Roberto Cortiñas

Specifying and implementing an eventual leader service for dynamic systems

The election of an eventual leader in an asynchronous system prone to process crashes is an important problem of fault-tolerant distributed computing. This problem is known as the implementation of the failure detector Omega. Nearly all papers that propose algorithms implementing such an eventual leader service consider a static system. In contrast this paper considers a dynamic system, i.e., a system in which processes can enter and leave. The paper has three contributions. It first proposes a specification of

Communication-efficient failure detection and consensus in omission environments

\Omega

On the implementation of communication-optimal failure detectors

suited to dynamic systems. Then, it presents and proves correct an algorithm implementing this specification. Finally, the paper discusses the notion of an eventual leader suited to dynamic systems. It introduces an additional property related to system stability. The design of an algorithm satisfying this last property remains an open challenging problem.

Secure Failure Detection and Consensus in TrustedPals

Failure detectors have been shown to be a very useful mechanism to solve the consensus problem in the crash failure model, for which a number of communication-efficient algorithms have been proposed. In this paper we deal with the definition, implementation and use of communication-efficient failure detectors in the general omission failure model, where processes can fail by crashing and by omitting messages when sending and/or receiving. We first define a new failure detector class for this model in terms of completeness and accuracy properties. Then we propose an algorithm that implements a failure detector of the proposed class in a communication-efficient way, in the sense that only a linear number of links are used to send messages forever. We also explain how the well-known consensus algorithm of Chandra and Toueg can be adapted in order to use the proposed failure detector.

Secure failure detection in TrustedPals

Several algorithms implementing failure detectors have been proposed in the literature. In particular, we have proposed a family of communication-efficient ⋄P algorithms, i.e., algorithms using n links to carry messages forever, being n the number of processes in the system. Moreover, we have recently proposed a ⋄P algorithm that uses only C links, being C the number of correct processes. In this paper, we show that C is the minimum number of links required to implement ⋄P. We also show that, assuming that there is at least one incorrect process, C is optimal not only for ⋄P but also for ⋄S and Ω. We revisit our Reliable Broadcast based communication-optimal ⋄P algorithm, and we show that, regarding QoS measures, it performs better than the communication-efficient algorithms.

Communication-optimal eventually perfect failure detection in partially synchronous systems ☆

We present a modular redesign of TrustedPals, a smart card-based security framework for solving Secure Multiparty Computation (SMC). Originally, TrustedPals assumed a synchronous network setting and allowed to reduce SMC to the problem of fault-tolerant consensus among smart cards. We explore how to make TrustedPals applicable in environments with less synchrony and show how it can be used to solve asynchronous SMC. Within the redesign we investigate the problem of solving consensus in a general omission failure model augmented with failure detectors. To this end, we give novel definitions of both consensus and the class oP of failure detectors in the omission model, which we call ◇P(om), and show how to implement ◇P(om) and have consensus in such a system with very weak synchrony assumptions. The integration of failure detection and consensus into the TrustedPals framework uses tools from privacy enhancing techniques such as message padding and dummy traffic.

An Evaluation of Efficient Leader Election Algorithms for Crash-Recovery Systems

This paper presents a modular redesign of TrustedPals, a smartcard-based security framework for solving secure multiparty computation (SMC). TrustedPals allows to reduce SMC to the problem of fault-tolerant consensus between smartcards. Within the redesign we investigate the problem of solving consensus in a general omission failure model augmented with failure detectors. To this end, we give novel definitions of both consensus and the class of ⋄P failure detectors in the omission model and show how to implement ⋄P and have consensus in such a system with some weak synchrony assumptions. The integration of failure detection into the TrustedPals framework uses tools from privacy enhancing techniques such as message padding and dummy traffic.

A Leader Election Service for Crash-Recovery and Omission Environments

Abstract Since Chandra and Toueg introduced the failure detector abstraction for crash-prone systems, several algorithms implementing failure detectors in partially synchronous systems have been proposed. Their performance can be measured by their Communication efficiency , defined as the number of links used forever. In this regard, in a communication-efficient algorithm only n links are used forever, n being the number of processes in the system. In this paper, we present communication optimality , a communication efficiency degree reached when only c links are used forever, c being the number of correct processes. We show that c is the minimum number of links used forever required to implement ◇ P and that c is also optimal for ◇ S and Ω when c n . Finally, we propose two communication-optimal ◇ P algorithms following respectively one-to-all and one-to-one communication patterns to manage suspicions, showing that there is a trade-off between detection latency and sporadic communication overhead.

Boosting Dependable Ubiquitous Computing: A Case Study

This paper presents an evaluation of three communication-efficient algorithms implementing the Omega class of failure detectors, which provides an eventual leader election functionality, in distributed systems where processes can crash and recover. Communication efficiency means that eventually only a correct process, i.e., the elected leader, keeps sending a message periodically to the rest of processes. The first algorithm relies on the use of stable storage to store the identity of the leader and an incarnation number. The second algorithm does not use stable storage, but requires a majority of correct processes. Also, it is near-communication-efficient, since besides the leader, unstable processes, i.e., those that crash and recover infinitely often, may send messages periodically before they receive a message from the leader. Finally, the third algorithm does neither use stable storage nor require a majority of correct processes, but assumes that each process has access to a nondecreasing and persistent local clock. Using the OMNeT++ network simulation framework, we evaluate the performance and the quality of service provided by these algorithms, in terms of the number of messages exchanged among processes and the capability of the failure detector to provide a single leader, respectively.

A Method for Dynamically Selecting the Best Frequency Hopping Technique in Industrial Wireless Sensor Network Applications

Leader election is a key service for many dependable ubiquitous systems. It eases the consistent management of replicas in current highly available computing scenarios. This paper presents our work on the design of a leader election service for crash-recovery and omission environments, in order to support fault-tolerant agreement protocols, e.g., Paxos.