Integration of Multidatabases in Heterogeneous Environment Using NDBC Protocol

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 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.

The GSM Recommendations give the standards for a Pan-European mobile radio telecommunication system. The Location Registers form a Distributed Data Base whose interworking with other entities of the GSM System provides an integrated directory function. The properties and functionality of the GSM Location Registers and the implementation of the Distributed Data Base in the system “ECR 900” are described.

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.

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.

Achieving high degree of concurrency in multidatabase transaction scheduling: MTOS

Several aspects are given of the Remote-Exchange project at USC, which focuses on the controlled sharing and exchange of information among autonomous, heterogeneous database systems. The spectrum of heterogeneity which may exist among the components in a federation of database systems is examined, and an approach to accommodating such heterogeneity is described. An overview of the Remote-Exchange experimental system is provided.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In recent past, the bookstores generate more data such as customer details, transaction details, book details, etc. for analyzing them to make data-driven decisions. These data are stored in a centralized database. In centralized database system, the data are stored in a single site or location and that are managed. The major drawback is that if the centralized system gets crashed the entire data are lost and also it makes access bottleneck when more users need to access the database. In order to overcome these limitations, the distributed database system is used to store the business data by locating them in different databases that are located in different sites in different location. The distributed database maintains the replication and duplication that ensure the availability and fault tolerance on the data. The distributed system also provides the high accessibility to the users. Thus, this paper presents a distributed database for a bookstore to improve availability, fault tolerance, and accessibility. This distribution also maintains the transparency and supports the database properties such as atomicity, consistency, inheritance and durability (ACID). Furthermore, this distributed database is designed with four sites and each site keeps the bookstore data for access and analyses. This distributed database is constructed and accessed using structured query language (SQL) with MySQL relational database management system (RDBMS). It also provides the security and work even in offline mode. Keywords: distributed databases; query processing; relational database management; structured query language; bookstore database;

An important problem in multidatabase systems is maintaining semantic integrity constraints among the multiple databases in the system. The paper investigates practical approaches to enforcing referential integrity, one of the most important classes of constraints. Referential integrity is important in its own right and also provides a good starting point for studying more general classes of constraints. The semantics of referential integrity are relatively well known and understood, yet they have much in common with more general classes of constraints. The paper considers what enforcement mechanisms can be used in the context of the transaction models or interaction models which are widely found in practice today: the queued message model, the remote procedure call model, the distributed transaction model, and the local transaction dialog model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

For a long time, there are a large number of heterogeneous databases on the network, and their heterogeneity is manifested in many aspects. With the development of enterprise informatization and e-government, the system database of each department constitutes a real heterogeneous database framework with its independence and autonomy in the network system of many different functional departments. This paper will design information sharing between heterogeneous databases of network database system of many similar functional departments by using XML data model. The solution of data sharing between heterogeneous databases can accelerate the integration of information systems with departments and businesses as the core among enterprises, form a broader and more efficient organic whole, improve the speed of business processing, broaden business coverage, and strengthen cooperation and exchange among enterprises. In addition, heterogeneous database sharing can avoid the waste of data resources caused by the heterogeneity of database, and promote the availability rate of data resources. Due to the advantages of XML data model, the system has good scalability.

Logic and algebraic languages for interoperability in multidatabase systems

Global deadlock detection for concurrency control in multidatabase systems

Research on query cost estimation for local database systems in a multidatabase system (MDBS) has attracted many researchers in the database area recently. Obtaining good query cost estimates is crucial for performing effective global query optimization in an MDBS. However, most techniques suggested so far, including database calibrating and query sampling, consider only a static multidatabase environment. Recently, we proposed a qualitative approach to developing cost models for a dynamic multidatabase environment. It has been shown that this approach is promising in estimating the cost of a query run in any given contention state for a dynamic environment. However, a large (cost) query in practice may experience multiple contention states during its execution, which cannot be directly handled by the qualitative approach. In this paper, we propose two new techniques, i.e., fractional analysis and probabilistic approach, to solve the problem. The fractional analysis technique, which is suitable for a system environment that changes contention states gradually and smoothly, estimates a query cost by analyzing its fractions. The probabilistic approach, which is suitable for a system environment that changes contention states rapidly and randomly, estimates a query cost based on the theory of Markov chains. Cost estimation formulas for both techniques are derived, and their properties are studied. Our experimental results demonstrate that the suggested techniques are quite promising in estimating costs for large queries in a dynamic multidatabase environment.

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.