Data replication is one of the main techniques by which database systems can hope to meet the stringent temporal constraints of current time-critical applications, especially Web-based directory and electronic commerce services. A pre-requisite for realizing the benefits of replication, however, is the development of high-performance concurrency control mechanisms. We present in this paper MIRROR (Managing Isolation in Replicated Realtime Object Repositories), a concurrency control protocol specifically designed for firm-deadline applications operating on replicated real-time databases. MIRROR augments the optimistic two-phase locking (O2PL) algorithm developed for non real-time databases with a novel and simple to implement state-based conflict resolution mechanism to fine-tune real-time performance. Using a detailed simulation model, we compare MIR-ROR’s performance against the real-time versions of a representative set of classical protocols for a range of transaction workloads and system configurations. Our performance studies show that (a) the relative performance characteristics of replica concurrency control algorithms in the real-time environment could be significantly different from their performance in a traditional (non-real-time) database system, (b) MIRROR provides the best performance in both fully and partially replicated environments for real-time applications with low to moderate update frequencies, and (c) MIRROR’s conflict resolution mechanism works almost as well as more sophisticated (and difficult to implement) strategies.

Typically, a real-time system consists of a a controlling system and a controlled system. In an automated factory, the controlled system is the factory floor with its robots, assembling stations, and the assembled parts, while the controlling system is the computer and human interfaces that manage and coordinate the activities on the factory

Software Transactional Memories (STMs) are emerging as a highly attractive programming model, thanks to their ability to mask concurrency management issues to the overlying applications. In this paper we are interested in dependability of STM systems via replication. In particular we present an extensive simulation study aimed at assessing the efficiency of some recently proposed database-oriented replication schemes, when employed in the context of STM systems. Our results point out the limited efficiency and scalability of these schemes, highlighting the need for redesigning ad-hoc solutions well fitting the requirements of STM environments. Possible directions for the re-design process are also discussed and supported by some early quantitative data. 1

Software Transactional Memories (STMs) are emerging as a potentially disruptive programming model. In this paper we are address the issue of how to enhance dependability of STM systems via replication. In particular we present AGGRO, an innovative Optimistic Atomic Broadcast-based (OAB) active replication protocol that aims at maximizing the overlap between communication and processing through a novel AGGRessively Optimistic concurrency control scheme. The key idea underlying AGGRO is to propagate dependencies across uncommitted transactions in a controlled manner, namely according to a serialization order compliant with the optimistic message delivery order provided by the OAB service. Another relevant distinguishing feature of AGGRO is of not requiring a-priori knowledge about read/write sets of transactions, but rather to detect and handle conflicts dynamically, i.e. as soon (and only if) they materialize. Based on a detailed simulation study we show the striking performance gains achievable by AGGRO (up to 6x increase of the maximum sustainable throughput, and 75 % response time reduction) compared to literature approaches for active replication of transactional systems. 1

Abstract. In the expanding e-society, mobile embedded systems are increasingly used to support transactions such as for banking, stock or database applications. Such systems entail a range of heterogeneous entities- both the embedded devices and the networks connecting them. While these systems are exposed to frequent and varied perturbations, the support of atomic distributed transactions is still a fundamental requirement to achieve consistent decisions. Guaranteeing atomicity and high performance in traditional fixed wired networks is based on the assumption that faults like node and link failures occur rarely. This assumption is not supported in current and future mobile embedded systems where the heterogeneity and mobility often result in link and node failures as a dominant operational scenario. In order to continue guaranteeing strict atomicity while providing for high efficiency (low resource blocking time and message overhead) and acceptable commit rate, transactional fault-tolerance techniques need to be revisited perhaps at the cost of transaction execution time. In this paper, a comprehensive classification of perturbations and their impact on the design of mobile transactions is provided. In particular we argue for the delay-awareness of mobile transactions to allow for the fault-tolerance mechanisms to ensure resilience to the various and frequent perturbations. Key words: Transactions, mobile database systems, dependability 1

Abstract. Distributed transactional systems require an atomic commitment protocol to preserve atomicity of the ACID properties. However, the industry leading standard, 2PC, is slow and adds a significant overhead to transaction processing. In this paper, a new atomic commitment protocol for main-memory primarybackup systems, C2PC, is proposed. It exploits replication to avoid disk-logging and performs the commit processing in a circular fashion. The analysis shows that C2PC has the same delay as 1PC, and reduces the total overhead compared to 2PC. 1

Abstract: Pervasive healthcare is an emerging discipline, where mobile embedded systems are increasingly used to support transactions. Such systems entail a range of heterogeneous entities- both the embedded devices and the networks connecting them. While these systems are exposed to frequent and varied perturbations, the support of atomic distributed transactions is still a fundamental requirement to achieve consistent decisions. Guaranteeing atomicity and high performance in traditional fixed wired networks is based on the assumption that faults like node and link failures occur rarely. This assumption is not supported in current and future mobile healthcare embedded systems where the heterogeneity and mobility often result in link and node failures as a dominant operational scenario. In this paper we summarize our work to provide for atomic commit protocols for mobile environments where consistency can not be compromised, for example healthcare systems. We present the implementation and experimental evaluation of a commit protocol showing its suitability for healthcare environments.

INTRODUCTION Many real-time database applications are inherently distributed in nature [24, 28]. These include the intelligent network services database described in [5] and the mobile telecommunication system discussed in [29]. More recent applications include the multitude of directory, datafeed and electronic commerce services that have become available on the World Wide Web. An essential requirement for most distributed real-time database system (DRTDBS) applications is transaction atomicity { this is satised by implementing a transaction commit protocol. Commit protocols typically require exchange of multiple messages, in multiple phases, between the participating sites where the distributed transaction executed. In addition, several log records are generated, some of which have to be \forced", that is, ushed to disk immediately in a synchronous manner. Due to this series of synchronous message and logging c

To ensure transaction atomicity, Distributed Database Systems implement transaction commit protocols. A commit protocol guarantees the uniform commitment of distributed transaction execution, that is, it ensures that all the participating sites agree on the final transaction outcome (commit or abort). Most importantly, this guarantee must be valid even in case of site or network failures. Database researchers have been working in this area for the last three decades and a variety of commit protocols have so far been proposed. These protocols include one-phase protocols like Early Prepare(EP), two-phase protocols like the classical Two Phase Commit(2PC), three-phase protocol Three Phase Commit(3PC) and many of their optimizations.

Abstract — In the expanding e-society, mobile embedded systems are increasingly used to support transactions such as for banking or database applications. Such systems entail a range of heterogeneous entities- both the devices and the networks connecting them. While these systems are exposed to frequent and varied perturbations, the support of atomic distributed transactions is still a fundamental requirement to achieve consistent decisions. Guaranteeing atomicity and high performance in traditional fixed wired networks is based on the assumption that node and link failures occur rarely. This assumption is often not supported in current and upcoming mobile environments where the heterogeneity and mobility often result in link and node failures as a dominant operational scenario. In order to continue guaranteeing strict atomicity while providing for high efficiency (low resource blocking time of transaction participants and message overhead) and acceptable commit rate, transactional fault-tolerance techniques need to be revisited perhaps at the cost of transaction execution time. In this paper, we provide a comprehensive classification of perturbations for a wide range of mobile environments including infrastructure-based, ad-hoc, and hybrid environments. We also investigate the impact of these perturbations on the design of mobile transactions. In particular we argue for the delay-awareness of mobile transactions to allow for the fault-tolerance mechanisms to ensure resilience to the various and frequent perturbations. Index Terms — Transactions, mobile database systems, dependability I.