Kerstin Schwarz

Transitive dependencies in transaction closures

Complex applications consist of a large set of transactions which are interrelated. There are different kinds of dependencies among transactions of a complex application, e.g. termination or execution dependencies which are constraints on the occurrence of significant transaction events. The authors analyze a set of (orthogonal) transaction dependencies. They do not follow traditional approaches which consider advanced transaction structures as a certain kind of nested transactions. They introduce the notion of transaction closure as a generalization of nested transactions. A transaction closure comprises all transactions which are (transitively) initiated by one (root) transaction. By specifying dependencies among transactions of a transaction closure they are then able to define well-known transaction structures like nested transactions as well as advanced activity structures, e.g. workflows, in a common framework. In particular they consider the transitivity property for all kinds of transaction dependencies discussed in the paper. Thus, they are able to conclude how two arbitrary transactions are transitively interrelated. This issue is fundamental for understanding the entire semantics of a complex application.

Design Support for Database Federations

Federated database systems provide a homogeneous interface to possibly heterogeneous local database systems. This homogeneous interface consists of a global schema which is the result of a logical integration of the schemata of the corresponding local database systems and file systems. In this paper, we sketch the integration process and a set of tools for supporting the design process. Besides the classical database schema integration, the design process for federated information systems requires the integration of other aspects like integrity rules, authorization policies and transactional processes. This paper reports on an integrated approach to tool support of several of these integration aspects. The different integration facets are linked via the database integration method GIM allowing a high degree of automatic integration steps.

Transactions and Database Dynamics

Federated transaction management (also known as multidatabase transaction management in the literature) is needed to ensure the consistency of data that is distributed across multiple, largely autonomous, and possibly heterogeneous component databases and accessed by both global and local transactions. While the global atomicity of such transactions can be enforced by using a standardized commit protocol like XA or its CORBA counterpart OTS, global serializability is not selfguaranteed as the underlying component systems may use a variety of potentially incompatible local concurrency control protocols. The problem of how to achieve global serializability, by either constraining the component systems or implementing additional global protocols at the federation level, has been intensively studied in the literature, but did not have much impact on the practical side. A major deficiency of the prior work has been that it focused on the idealized correctness criterion of serializability and disregarded the subtle but important variations of SQL isolation levels supported by most commercial database systems. This paper reconsiders the problem of federated transaction management, more specifically its concurrency control issues, with particular focus on isolation levels used in practice, especially the popular snapshot isolation provided by Oracle. As pointed out in a SIGMOD 1995 paper by Berenson et al., a rigorous foundation for reasoning about such concurrency control features of commercial systems is sorely missing. The current paper aims to close this gap by developing a formal framework that allows us to reason about local and global transaction executions where some (or all) transactions are running under snapshot isolation. The paper derives criteria and practical protocols for guaranteeing global snapshot isolation at the federation level. It further generalizes the well-known ticket method to cope with combinations of isolation levels in a federated system.

Extending Transaction Closures by N-ary Termination Dependencies

Transaction dependencies have been recognized as a valuable method in describing restrictions on the executions of sets of transactions. A transaction closure is a generalized transaction structure consisting of a set of related transactions which are connected by special dependencies. Traditionally, relationships between transactions are formulated by binary dependencies. However, there are applications scenarios where dependencies must be specified among more than two transactions. Since n-ary dependencies cannot be expressed by binary dependencies, appropriate extensions are required. In this paper, we extend the concept of transaction closure by ternary termination dependencies. We show how n-ary termination dependencies can be expressed by binary and ternary termination dependencies. As a result, we present rules for reasoning about the combination of these termination dependencies.

Execution dependencies in transaction closures

Activities of advanced applications can be modeled by interrelated transactions. These relations can be described by different kinds of transaction dependencies. The notion of transaction closure is a generalization of nested transactions providing means to describe complex activities such as transactional workflows. In this paper our main focus lies on execution dependencies for describing certain control flows among related transactions of transaction closures. In particular we consider the transitivity property for all kinds of transaction execution dependencies and discuss their relationship to other kinds of dependencies such as transaction termination dependencies. We point out that some of these dependency combinations are incompatible. As a result we present rules for reasoning about the transitivity of execution dependencies. Thus, we are able to conclude how arbitrary transactions of a transaction closure are transitively interrelated.

INTEGRATING EXECUTION DEPENDENCIES INTO THE TRANSACTION CLOSURE FRAMEWORK

The transaction closure framework provides means to describe and reason about different kind of dependencies between interrelated transactions. In this paper, we investigate execution dependencies for describing certain control flows among related transactions of a transaction closure. In particular, we consider the transitivity property for all kinds of transaction execution dependencies and present a complete and minimal set of rules for reasoning about the transitivity of execution dependencies. Furthermore, we analyze the relationship between execution and termination dependencies and point out that some dependency combinations are incompatible. Using derived transitive dependencies, we are able to conclude how arbitrary transactions of a transaction closure are transitively interrelated and, thus, to detect contradictory dependency specifications as well as superfluous transactions.

Wechselwirkungen von Abhängigkeiten in transaktionalen Workflows

Workflows bestehen aus Tasks mit vielschichtigen Beziehungen untereinander. Beziehungen zwischen wenigen Tasks sind meist offensichtlich, haben aber haufig einen indirekten Einflus auf andere Tasks. Bereits die Verknupfung mehrerer Tasks kann zu unuberschaubaren Spezifikationen mit komplexen Wechselwirkungen zwischen den Tasks fuhren. Deshalb ist es fur den Designer oft schwierig, die transitiven Beziehungen zwischen beliebigen Tasks des Workflows zu erkennen und Ruckschlusse bezuglich einer korrekten, der Real-Welt entsprechenden Modellierung zu ziehen. Der Designer benotigt eine Unterstutzung des Entwurfprozesses auf konzeptioneller Ebene.

Abhängigkeiten zwischen Transaktionen in föderierten Datenbanksystemen

In foderierten Datenbanksystemen werden globale Transaktionen zur Ausfuhrung auf lokalen Datenbanksystemen in globale Subtransaktionen aufgeteilt. Die Ergebnisse dieser Subtransaktionen werden anschliesend global zusammengefuhrt und an den Nutzer weitergeleitet. Zwischen den globalen Subtransaktionen und der globalen Transaktion bestehen je nach Anfrageart und Auspragung des betroffenen Schemas unterschiedliche Abhangigkeiten. In einigen Fallen wird eine globale Transaktion als erfolgreich ausgefuhrt angesehen, wenn mindestens eine globale Subtransaktion erfolgreich war. In anderen Fallen darf beispielsweise nur genau eine globale Subtransaktion erfolgreich beendet werden oder alle globalen Subtransaktionen mussen erfolgreich zum Ende gefuhrt werden. In diesem Papier untersuchen wir, welche Terminierungsabhangigkeiten zwischen den jeweiligen globalen Subtransaktionen und der globalen Transaktion sinnvoll sind. Das Wissen uber diese Abhangigkeiten schafft die Grundlage fur die Entwicklung neuer Commit-Protokolle zur Unterstutzung unterschiedlicher globaler Einfuge-, Losch- und Anderungssemantiken.

Computing Rules for Detecting Contradictory Transaction Termination Dependencies

Complex applications consist of a large set of interrelated transactions. Often, it is very difficult for the application/transaction designer to get a grasp of the transitive relationships among the transactions of a complex application. In this paper, we discuss transitive termination dependencies under consideration of transaction compensation and present an algorithm to derive a set of rules for reasoning about transitive dependencies. These rules may help the designer in understanding the entire semantics of a complex application and detecting contradictory dependency specifications.