Optimistic Recovery is a new technique supporting application-independent transparent recovery from processor failures in distributed systems. In optimistic recovery communication, computation and checkpointing proceed asynchronously. Synchronization is replaced by causal dependency trock-ing, which enables a posteriori reconstruction of a consistent distributed system state following a failure using process rollback and message replay. Because there is no synchronization among computation, communication, and checkpointing, optimistic recovery can tolerate the failure of an arbitrary number of processors and yields better throughput and response time than other general recovery techniques whenever failures are infre-quent.

In this paper, a terminological framework is provided for describing different transaction-oriented recovery schemes for database systems in a conceptual rather than an implementation-dependent way. By introducing the terms materialized database, propagation strategy, and checkpoint, we obtain a means for classifying arbitrary

Concurrency control has been thoroughly studied in the context of traditional database applications such as banking and airline reservations systems. There are relatively few studies, however, that address the concurrency control issues of advanced database applications such as CAD/CAM and software development environments. The

Compensating transactions are intended to handle situations where it is required to undo either committed or uncommitted transactions that affect other transactions, without resorting to cascading aborts. This stands in sharp contrast to the standard approach to transaction recovery where cascading aborts are avoided by requiring transactions to read only committed data, and where committed transactions are treated as permanent and irreversible. We argue that this standard approach to recovery is not suitable for a wide range of advanced database applications, in particular those applications that incorporate long-duration or nested transactions. We show how compensating transactions can be effectively used to handle these types of applications. We present a model that allows the definition of a variety of types of correct compensation. These types of compensation range from traditional undo, at one extreme, to application-dependent, special-purpose compensating transactions, ...

Introduction This chapter gives an overview on multilevel transactions and its generalization toward open nested transactions. The main features of these transaction models are the following: first, semantic properties of operations can be exploited to relax the isolation of concurrent transactions; second, as a consequence, atomicity is achieved by compensation rather than state-based undo; and third, subtransactions can be made persistent independently of their commit state, that is, global visibility of their updates. Advanced transaction models and new correctness criteria for transaction executions have been proposed for the following reasons (and possibly further reasons that are not mentioned here): 1. to provide better support for long-lived activities in advanced DBMS applications, 2. to relax the classical ACID paradigm, for example, provide more flexibility as to when updates are made visible to concurrent transactions, 3. to support cooperation between the members

Synchronization of accesses to shared data and recovering the state of such data in the case of failures are really two aspects of the same problem--implementing atomic actions on a related set of data items. In this paper a mechanism that solves both problems simultaneously in a way that is compatible with requirements of decentralized systems is described. In particular, the correct construction and execution of a new atomic action can be accomplished without knowledge of all other atomic actions in the system that might execute concurrently. Further, the mechanisms degrade gracefully if parts of the system fail: only those atomic actions that require resources in failed parts of the system are prevented from executing, and there is no single coordinator that can fail and bring down the whole system.

The Coordinated Atomic Action (or CA action) concept is a unified scheme for coordinating complex concurrent activities and supporting error recovery between multiple interacting objects in a distributed object-oriented system. It provides a conceptual framework for dealing with different kinds of concurrency and achieving fault tolerance by extending and integrating two complementary concepts --- conversations and transactions. Conversations (enhanced with concurrent exception handling) are used to control cooperative concurrency and to implement coordinated error recovery whilst transactions are used to maintain the consistency of shared resources in the presence of failures and competitive concurrency. This paper explains the CA action concept in detail and then addresses related design issues such as multi-thread coordination, exception handling and resolution, coordinated access to shared objects and provision of software fault tolerance. Finally, brief details are given of a numb...

Open-ended activities are characterized by uncertain duration, unpredictable developments, and interactions with other concurrent activities. Like other database applications, they require consistent concurrent access and fault-tolerance, but their unconventional characteristics are incompatible with the conventional database mechanisms of concurrency and failure atomicity. We present the split-transaction and join-transaction operations for restructuring in-progress transactions, as an approach to consistent concurrent access and fault-tolerance for open-ended activities. Split-transaction divides an on-going transaction into two or more transactions that are serializable with respect to each other and all other transactions, and each of the new transactions is later committed or aborted independently of the others. Jointransaction merges two or more transactions that are serializable with respect to each other into a single transaction as if they had always been part of the same tra...

This chapter surveys extended transaction models proposed to support long duration, interactive and/or cooperative activities in the context of multi-user software development and CAD/CAM environments. Many of these are variants of the checkout model, which addresses the long duration and interactive nature of the activities supported by environments but still isolates environment users, making it difficult for them to collaborate while their activities are in progress. However, a few cooperative transaction models have been proposed to facilitate collaboration, usually while maintaining some guarantees of consistency.

This paper has two main contributions. First, we present a comprehensive model for nested transaction systems. The model allows rigorous proofs of a wide variety of transaction-processing algorithms in a single uniform framework. The model generalizes most previous work on concurrency control to encompass nested transactions and type-specific concurrency control algorithms. We used the model to define correctness for nested transaction systems and also to discuss alternative correctness criteria. Second, we present a new concurrency control and recovery algorithm for abstract data types in a nested transaction system and prove it correct. The algorithm, which generalizes an algorithm developed by Weihl [42, 39] to handle nested transactions, uses commutativity properties of operations to achieve high levels of concurrency. The results of operations, in addition to their names and arguments, can be used in checking for conflicts, further increasing concurrency. As part of our development of the general model, we present a theorem that provides a general sufficient condition for a transaction-processing algorithm to be correct. This condition is analogous to the "absence of cycles" condition used in the more classical work on concurrency control (e.g., see [7]). We use the condition as the basis of the correctness proof of the algorithms presented in this paper. We have also used it in other work to prove the correctness of other algorithms. For example, in [2], we prove the correctness of Reed's multi-version timesta?ping algorithm [34] and of a type-specific variation of Reed's algorithm that uses the semantics of operations to permit more concurrency. The description and correctness proof of our algorithm are modular. We consider a system structure consisting of many o...