Probabilistic Quorum Research Articles

Relaxing data consistency along different dimensions for increasing operation availabilities

Pessimistic quorum-based data replication strategies have strictly upper-bounded operation availabilities. The probabilistic relaxation of their strict consistency notion along different dimensions permits to overcome this upper-bound. The resulting probabilistic replication strategies allow the exploitation of the data consistency versus operation availabilities trade-off. In our approaches, we add a certain number of probabilistically intersecting quorums to the originally strictly intersecting partially ordered quorum system in order to ‘closely conserve’ its original characteristic behaviour. As one expects, the number and nature of probabilistic quorums added have a strong impact on the particular trade-off. We present two algorithms for the generation of probabilistic quorums based on input strict quorums. Furthermore, the order in which probabilistic and strict quorums are to be probed and used for executing operations is also very important. We relate three general strategies of ordering the probabilistic and strict quorums of those probabilistic quorum systems (PQSs), evaluate their impact on the data consistency versus operation availabilities trade-off and compare them in terms of this trade-off. The evaluation uses Markov chain-based steady-state analysis applied to a representative instance of PQSs with adequate partial orderings.

Probabilistic quorum systems in wireless Ad Hoc networks

Quorums are a basic construct in solving many fundamental distributed computing problems. One of the known ways of making quorums scalable and efficient is by weakening their intersection guarantee to being probabilistic. This article explores several access strategies for implementing probabilistic quorums in ad hoc networks. In particular, we present the first detailed study of asymmetric probabilistic biquorum systems, that allow to mix different access strategies and different quorums sizes, while guaranteeing the desired intersection probability. We show the advantages of asymmetric probabilistic biquorum systems in ad hoc networks. Such an asymmetric construction is also useful for other types of networks with nonuniform access costs (e.g, peer-to-peer networks). The article includes a formal analysis of these approaches backed up by an extensive simulation-based study. The study explores the impact of various parameters such as network size, network density, mobility speed, and churn. In particular, we show that one of the strategies that uses random walks exhibits the smallest communication overhead, thus being very attractive for ad hoc networks.

Analysis of Probabilistic Trapezoid Protocol for Data Replication

Maintaining replicated data between nodes can improve the dependability of data. We propose a probabilistic trapezoid protocol for replicated data that combines the trapezoid protocol with the concept of a probabilistic quorum system. We analyzed the read availability, the latest version's read availability and the average number of nodes accessed for the protocol. Our numerical evaluations demonstrated that it improves not only the read availability but also the latest version's read availability. Furthermore, when the number of nodes is greater than 100, it could effectively reduce the system's load. We designed and implemented a file transfer protocol to replicate data. Experimental results proved that the trapezoid protocol could achieve a better throughput than the voting system or the grid protocol. Despite node failure, the probabilistic trapezoid protocol also achieved a relatively better throughput.

Randomized leader election

We present an efficient randomized algorithm for leader election in large-scale distributed systems. The proposed algorithm is optimal in message complexity (O(n) for a set of n total processes), has round complexity logarithmic in the number of processes in the system, and provides high probabilistic guarantees on the election of a unique leader. The algorithm relies on a balls and bins abstraction and works in two phases. The main novelty of the work is in the first phase where the number of contending processes is reduced in a controlled manner. Probabilistic quorums are used to determine a winner in the second phase. We discuss, in detail, the synchronous version of the algorithm, provide extensions to an asynchronous version and examine the impact of failures.

Probabilistic quorums for dynamic systems

A quorum system is a set of sets such that every two sets in the quorum system intersect. Quorum systems are well known building blocks for maintaining information in a distributed system while providing high availability and good load balance. Probabilistic Quorum Systems (PQS) are variants of quorum systems that relax the strict intersection requirement. They are particularly attractive for large scale systems due to their simplicity and highly efficient availability--load balance tradeoff. We introduce scalable techniques that maintain a PQS in a highly decentralized and highly dynamic setting. We address two challenges. First we show how PQS can be realized efficiently even when each process maintains knowledge of only a constant number of other members. Second, we provide algorithms that adaptively evolve the quorums to adjust to the changes in the system caused by processes leaving and joining the system over time.

Pilot: probabilistic lightweight group communication system for ad hoc networks

Providing reliable group communication is an ever recurring topic in distributed settings. In mobile ad hoc networks, this problem is even more significant since all nodes act as peers, while it becomes more challenging due to highly dynamic and unpredictable topology changes. In order to overcome these difficulties, we deviate from the conventional point of view, i.e., we fight fire with fire, by exploiting the nondeterministic nature of ad hoc networks. Inspired by the principles of gossip mechanisms and probabilistic quorum systems, we present in this paper PILOT (probabilistic lightweight group communication system) for ad hoc networks, a two-layer system consisting of a set of protocols for reliable multicasting and data sharing in mobile ad hoc networks. The performance of PILOT is predictable and controllable in terms of both reliability (fault tolerance) and efficiency (overhead). We present an analysis of PILOT's performance, which is used to fine-tune protocol parameters to obtain the desired trade off between reliability and efficiency. We confirm the predictability and tunability of PILOT through simulations with ns-2.

Probabilistic Quorum Systems

We initiate the study of probabilistic quorum systems, a technique for providing consistency of replicated data with high levels of assurance despite the failure of data servers. We show that this technique offers effective load reduction on servers and high availability. We explore probabilistic quorum systems both for services tolerant of benign server failures and for services tolerant of arbitrary (Byzantine) ones. We also prove bounds on the server load that can be achieved with these techniques.

Probabilistic quorum protocols for biometrical user authentication in OLTP

A statistical zero-knowledge authentication scheme is described for security control in on-line database transaction processing systems (OLTP). This scheme uses probabilistic quorum protocols to validate users using their biometrical characteristics (such as speech, handwriting and keyboard characteristics). This authentication scheme can be implemented using the present-day smart card technology.