need for integration of multidatabase system by using NDBC protocol and proposes the algorithm for the implementation of xDB system components.

Cost parameters and database statistics are the basis of query optimization techniques. However, in distributed and heterogeneous database systems, acquiring and treating information in order to help the optimization process are often tasks of a global query processor, which adapts its functionalities to a specific system architecture. Moreover, this acquisition process involves a large number of parameters and requires customized methods to retrieve data from specific sources. DIG (Distributed Information Gatherer) is a provider of data statistics and query costs that, through an independent and flexible service, aims to support global query optimization processing in distributed and heterogeneous database systems over autonomous data sources. We have developed a DIG prototype and experimented it with specific wrappers for a query middleware on both semi-structured data sources and an object DBMS.

Development of a dynamic data interchange scheme to support product design in agile manufacturing

The SIRIUS-DELTA architecture: A framework for co-operating database systems

Given the large number of conceptual schema design tools and methodologies in use today, conversion efforts need strategies by which any source schemas can be easily integrated in heterogeneous database systems. We discuss the design and analysis of strategies for multiple schema/view integration with an extensible redesign system that is object-based, as defined herein. Our focus is on well-defined, clearly identifiable objects; E-R schemas describable in linear form, or in graphical (E-R Diagram) form with annotations. E-R schemas are a composite class of basic objects: E-sets (of entities) and R-sets (of relationships), each of which is extended. E-sets are described by structured sets of keys, lists of attributes (with domains), indexes, and textual constraints. R-sets are structured lists of E-set names with a description of their participation in the R-set [“one” or “many”], and textual constraints, e.g. foreign keys. We propose a design for the missing step: a Graphic Schema Algebra to merge various conceptual schemas, to provide a leverage point for interoperation.KeywordsConceptual SchemaIntegration RuleLeverage PointAbstract Data TypeSource SchemaThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.