The basic transaction model has evolved over time to incorporate more complex transaction structures and to selectively modify the atomicity and isolation properties. In this chapter we discuss the application of transaction concepts to activities that involve coordinated execution of multiple tasks (possibly of different types) over different processing entities. Such applications are referred to as transactional workflows. In this chapter we discuss the specification of such workflows and the issues involved in their execution. 1 What is a Workflow? Workflows are activities involving the coordinated execution of multiple tasks performed by different processing entities. A task defines some work to be done and can be specified in a number of ways, including a textual description in a file or an email, a form, a message, or a computer program. A processing entity that performs the tasks may be a person or a software system (e.g., a mailer, an application program, a database mana...

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 main difficulty in enforcing global serializability in a multidatabase environment lies in resolving indirect (transitive) conflicts between multidatabase transactions. Indirect conflicts introduced by local transactions are difficult to resolve because the the behavior or even the existence of local transactions is not known to the multidatabase system. To overcome these problems, we propose to incorporate additional data manipulationoperations 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 a multidatabase transaction management method that requires the local database systems to ensure only local serializability. The proposed method and its refinements do not violate the autonomy of the local database systems an...

To enforce global serializability in a multidatabase environment themultidatabase 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 assure 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.

Global transaction management requires cooperation from local sites to ensure the consistent and reliable execution of global transactions in a distributed database system. In a heterogeneous distributed database (or multidatabase) environment, various local sites make conflicting assertions of autonomy over the execution of global transactions. A flexible transaction model for the specification of global transactions makes it possible to deal robustly with these conflicting requirements. This paper presents an approach that preserves the semi-atomicity (a weaker form of atomicity) of flexible transactions, allowing local sites to autonomously maintain serializability and recoverability. We offer a fundamental characterization of the flexible transaction model and precisely define the semi-atomicity. We investigate the commit dependencies among the subtransactions of a flexible transaction. These dependencies are used to control the commitment order of the subtransactions. We next iden...

Future information processing environments will consist of a vast network of heterogeneous, autonomous, and distributed computing resources, including computers (from mainframe to personal), information-intensive applications, and data (files and databases). A key challenge in this environment is providing capabilities for combining this varied collection of resources into an integrated distributed system, allowing resources to be flexibly combined, and their activities coordinated, to address challenging new information processing requirements. In this paper, we describe the concept of distributed object management, and identify its role in the development of these open, interoperable systems. We identify the key aspects of system architectures supporting distributed object management, and describe specific elements of a distributed object management system being developed at GTE Laboratories. 1. Introduction Today, computer usage is expanding into all parts, and all functions, of lar...

Composite systems are collections of autonomous, heterogeneous, and distributed software applications. In these systems, data dependencies are continuously violated by local operations and therefore, coordination processes are necessary to guarantee overall correctness and consistency. Such coordination processes must be endowed with some form of execution guarantees, which require the participating subsystems to have certain database functionality (such as atomicity of local operations, order-preservation and either compensation of operations or the deferment of their commit). However, this functionality is not present in many applications and must be implemented by a transactional coordination agent coupled with the application. In this paper, we discuss the requirements to be met by the applications and their associated transactional coordination agents. We identify a minimal set of functionality the applications must provide in order to participate in transactional coordination pro...

In many applications in a multidatabase environment global serializability is needed to assure correctness of concurrent execution of transactions. The serializability of all local schedules is, by itself, not sufficient to ensure the global serializability, since the (relative) local serialization orders of subtransactions of global transactions must be the same at all systems the global transactions execute. In this paper, we present a fully decentralized global concurrency control method in which the concurrency control decisions concerning global transactions can be made at each site, based on the information that is locally available. The method uses a top-down approach to enforce the same serialization order at all sites a global transaction is executed. The proposed method uses forced local conflicts to prevent unacceptable local schedules while assuring deadlock free execution. 1 Introduction A Multidatabase System (MDBS) is a federation of autonomous and possibly heterogeneou...

In recent years, databases have started to be used as intelligent repositories for a variety of semantically-richer systems. A consequence of such architectures is that transaction scheduling takes place throughout composite systems consisting of layered subsystems. Such transaction architectures have been studied extensively. Existing theory, however, limits the degree of parallelism, and makes a number of simplifying assumptions which cannot be taken for granted in practice. This paper proposes a new model and correctness criterion, stack conflict consistency, for composite transactional systems. The main contribution of the new model is to establish the correctness conditions under which higher degrees of parallelism can be achieved between operations of the same transaction, as well as between conflicting operations of different transactions, in a uniform way. This possibility, although hinted at previously, has not yet been exploited in practical composite systems. Hence, we hope...

This paper surveys recent workon extended transaction management. It focusses on transaction--oriented applications in large distributed and heterogeneous information systems where applications often span multiple autonomous databases. In such an environment, the challenge is to reconcile data consistency and acceptable performance to the best extent possible, which may imply trading off consistency versus performance in a controlled manner. One approach is to decompose a long user interaction into several independently executed transactions and to provide an additional control sphere as the "glue" between the transactions. This additional control sphere is usually referred to as an activity or a transactional workflow. Workflow management serves to specify and enforce certain types of execution dependencies between the transactions of a workflow, and thus captures behavioral consistency properties of an application. A second approach is to execute a long user interaction as a transact...