Autonomous Database Systems Research Articles

Formalization of Federated Schema Architectural Style Variability

Data integration requires managing heterogeneous schema information. A federated database system integrates heterogeneous, autonomous database systems on the schema level, whereby both local applications and global applications accessing multiple component database systems are supported. Such a federated database system is a complex system of systems which requires a well-designed organization at the system and software architecture level. A specific challenge that federated database systems face is the organization of schemas into a schema architecture. This paper provides a detailed, formal investigation of variability in the family of schema architectures, which are central components in the architecture of federated database systems. It is shown how the variability of specific architectures can be compared to the reference architecture and to each other. To achieve this, we combine the semi-formal object-oriented modeling language UML with the formal object-oriented specification language Object-Z. Appropriate use of inheritance in the formal specification, as enabled by Object-Z, greatly supports specifying and analyzing the variability among the studied schema architectures. The investigation also serves to illustrate the employed specification techniques for analyzing and comparing software architecture specifications.

Semantic Enrichment of GSM-Based Artifact-Centric Models

We provide a comprehensive framework for semantic GSM artifacts, discuss in detail its properties, and present main software engineering architectures it is able to capture. The distinguishing aspect of our framework is that it allows for expressing both the data and the lifecycle schema of GSM artifacts in terms of an ontology, i.e., a shared and formalized conceptualization of the domain of interest. To guide the modeling of data and lifecycle we provide an upper ontology, which is specialized in each artifact with specific lifecycle elements, relations, and business objects. The framework thus obtained allows to achieve several advantages. On the one hand, it makes the specification of conditions on data and artifact status attribute fully declarative and enables semantic reasoning over them. On the other, it fosters the monitoring of artifacts and the interoperation and cooperation among different artifact systems. To fully achieve such an interoperation, we enrich our framework by enabling the linkage of the ontology to autonomous database systems through the use of mappings. We then discuss two scenarios of practical interest that show how mappings can be used in the presence of multiple systems. For one of these scenarios we also describe a concrete instantiation of the framework and its application to a real-world use case in the energy domain, investigated in the context of the EU project ACSI.

Query optimization in distributed networks of autonomous database systems

Large-scale distributed environments, where each node is completely autonomous and offers services to its peers through external communication, pose significant challenges to query processing and optimization. Autonomy is the main source of the problem, as it results in lack of knowledge about any particular node with respect to the information it can produce and its characteristics, for example, cost of production or quality of produced results. In this article, inspired by e-commerce technology, we recognize queries as commodities and model query optimization as a trading negotiation process. Subquery answers and subquery operator execution jobs are traded between nodes until deals are struck with some nodes for all of them. Such trading may also occur recursively, in the sense that some nodes may play the role of intermediaries between other nodes (subcontracting). We identify the key parameters of the overall framework and suggest several potential alternatives for each one. In comparison to trading negotiations for e-commerce, query optimization faces unique new challenges that stem primarily from the fact that queries have a complex structure and can be broken into smaller parts. We address these challenges through a particular instantiation of our framework focusing primarily on the optimization algorithms run on “buying” and “selling” nodes, the evaluation metrics of the queries, and the negotiation strategy. Finally, we present the results of several experiments that demonstrate the performance characteristics of our approach compared to those of traditional query optimization.

Design of an SHD-distributed database over IP to support telemedicine.

We have conducted tests utilizing superhigh-definition CRTs (hereafter, referred to as a "super high-definition image system," or "SHD ": 2,048 x 2,048 pixels, 24-bit RGB gradation, noninterlace system) for autonomous and distributed medical image database systems. The system is capable of providing six times the image definition of a hi-vision television (HDTV). The aim in designing and testing the system is to enable gigabit network connectivity while displaying images on a DOS- V machine. This paper discusses how we devised an integrated environment for managing and operating distributed an SHD image database to support Telemedicine linking with ATM, with INS-1500, and with dial-up IP connection.

Global scheduling for flexible transactions in heterogeneous distributed database systems

A heterogeneous distributed database environment integrates a set of autonomous database systems to provide global database functions. A flexible transaction approach has been proposed for the heterogeneous distributed database environments. In such an environment, flexible transactions can increase the failure resilience of global transactions by allowing alternate (but in some sense equivalent) executions to be attempted when a local database system fails or some subtransactions of the global transaction abort. We study the impact of compensation, retry, and switching to alternative executions on global concurrency control for the execution of flexible transactions. We propose a new concurrency control criterion for the execution of flexible and local transactions, termed F-serializability, in the error-prone heterogeneous distributed database environments. We then present a scheduling protocol that ensures F-serializability on global schedules. We also demonstrate that this scheduler avoids unnecessary aborts and compensation.

An approach to interoperation between autonomous database systems

In this paper we present an approach to support interoperation between autonomous database systems. In particular, we concentrate on distributed information discovery and access for systems with a large number of databases. We avoid the need for integrated global schemas or centralized structures containing information on the available data and its location. We instead provide an architecture that supports data distribution, autonomy and heterogeneity. The architecture also supports system evolution by the addition and removal of databases. A distributed information discovery algorithm is provided to perform data requests, database location and data access. A feature of our approach is to distribute the information about database contents using simple hierarchical information structures composed of special terms. A prototype has been developed to demonstrate and evaluate the approach. A hospital case study is used to illustrate its feasibility and applicability.

Schema integration and transaction management for multidatabases

In this paper we propose a schema integration technique to integrate multiple autonomous database systems. We adopt the unified model, which is basically a relational model with object-oriented features, as the global model for integrating schemas of multiple databases in the relational, hierarchical, network, and object-oriented models. The features and the advantages of the unified model as the global model are discussed, then we describe the methodologies to integrate heterogeneous databases using this model and show how various types of conflicts are resolved. We also show how a global query on the integrated schema is handled in heterogeneous environment. We also present a transaction management scheme which uses semantic recovery techniques. Our scheme does not impose any restrictions on data items that transactions can read and write and does not violate local autonomy.

Concurrency control in multidatabase systems: A performance study

Abstract A multidatabase system (MDBS) is a federation of multiple autonomous local database systems (LDBSs). Over the years, it has been recognized that the most challenging issue of concurrency control in an MDBS has been a concern about how to resolve indirect conflicts and thereby to ensure global serializability (GSR), without sacrificing the local autonomy. Even though the global serializability issue is well understood and many concurrency control algorithms have been proposed, it is noticeable that the performance trade-offs of those algorithms are not yet understood. To address issues about the performance of concurrency control algorithms in an MDBS, we examine two MDBS concurrency control algorithms through simulation: optimistic ticket method (OTM) and global ticket method (GTM) that we have proposed. In OTM, the GSR is assured by way of accessing a local object (ticket), by which any indirect conflicts that may cause non-serializable execution are resolved. The global transaction manager (Global_TM) in GTM maintains a global variable (ticket), which is similar to timestamp, and the commit order of a GT is predefined by the order of assigned global ticket value. We examine the performance trade-offs of the two concurrency control methods (CCMs) with respect to ensuring GSR in particular. To ensure GSR, the (GTs) are expected to frequently restart in OTM and block in GTM. In this study, we also investigate the implications of time-out interval on the performance as we assume that the time-out approach is used as a global deadlock resolution method for both algorithms. Our experimental results showed that GTM outperforms OTM in cases that short time-out values are given. However, in case that the time-out value relatively becomes long, our results demonstrated that OTM outperforms GTM.

Temporal Interoperability in Multi+Temporal Databases

A multidatabase system allows applications and users to access heterogeneous information through a common interface. Information is shared and exchanged in a transparent way within a federation of autonomous database systems. In a multi+temporal database, the connected systems may be standard (snapshot) databases, temporal databases (either valid-time, transaction-time or bitemporal) or even single multitemporal systems in which data with different temporal formats may coexist. In this paper we consider semantic interoperability problems arising from the interaction of temporally heterogeneous data in a multi+temporal environment. In particular, we focus on the evaluation of multitemporal algebraic expressions for the support of an SQL-like query interface. The basic requirement of a multitemporal algebra is that all the operations involved must be carefully designed in order to retrieve the largest amount of certain stored information and avoid the creation of spurious information during the interaction of heterogeneous data. The main difficulties underlying the definition of such operations are presented in the paper, together with solutions to overcome them. They include conversion of data from one temporal format into another, definition of lossless operations on temporally incomplete information, and semantic optimization of multitemporal algebraic expressions.

An experimental object-based sharing system for networked databases

An approach and mechanism for the transparent sharing of objects in an environment of interconnected (networked), autonomous database systems is presented. An experimental prototype system has been designed and implemented, and an analysis of its performance conducted. Previous approaches to sharing in this environment typically rely on the use of a global, integrated conceptual database schema; users and applications must pose queries at this new level of abstraction to access remote information. By contrast, our approach provides a mechanism that allows users to import remote objects directly into their local database transparently; access to remote objects is virtually the same as access to local objects. The experimental prototype system that has been designed and implemented is based on the Iris and Omega object-based database management systems; this system supports the sharing of data and meta-data objects (information units) as well as units of behavior. The results of experiments conducted to evaluate the performance of our mechanism demonstrate the feasibility of database transparent object sharing in a federated environment, and provide insight into the performance overhead and tradeoffs involved.

Object orientation in multidatabase systems

A multidatabase system (MDBS) is a confederation of preexisting distributed, heterogeneous, and autonomous database systems. There has been a recent proliferation of research suggesting the application of object-oriented techniques to facilitate the complex task of designing and implementing MDBSs. Although this approach seems promising, the lack of a general framework impedes any further development. The goal of this paper is to provide a concrete analysis and categorization of the various ways in which object orientation has affected the task of designing and implementing MDBSs. We identify three dimensions in which the object-oriented paradigm has influenced this task: the general system architecture, the schema architecture, and the heterogeneous transaction management. Then we provide a classification and a comprehensive analysis of the issues related to each of the above dimensions. To demonstrate the applicability of this analysis, we conclude with a comparative review of existing multidatabase systems.

Serializability by commitment ordering

A new serializability concept, Commitment Ordering (CO), allows to effectively achieve global serializability across multiple autonomous Database Systems, that may use different (any) concurrency control mechanisms. CO generalizes the popular Strong-Strict Two Phase Locking (S-S2PL) concept.

A TWO-LEVEL METADATA DICTIONARY APPROACH FOR SEMANTIC QUERY PROCESSING IN MULTIDATABASE SYSTEMS

A multidatabase system (MDBS) is a system that integrates several autonomous database systems and provides users with a uniform access to all the databases. In this paper we developed a two-level active metadata dictionary approach for semantic query processing. To capture the global view of data schemas of participating databases which may be heterogeneous, a Hornclause data model is used. The lower-level metadata dictionaries (LLMDs) keep metadata for each corresponding local database in MDBS. The higher-level metadata dictionary (HLMD) integrates the metadata about all LLMDs. The database integration strategy includes two phases: schema translation and schema integration. It is a bottom-up approach integrating schema from the underlying database schemas. The evaluation strategy is a top-down approach. It starts with a query as a global goal to be achieved, unifies and optimizes the query to decompose the goal into subgoals that can be evaluated against extensional database, then translates these subgoals into corresponding queries against underlying DBMSs. To solve the control problem, we employ a G-net model for procedure control and inference control. An experimental implementation in Prolog is described.

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.