Monotonic Prefix Consistency in Distributed Systems

We study the issue of data consistency in distributed systems. Specifically, we consider a distributed system that replicates its data at multiple sites, which is prone to partitions, and which is assumed to be available (in the sense that queries are always eventually answered). In such a setting, strong consistency, where all replicas of the system apply synchronously every operation, is not possible to implement. However, many weaker consistency criteria that allow a greater number of behaviors than strong consistency, are implementable in available distributed systems. We focus on determining the strongest consistency criterion that can be implemented in a convergent and available distributed system that tolerates partitions. We focus on objects where the set of operations can be split into updates and queries. We show that no criterion stronger than Monotonic Prefix Consistency (MPC) can be implemented.Comment: Submitted pape

Democratrizing the Parliament

This paper presents a replication algorithm that implements a highly-available, non-deterministic state machine. Our algorithm generalizes the Paxos parliament algorithm of Lamport to cope with non-deterministic computations, while preserving its nice resilience and efficiency properties. The algorithm is surprisingly simple, thanks to the use of two powerful underlying abstractions: weak consensus and weak leader election, together with a generic data structure: consensus bag. As a side-effect of our work, we discuss some similarities and differences between replicating deterministic and non-deterministic state machines. Indirectly, we revisit the traditional classification between state-machine replication and primary-backup

Adaptive gossip-based broadcast

This paper presents a novel adaptation mechanism that allows every node of a gossip-based broadcast algorithm to adjust the rate of message emission 1) to the amount of resources available to the nodes within the same broadcast group and 2) to the global level of congestion in the system. The adaptation mechanism can be applied to all gossip-based broadcast algorithms we know of and makes their use more realistic in practical situations where nodes have limited resources whose quantity changes dynamically with time without decreasing the reliability.(undefined

The Driving Philosophers

We introduce a new synchronization problem in mobile ad-hoc systems: the Driving Philosophers. In this problem, an unbounded number of driving philosophers (processes) access a round-about (set of shared resources organized along a logical ring). The crux of the problem is to ensure, beside traditional mutual exclusion and starvation freedom at each particular resource, gridlock freedom (i.e., cyclic waiting chain amongst processes). The problem captures explicitly the very notion of process mobility and the underlying model does not involve any assumption on the total number of (participating) processes or the use of shared memory, i.e., the model conveys the ad-hoc environment. We present a generic algorithm that solves the problem in a synchronous model. Instances of this algorithm can be fair but not concurrent, or concurrent but not fair. We derive the impossibility of achieving fairness and concurrency at the same time as well as the impossibility of solving the problem in an asynchronous model. We also conjecture the impossibility of solving the problem in an ad-hoc network model with limited-range communication

Mobility Friendly Publish/Subscribe

This paper describes an event dissemination algorithm that implements a topic-based publish/subscribe abstraction in mobile ad-hoc networks (MANETs). Our algorithm relies on (1) the mobility of the processes and (2) the validity period of the events to ensure the reliability of the dissemination (under reasonable conditions) with a thrifty usage of the memory. The algorithm is inherently portable and does not assume any specific routing protocol. Old events are collected to save the memory and the energy consumption is, in some sense, related to the size of the event scope a subscriber is interested in. We give simulation results in different mobility models and highlight the advantages/drawbacks of our approach as well as we expose some interesting relations between validity periods and reliability

Of Malicious Motes and Suspicious Sensors

How efficiently can a malicious device disrupt communication in a wireless network? Imagine a basic game involving two honest players, Alice and Bob, who want to exchange information, and an adversary, Collin, who can disrupt communication using a limited budget of B broadcasts. How long can Collin delay Alice and Bob from communicating? In fact, the trials and tribulations of Alice and Bob capture the fundamental difficulty shared by several n-player problems, including reliable broadcast, leader election, static k-selection, and t-resilient consensus. We provide round complexity lower bounds—and (nearly) tight upper bounds—for each of those problems. These results imply bounds on adversarial efficiency, which we analyze in terms of jamming gain and disruption-free complexity

Data-Aware Multicast

This paper presents a multicast algorithm for peer-to-peer dissemination of events in a distributed topic-based publish-subscribe system, where processes publish events of certain topics, organized in a hierarchy, and expect events of topics they subscribed to. Our algorithm is ``data-aware'' in the sense that it exploits information about process subscriptions and topic inclusion relationships to build dynamic groups of processes and efficiently manage the flow of information within and between these process groups. This ``data-awareness'' helps limit the membership information that each process needs to maintain and preserves processes from receiving messages related to topics they have not subscribed to. It also provides the application with means to control, for each topic in a hierarchy, the trade-off between the message complexity and the reliability of event dissemination. We convey this trade-off through both analysis and simulation

Pragmatic Type Interoperability

Providing type interoperability consists in ensuring that, even if written by different programmers, possibly in different languages and running on different platforms, types that are supposed to represent the same software module are indeed treated as one single type. This form of interoperability is crucial in modern distributed programming. We present a pragmatic approach to deal with type interoperability in a dynamic and distributed environment. Our approach is based on an optimistic transport protocol, specific serialization mechanisms and a set of implicit type conformance rules. We experiment the approach over the .NET platform which we indirectly evaluate

GosSkip: a Gossip-based Structured Overlay Network for Efficient Content-based Filtering

It is appealing to implement publish/subscribe systems in a peer to peer (P2P) manner to circumvent the scalability issues of broker-based (semi-decentralized) systems and simplify the deployment. While existing P2P generic infrastructures provide a scalable support for topic-based publish-subscribe systems, they are not well adapted to content-based ones. In this paper, we advocate the need for dedicated overlay networks -where the overlay structure reflects the actual structure of the underlying application properties- to implement efficient content-based publish-subscribe infrastructures. We propose a new scalable P2P event filtering mechanism where filters are arranged according to a dictionary-based semantics. This structured overlay, which we call \textit{GosSkip}, relies on gossip messages to construct a structure eventually similar to a perfect Skip list, preserving the semantic locality of the items stored in the overlay. In GosSkip, events are delivered to matching subscriptions in O(log N) routing hops, N being the total number of subscriptions. The very same approach can be extended to support range queries. Preliminary implementation results based on a real P2P trace convey the scalability and the efficiency of the approach both in static and dynamic scenarios

An architecture for reliable distributed computer-controlled systems

In Distributed Computer-Controlled Systems (DCCS), both real-time and reliability requirements are of major concern. Architectures for DCCS must be designed considering the integration of processing nodes and the underlying communication infrastructure. Such integration must be provided by appropriate software support services. In this paper, an architecture for DCCS is presented, its structure is outlined, and the services provided by the support software are presented. These are considered in order to guarantee the real-time and reliability requirements placed by current and future systems