In fault-tolerant distributed computing, an atomic broadcast or total order broadcast is a broadcast where all correct processes in a system of multiple processes receive the same set of messages in the same order; that is, the same sequence of messages. The broadcast is termed "atomic" because it either eventually completes correctly at all participants, or all participants abort without side effects. Atomic broadcasts are an important distributed computing primitive. Properties The following properties are usually required from an atomic broadcast protocol:

Validity: if a correct participant broadcasts a message, then all correct participants will eventually receive it.

Uniform Agreement: if one correct participant receives a message, then all correct participants will eventually receive that message.

Uniform Integrity: a message is received by each participant at most once, and only if it was previously broadcas

Atomic broadcast

Nils Richard Ekwall

Within only a couple of generations, the so-called digital revolution has taken the world by storm: today, almost all human beings interact, directly or indirectly, at some point in their life, with a computer system. Computers are present on our desks, computer systems control the antilock braking system and the stability control in cars, they collect usage statistics in elevators in order to anticipate maintenance and repair operations. Computer systems also operate critical systems, such as nuclear power plants, airplane control systems or space rockets. Furthermore, computer systems are not only omnipresent, but also increasingly networked. As the use of computer systems has increased dramatically over the past decades, the needs and expectations associated with these systems have also increased. In particular, one of the critical points of a system is its availability (the fraction of the time during which the system provides a service to the users): the costs and negative publicity of a system outage (be it a commercial web site or a stock exchange for example) are often considerable. Fault tolerance is one of the approaches to designing a highly-available system: a fault tolerant system is designed in such a way that the failure of one of the components of the system does not compromise the functionality of the system as a whole. Replication is one of the common fault tolerance techniques. Instead of having a single machine (a replica) providing a service, the system is composed of several replicas running the service and connected through a network. If one of the replicas fails, the service is still provided by the remaining replicas. The replication technique is interesting as it can be achieved by using software running on commodity hardware, thus avoiding the high cost of special purpose hardware. Replication, although intuitive to understand, is complex to implement in practice, as the replicas have to interact in order to ensure the consistency of the system as a whole. Group communication simplifies the replication problem, by hiding issues such as the communication between the replicas, the crashes of one or several replicas and the synchronization of the replicas. In this thesis, we start by comparing two replication techniques – group communication and quorum systems – and identifying in which case either technique should be used. Atomic broadcast (a group communication primitive at the heart of this work) allows replicas to broadcast messages to each other and then deliver them in the same total order, even if replicas broadcast messages quasi simultaneously. Atomic broadcast is especially useful for replication: since all replicas deliver messages in the same order, their state is kept consistent. After the comparison between the replication techniques, we present an atomic broadcast algorithm designed to perform well when the system is heavily loaded and that allows to quickly detect crashed replicas (by minimizing the consequences of wrongly suspecting a non-crashed replica). The presentation of the algorithm includes simulation results comparing the performance of the new algorithm to previously proposed atomic broadcast algorithms. The second part of the thesis focuses on the experimental performance evaluation of the new algorithm in several settings. We start by comparing four atomic broadcast algorithms in a local area network. We then compare three of the four algorithms in a wide area network, with sites in Switzerland, Japan and France, and where the round trip time between the sites varies between 4 and 300 ms. Finally, we evaluate the impact of the size of the system (the number if replicas) on the performance of the algorithms.

EPFL2007

Evaluating the performance of distributed agreement algorithms

Nowadays, networked computers are present in most aspects of everyday life. Moreover, essential parts of society come to depend on distributed systems formed of networked computers, thus making such systems secure and fault tolerant is a top priority. If the particular fault tolerance requirement is high availability, replication of components is a natural choice. Replication is a difficult problem as the state of the replicas must be kept consistent even if some replicas fail, and because in distributed systems, relying on centralized control or a certain timing behavior is often not feasible. Replication in distributed systems is often implemented using group communication. Group communication is concerned with providing high-level multipoint communication primitives and the associated tools. Most often, an emphasis is put on tolerating crash failures of processes. At the heart of most communication primitives lies an agreement problem: the members of a group must agree on things like the set of messages to be delivered to the application, the delivery order of messages, or the set of processes that crashed. A lot of algorithms to solve agreement problems have been proposed and their correctness proven. However, performance aspects of agreement algorithms have been somewhat neglected, for a variety of reasons: the lack of theoretical and practical tools to help performance evaluation, and the lack of well-defined benchmarks for agreement algorithms. Also, most performance studies focus on analyzing failure free runs only. In our view, the limited understanding of performance aspects, in both failure free scenarios and scenarios with failure handling, is an obstacle for adopting agreement protocols in practice, and is part of the explanation why such protocols are not in widespread use in the industry today. The main goal of this thesis is to advance the state of the art in this field. The thesis has major contributions in three domains: new tools, methodology and performance studies. As for new tools, a simulation and prototyping framework offers a practical tool, and some new complexity metrics a theoretical tool for the performance evaluation of agreement algorithms. As for methodology, the thesis proposes a set of well-defined benchmarks for atomic broadcast algorithms (such algorithms are important as they provide the basis for a number of replication techniques). Finally, three studies are presented that investigate important performance issues with agreement algorithms. The prototyping and simulation framework simplifies the tedious task of developing algorithms based on message passing, the communication model that most agreement algorithms are written for. In this framework, the same implementation can be reused for simulations and performance measurements on a real network. This characteristic greatly eases the task of validating simulation results with measurements (or vice versa). As for theoretical tools, we introduce two complexity metrics that predict performance with more accuracy than the traditional time and message complexity metrics. The key point is that our metrics take account for resource contention, both on the network and the hosts; resource contention is widely recognized as having a major impact on the performance of distributed algorithms. Extensive validation studies have been conducted. Currently, no widely accepted benchmarks exist for agreement algorithms or group communication toolkits, which makes comparing performance results from different sources difficult. In an attempt to consolidate the situation, we define a number of benchmarks for atomic broadcast. Our benchmarks include well-defined metrics, workloads and failure scenarios (faultloads). The use of the benchmarks is illustrated in two detailed case studies. Two widespread mechanisms for handling failures are unreliable failure detectors which provide inconsistent information about failures, and a group membership service which provides consistent information about failures, respectively. We analyze the performance tradeoffs of these two techniques, by comparing the performance of two atomic broadcast algorithms designed for an asynchronous system. Based on our results, we advocate a combined use of the two approaches to failure handling. In another case study, we compare two consensus algorithms designed for an asynchronous system. The two algorithms differ in how they coordinate the decision process: the one uses a centralized and the other a decentralized communication schema. Our results show that the performance tradeoffs are highly affected by a number of characteristics of the environment, like the availability of multicast and the amount of contention on the hosts versus the amount of contention on the network. Famous theoretical results state that a lot of important agreement problems are not solvable in the asynchronous system model. In our third case study, we investigate how these results are relevant for implementations of a replicated service, by conducting an experiment in a local area network. We exposed a replicated server to extremely high loads and required that the underlying failure detection service detects crashes very fast; the latter is important as the theoretical results are based on the impossibility of reliable failure detection. We found that our replicated server continued working even with the most extreme settings. We discuss the reasons for the robustness of our replicated server.

EPFL2003