Xavier Défago

Semi-passive replication

This paper presents the semi-passive replication technique, a variant of passive replication, that can be implemented in the asynchronous system model without requiring a membership service to agree on a primary. Passive replication is a popular replication technique since it can tolerate non-deterministic servers (e.g., multi-threaded servers) and uses little processing power when compared to other replication techniques. However, passive replication suffers from a high reconfiguration cost in case of the failure of the primary. The semi-passive replication technique presented in the paper benefits from the same advantages as passive replication. However, since it does not require a group membership service, semi-passive replication has a considerably lower cost in case of failure. As explained in the paper, this technique can benefit from an aggressive time-out value significantly lower than what a group membership allows. As a result, the reaction to crashes is greatly improved. The semi-passive replication algorithm uses failure detectors. The algorithm given in the paper is analysed in the failure free case and in the case of one server crash. The response time (for the client) of these two scenarios is analysed through simulation.

Circle formation for oblivious anonymous mobile robots with no common sense of orientation

This paper proposes a distributed algorithm by which a collection of mobile robots roaming on a plane move to form a circle. The algorithm operates under the premises that robots (1) are unable to recall past actions and observations (i.e., oblivious), (2) cannot be distinguished from each others (i.e., anonymous), (3) share no common sense of direction, and (4) are unable to communicate in any other ways than by observing each others position.

An energy efficient routing scheme for mobile wireless sensor networks

Research on wireless sensor networks has recently received much attention as they offer an advantage of monitoring various kinds of environment by sensing physical phenomenon. Among various issues, energy consumption is one of the most important criteria for routing protocol in wireless sensor networks (WSNs). This paper introduces an energy efficient clustering algorithm for mobile sensor network based on the LEACH protocol. The proposed protocol adds feature to LEACH to support for mobile nodes and also reduces the consumption of the network resource in each round. The proposed protocol is simulated and the results show a significant reduction in network energy consumption compared to LEACH.

Failure detectors as first class objects

One of the fundamental differences between a centralized system and a distributed one is the notion of partial failures. The ability to efficiently and accurately detect failures is a key element underlying reliable distributed computing. In current distributed systems, however, failure detection is either left to the application developer or hidden from the programmer and provided in an ad-hoc manner behind the scenes. We plead for an intermediate approach where failure detectors are first-class objects. We view failure detection as an abstraction, the complexity of which is encapsulated behind well-defined interfaces. The various roles of a failure detection service are all represented as first-class objects. Following our approach, one can reuse existing failure detection protocols as they are, or, through composition or refinement, one can define new protocols that match the application requirements. We describe an interesting result of a composition that mixes push and pull failure monitoring, and we show how scalability issues may be addressed by using a hierarchical failure detection configuration. We also discuss the implementation of our failure service both in CORBA and in Java.

Fault-tolerant and self-stabilizing mobile robots gathering

Gathering is a fundamental coordination problem in cooperative mobile robotics. In short, given a set of robots with arbitrary initial location and no initial agreement on a global coordinate system, gathering requires that all robots, following their algorithm, reach the exact same but not predetermined location. Gathering is particularly challenging in networks where robots are oblivious (i.e., stateless) and the direct communication is replaced by observations on their respective locations. Interestingly any algorithm that solves gathering with oblivious robots is inherently self-stabilizing. In this paper, we significantly extend the studies of deterministic gathering feasibility under different assumptions related to synchrony and faults (crash and Byzantine). Unlike prior work, we consider a larger set of scheduling strategies, such as bounded schedulers, and derive interesting lower bounds on these schedulers. In addition, we extend our study to the feasibility of probabilistic gathering in both fault-free and fault-prone environments. To the best of our knowledge our work is the first to address the gathering from a probabilistic point of view.

Neko: a single environment to simulate and prototype distributed algorithms

Designing, tuning, and analyzing the performance of distributed algorithms and protocols are complex tasks. A major factor that contributes to this complexity is the fact that there is no single environment to support all phases of the development of a distributed algorithm. This paper presents Neko, an easy to use Java platform that provides a uniform and extensible environment for the various phases of algorithm design and performance evaluation: prototyping, tuning, simulation, deployment, etc.

Gathering asynchronous mobile robots with inaccurate compasses

This paper considers a system of asynchronous autonomous mobile robots that can move freely in a two-dimensional plane with no agreement on a common coordinate system. Starting from any initial configuration, the robots are required to eventually gather at a single point, not fixed in advance (gathering problem). Prior work has shown that gathering oblivious (i.e., stateless) robots cannot be achieved deterministically without additional assumptions. In particular, if robots can detect multiplicity (i.e., count robots that share the same location) gathering is possible for three or more robots. Similarly, gathering of any number of robots is possible if they share a common direction, as given by compasses, with no errors. Our work is motivated by the pragmatic standpoint that (1) compasses are error-prone devices in reality, and (2) multiplicity detection, while being easy to achieve, allows for gathering in situations with more than two robots. Consequently, this paper focusses on gathering two asynchronous mobile robots equipped with inaccurate compasses. In particular, we provide a self-stabilizing algorithm to gather, in a finite time, two oblivious robots equipped with compasses that can differ by as much as π/4.

Using eventually consistent compasses to gather memory-less mobile robots with limited visibility

Reaching agreement among a set of mobile robots is one of the most fundamental issues in distributed robotic systems. This problem is often illustrated by the gathering problem, where the robots must self-organize and meet at some location not determined in advance, and without the help of some global coordinate system. While very simple to express, this problem has the advantage of retaining the inherent difficulty of agreement, namely the question of breaking symmetry between robots. In previous works, it has been proved that the gathering problem is solvable in asynchronous model with oblivious (i.e., memory-less) robots and limited visibility, as long as the robots share the knowledge of some direction, as provided by a compass. However, the problem has no solution in the semi-synchronous model when robots do not share a compass, or when they cannot detect multiplicity. In this article, we define a model in which compasses may be unreliable, and study the solvability of gathering oblivious mobile robots with limited visibility in the semi-synchronous model. In particular, we give an algorithm that solves the problem in finite time in a system where compasses are unstable for some arbitrary long periods, provided that they stabilize eventually. In addition, we show that our algorithm solves the gathering problem for at most three robots in the asynchronous model. Our algorithm is intrinsically self-stabilizing.

The Gathering Problem for Two Oblivious Robots with Unreliable Compasses

Anonymous mobile robots are often classified into synchronous, semi-synchronous, and asynchronous robots when discussing the pattern formation problem. For semi-synchronous robots, all patterns formable with memory are also formable without memory, with the single exception of forming a point (i.e., the gathering) by two robots. (All patterns formable with memory are formable without memory for synchronous robots, and little is known for asynchronous robots.) However, the gathering problem for two semi-synchronous robots without memory (called oblivious robots in this paper) is trivially solvable when their local coordinate systems are consistent, and the impossibility proof essentially uses the inconsistencies in their coordinate systems. Motivated by this, this paper investigates the magnitude of consistency between the local coordinate systems necessary and sufficient to solve the gathering problem for two oblivious robots under semi-synchronous and asynchronous models. To discuss the magnitude of consistency, we assume that each robot is equipped with an unreliable compass, the bearings of which may deviate from an absolute reference direction, and that the local coordinate system of each robot is determined by its compass. We consider two families of unreliable compasses, namely, static compasses with (possibly incorrect) constant bearings and dynamic compasses the bearings of which can change arbitrarily (immediately before a new look-compute-move cycle starts and after the last cycle ends). For each of the combinations of robot and compass models, we establish the condition on deviation

Contention-aware metrics for distributed algorithms: comparison of atomic broadcast algorithms

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