João Lourenço

Database systems are a key component behind many of today's computer systems. As a consequence, it is crucial that database systems provide correct and continuous service despite unpredictable circumstances, such as software bugs or attacks. This paper presents the design of Byzantium, a Byzantine fault-tolerant database replication middleware that provides snapshot isolation (SI) semantics. SI is very popular because it allows increased concurrency when compared to serializability, while providing similar behavior for typical workloads. Thus, Byzantium improves on existing proposals by allowing increased concurrency and not relying on any centralized component. Our middleware can be used with off-the-shelf database systems and it is built on top of an existing BFT library.

Transactional memory (TM) is an increasingly popular technique for synchronising threads in multi-threaded programs. To address both correctness and performance-related issues of TM programs, one needs to monitor and analyse their execution. However, monitoring concurrent programs (including TM programs) may have a non-negligible impact on their behaviour, which may hamper the objectives of the intended analysis. In this paper, we propose several approaches for monitoring TM programs and study their impact on the behaviour of the monitored programs. The considered approaches range from specialised lightweight monitoring to generic heavyweight monitoring. The implemented monitoring tools are publicly available to the scientific community, and the implementation techniques used for lightweight monitoring of TM programs may be used as an inspiration for developing other specialised lightweight monitors.

This special issue concerns a range of issues related to the development of concurrent programs. This is an important topic, because many systems are now either multi-threaded or distributed, and it is well known that concurrency makes testing, analysis and debugging significantly more complicated. Essentially, the alternative interleavings of events can lead to different behaviours, and so any analysis, debugging or testing technique must consider these interleavings. The interest in this topic is reflected in the larger than normal issue, which contains five papers. The papers fall into three groups: we start with a paper on debugging, then have two on static analysis techniques and finally have two on testing. All papers were reviewed in the normal way.

In software, hardware, and embedded system domains, debugging is the process of locating and correcting faults in a system. Depending on the context, the various characteristics of debugging induce different challenges and solutions. Post-silicon hardware debugging, for example, needs to address issues such as limited visibility and controllability, while debugging software entails other issues, such as the handling of distributed or non-deterministic computation. The challenges that accompany such issues are the focus of many current research efforts. Solutions for debugging range from interactive tools to highly analytic techniques. We have seen great advances in debugging technologies in recent years, but bugs continue to occur, and debugging still encompasses significant portions of the life-cycles of many systems. The session covered state-of-the-art approaches as well as promising new research directions in both the hardware and software domains.

Transactional Memory [3] (TM) is a new paradigm for concurrency control that brings the concept of transactions, widely known from the Databases community, into the management of data located in main memory. TM delivers a powerful semantics for constraining concurrency and provides the means for the extensive use of the available parallel hardware. TM uses abstractions that promise to ease the development of scalable parallel applications by achieving performances close to fine-grained locking while maintaining the simplicity of coarse-grained locking.