Using tickets to enforce the serializability of multidatabase transactions

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 reliable global atomic commitment protocol for distributed multidatabase systems

Achieving high degree of concurrency in multidatabase transaction scheduling: MTOS

Global deadlock detection for concurrency control in multidatabase systems

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 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.

Compensation is the process by which a committed transaction in a database is undone by running the semantic inverse of that transaction on the database. Compensation has been proposed as a technique for undoing committed work in various situations where strict atomicity cannot be maintained (H. Garcia-Molina, K. Salem, 1987; J.G. Mullen, 1991). We discuss compensation in long-running multidatabase transactions. We define the step approach to integrating local database schemas into a multidatabase. In the step approach, each local database is encapsulated by a set of procedures (steps). Steps can be grouped into atomic global transactions. Each step also has an associated compensating step, which is called if the compensating transaction is run. We examine two areas of multidatabase transaction management where compensation is required. The first is implementing compensation as a recovery technique when an open, nested transaction is aborted. The second is in backing out the effects of an atomic mutlidatabase transaction when some local database transaction commits before a global abort decision is made. >

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.

Transaction management for global serializability and local autonomy 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.

The identification of missing persons and human remains is a worldwide problem which has been exacerbated with increased migrations and rampant human trafficking and smuggling cases. DNA typing and DNA databases are primary tools and resources used to help identify human remains and missing persons. The foundation of most, if not all, national DNA database systems, e.g., CODIS, is law enforcement identification. With such database systems, compliance with statutory and operational requirements is necessary to ensure the integrity of the databases. However, because of conditions in their homelands, relatives of missing persons at times may not trust the government and may be reluctant to contact a law enforcement agency, making it difficult to satisfy the law enforcement nexus necessary for entry into a national DNA database. A potential solution to increase the identification of unidentified human remains found within the USA, such as those that may be of foreign nationals, the University of North Texas Center for Human Identification (UNTCHI) has created a Humanitarian DNA Identification DNA Database (HDID) that enables family reference sample DNA profiles from non-US citizens to be compared with the DNA profiles from unidentified human remains within its local database system. This short communication describes the needs, basis, policies, and practices to inform the scientific, investigative, and legal communities and the public so that various entities may become aware and consider submitting family reference sample (FRS) profiles from foreign nationals for the purpose of searching against UNTCHI's HDID. It is our hope that by creating this HDID, another vehicle is available to support identification of human remains within the USA and to bring much needed answers to the family members of missing persons. The HDID will merge high forensic quality and best practices with the broader accessibility for non-US families to voluntarily donate DNA profiles for searching for missing loved ones.

For efficient construction of the distributed database from media failure, a transaction-consistent checkpointing algorithm is proposed for heterogeneous distributed database systems. For full design autonomy and increased availability on the heterogeneous distributed database systems, the proposed algorithm never enforces termination of normal operations of transactions and changes of checkpointing algorithms in local database systems. The global checkpoints generated by the algorithm can be used to reconstruct the previous consistent states of a database efficiently. >

In a multidatabase system, the participating databases are autonomous. The schemas of these databases may be different in various ways, while the same information is represented. A global query issued against the global database needs to be translated to a proper form before it can be executed in a local database. Since data requested by a query (or a part of a query) is sometimes available in multiple sites, the site (database) that processes the query with the least cost is the desired query processing site. The authors study the effect of differences in schemas on the cost of query processing in a multidatabase environment. They first classify schema conflicts to different types. For each type of conflict, they show how much more or less complex a translated query can become in comparison with the originally user-issued global query. Based on this observation, they propose an analytical method that considers the conflicts between local databases and finds the database(s) that renders the least execution cost in processing a global query. This research introduces a new level of query optimization (termed the schema-level optimization) in multidatabase environments. The results provide a new dimension of enhancement for the capability of a query optimizer in multidatabase systems.

Middleware transaction manager in multidatabase management systems

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.