Optimistic Methods for Concurrency Control in Distributed Database Systems

Recently, methods for concurrency control have been proposed which were called "optimistic". These methods do not detect access conflicts when they occur; instead, a transaction always proceeds, and at its end a check is performed whether a conflict has happened. If so, the transaction is backed up. This basic approach is investigated in two directions: First, a method is developed which frees read transactions from any consideration of concurrency control; all responsibility for correct synchronization is assigned to the update transactions. This method has the great advantage that, in case of conflicts between read transactions and update transactions, no backup is performed. Then, the application of optimistic solutions in distributed database systems is discussed, a solution is presented

An Object-Oriented Programming Environment for Advanced Database Applications

For vartous reasons conventional database systems fail to provide appropriate support for a wide vartety of applications. Therefore, new generation database systems are being developed in some recent research efforts, among them OOPS. OOPS is an object-oriented programming system with integrated datamanagement facilities. lt is designed to especially support advanced applications, like e. g. engineering and office applications. Extensive data modelling facilities, as they are typically offered by the abstract data type concept of object-oriented languages, are enriched by advanced data management concepts. Therefore, not only abstraction mechanisms, like e.g. the concepts of classification, generalization and aggregation, are provided but also modulartty (keeping logically related things together), information hiding (hiding the implementation / internal representation of objects) and extensibility and flexibility (by allowing the modification of existing and the addition of new data types (classes) and operations). Furthermore, to especially support the semantics of applications, complex integrity constraints, exceptions, and triggers can be specified. Since all these facilities are integrated into a single language the artificial distinction between database language and programming language is eleminated

Concurrency Control in Integrated Information Systems: A Survey of Problems

A main stream of research in the area of database systems is the extension of these systems to support non-classical applications like CAD, information-retrieval or office information systems. The usage of these systems is different in some essential aspects from classical database systems. It turns out that these different usage patterns have direct consequences for concurrency control. This paper gives a summary of the research problems to be solved for these integrated information systems with respect to concurrency control and recovery. It is not the intention of the paper to present special solutions, but to characterize the open issues

Data Modeling for Hypermedia

Hypermedia systems are in a fair way to become an important information system class for a wide range of new and fascinating applications. But current systems still have some very unpleasant restrictions. For example, only very few hypermedia systems support the design of higher level hypermedia objects beyond the basic node-link paradigm; there are further restrictions concerning the modularization of the overall design and the reuse (or sharing) of hypermedia resources. In this paper we address the problems outlined above from a design-oriented point of view and present an object-oriented approach to hypermedia data modeling. In general, object-oriented (database) systems are good candidates to map the complex nature of hypermedia applications. We introduce link types with particular behaviour, a general mechanism for the sharing of hypermedia resources and the SBL-node concept as fundamental extensions of the basic hypermedia data model. SBL-nodes are composite nodes determinining structural and hehavioural properties of hypermedia subnetworks in a local environment. The hypermedia model proposed in this paper provides the basis for the implementation of a hypermedia engine prototype on top of an object-oriented database management system

Improved optimistic concurrency control and its use in distributed database systems

The original approach of optimistic concurrency control has some serious weaknesses with respect to the validation technique, the treatment of long transactions and the problem of starvation. In the first part of this paper some design alternatives of the validation-phase are considered which not only avoid these disadvantages but additionally improve the performance of the original approach. Very promising is a new validation technique which especially supports readtransactions, therefore decreasing the number of backups substantially. In the second part of this paper an adaptation of the new optimistic concurrency control schemes to a distributed environment is presented. Although a lot of algorithms based on two-phase locking have been proposed, concurrency control for distributed databases is still the subject of intensive research because most locking approaches have the common disadvantage that deadlocks may occur. As deadlock detection is a difficult and expensive task in a distributed database system, nonblocking algorithms like the time-stamp approach or the optimistic approach are promising alternatives. We present an optimistic concurrency control algorithm for a distributed database system, which is based on a global coordinator. This algorithm benefits from the improved validation techniques presented in the first part. Its advantage is high flexibility and good performance due to high parallelism and small message overhead. Of course, the algorithm also is deadlock-free. lt will be shown that the main disadvantage, the existence of a global coordinator, does not have that much weight

Active Relationships: A Means for Controlled Propagation of Information and Activities in Databases

In many complex applications (design, manufacturing, workflow management, etc.) information about activities on specific objects affect related objects and result in subsequent activities on distant objects. The way in which objects are influenced by actions on related objects is dependent on the operational semantics of the relationship. To capture this semantics, we introduce the concept of "active relationships" which contain a description of the activity propagation as an integral part of the relationship type definition. Active relationships allow to separate the specification of local activities and their global consequences and to solve typical problems concerning consistency control, configuration management, resource activation, etc. The paper discusses the features of active relationships and their integration into an object-oriented environment by means of several application examples. In addition, it describes an implementation of active relationships based on an object-oriented database and the use of this implementation for coupling two subsystems to an integrated system

Complex and Composite Objects in CAD/CAM Databases

An essential modelling concept for CAD/CAM databases is the composition relationship which relates a superior composite object to its components and makes the component's data visible in the composite object. In order to import only the necessary information into the composite object, often an interface object is defined which is copied into the composite object as an abstraction of the component. In this paper an object oriented model is introduced which comprises an inheritance relationship between an inheritor object and a transmitter object. The inheritor object inherits (the existence of) attributes of the transmitter object, but moreover it also may inherit the attributes' values. This inheritance relationship can be used to model the notion of interface of an object as well as the composition relationship. Thus, by using the only concept of value inheritance it is possible to model all aspects of the composition of objects. Furthermore, some severe drawbacks of a rigid interface concept can be overcome by the inheritance relationship

Redesign of Optimistic Methods: Improving Performance and Applicability

The original approach of optimistic concurrency control /2/ has some serious weaknesses with respect to validation, long transactions and starvation. This paper first discusses design alternatives which avoid these disadvantages. Essential improvements can be reached by a new validation scheme which is called snapshot validation. The paper then discusses a validation scheme which especially supports read transactions, so that the number of backups decreases substantially. Finally a very simple solution for the starvation problem is presented. The proposal is perfectly consistent with the underlying optimistic approach

Information models for connectivity in electronic design

Within the last years, the need for information modelling was more and more recognized in the world of electronic design. During 1990 both the EDIF European TSC Information Modelling Working Group (IMG) and the CFI Technical Subcommittee Design Representation (DR) have started to develop an information model for VLSI design representation data using the language Express. While the DR model is accompanied by a programming interface (PI) for tool access, the primary goal of the IMG model is to contribute to an understanding of the semantical concepts underlying EDIF. Both groups started their efforts with modelling connectivity. While the DR model is still restricted to that area, i.e. it covers approximately the EDIF netlist view, the IMG model also includes parts of other views, namely schematics and masklayout. This paper compares the connectivity parts of both models to find out, where they harmonize and in which points they differ. The comparison will show, that the basic entities (cell, view, port) are defined similar in both models, but that there are some differences in the connectivity of complex ports, which represent the concept of busses. Although there are differences in style and wording between the two models according to their primary goals, the commonly used language Express facilitates the comparison of the overlapping areas of both models. Thus it becomes clear, that the use of Express contributes to communication and clarification in the area of electronic design