Programming with process groups: Group and multicast semantics

Process groups are a natural tool for distributed programming and are increasingly important in distributed computing environments. Discussed here is a new architecture that arose from an effort to simplify Isis process group semantics. The findings include a refined notion of how the clients of a group should be treated, what the properties of a multicast primitive should be when systems contain large numbers of overlapping groups, and a new construct called the causality domain. A system based on this architecture is now being implemented in collaboration with the Chorus and Mach projects

VCube-PS: A Causal Broadcast Topic-based Publish/Subscribe System

In this work we present VCube-PS, a topic-based Publish/Subscribe system built on the top of a virtual hypercube-like topology. Membership information and published messages are broadcast to subscribers (members) of a topic group over dynamically built spanning trees rooted at the publisher. For a given topic, the delivery of published messages respects the causal order. VCube-PS was implemented on the PeerSim simulator, and experiments are reported including a comparison with the traditional Publish/Subscribe approach that employs a single rooted static spanning-tree for message distribution. Results confirm the efficiency of VCube-PS in terms of scalability, latency, number and size of messages.Comment: Improved text and performance evaluation. Added proof for the algorithms (Section 3.4

Virtual Central Control.

A distributed system is a set of cooperating computers (processes) communicating with each other to achieve a common goal. They are broadly classified as centralized and decentralised systems. In a centralized system, a single computer plays the role of central coordinator and controls all the system activities. Whereas in a decentralized system, all the cooperating processes have an equal role to play, therefore solutions to problems involve instructing all the processes and coordinating their actions. Although a centralized system facilitates program development, it has serious drawbacks. If the coordinator fails, the system effectively breaks down. Also, the coordinator can become a performance bottleneck. On the other hand, a decentralized system does not suffer from these shortcomings, but program development is more difficult. In this research, we develop a paradigm for a distributed system that provides the view of a centralized system even though the underlying system is decentralized. Special processes called agents reside on all participating computers. One of the agents acts as a leader and coordinates activities of other agents. Agents communicate through a fault-tolerant agent-to-agent protocol. The paradigm defines a one-shot computation, a construct that enables the expression of programs in a simulated centralized environment, thus making program development easier. Programs, written in terms of one-shot computations that are encapsulated in templates which hide all the lower level details from the programmer. We show that every computation can be expressed as a one-shot computation. The proposed paradigm has a high degree of fault-tolerance. It tolerates up to $n-1$ failures in a system of n processors. A prototype has been implemented using PVM

ZeroComm: Decentralized, Secure and Trustful Group Communication

In the context of computer networks, decentralization is a network architecture that distributes both workload and control of a system among a set of coequal participants. Applications based on such networks enhance trust involved in communication by eliminating the external author- ities with self-interests, including governments and tech companies. The decentralized model delegates the ownership of data to individual users and thus mitigates undesirable behaviours such as harvesting personal information by external organizations. Consequently, decentral- ization has been adopted as the key feature in the next generation of the Internet model which is known as Web 3.0. DIDComm is a set of abstract protocols which enables secure messaging with decentralization and thus serves for the realization of Web 3.0 networks. It standardizes and transforms existing network applications to enforce secure, trustful and decentralized com- munication. Prior work on DIDComm has only been restricted to pair-wise communication and hence it necessitates a feasible strategy for adapting the Web 3.0 concepts in group-oriented networks. Inspired by the demand for a group communication model in Web 3.0, this study presents Zero- Comm which preserves decentralization, security and trust throughout the fundamental opera- tions of a group such as messaging and membership management. ZeroComm is built atop the publisher-subscriber pattern which serves as a messaging architecture for enabling communi- cation among multiple members based on the subjects of their interests. This is realized in our implementation through ZeroMQ, a low-level network library that facilitates the construction of advanced and distributed messaging patterns. The proposed solution leverages DIDComm protocols to deliver safe communication among group members at the expense of performance and efficiency. ZeroComm offers two different modes of group communication based on the organization of relationships among members with a compromise between performance and security. Our quantitative analysis shows that the proposed model performs efficiently for the messaging operation whereas joining a group is a relatively exhaustive procedure due to the es- tablishment of secure and decentralized relationships among members. ZeroComm primarily serves as a low-level messaging framework but can be extended with advanced features such as message ordering, crash recovery of members and secure routing of messages

A Publish/Subscribe System Using Causal Broadcast Over Dynamically Built Spanning Trees

International audienceIn this paper we present VCube-PS, a topic-based Publish/Subscribe system built on the top of a virtual hypercube-like topology. Membership information and published messages to subscribers (members) of a topic group are broadcast over dynamically built spanning trees rooted at the message's source. For a given topic, delivery of published messages respects causal order. Performance results of experiments conducted on the PeerSim simulator confirm the efficiency of VCube-PS in terms of scalability, latency, number, and size of messages when compared to a single rooted, not dynamically, tree built approach

Explaining decision-making in government: the neo-Durkheimian institutional framework

In understanding styles of political judgement in government decision-making, explanatory limitations of rational choice, prospect theoretic, historical institutional, groupthink, and other approaches suggest that there is space for developing other frameworks. This article argues that the neo-Durkheimian institutional theoretical framework deserves serious consideration. It shows that it offers a powerful causally explanatory framework for generating theories of decision-making in government which can be examined using historical comparative research designs. The value of the concept of a ‘thought style’ for understanding political judgement is demonstrated, and contrasted sharply with ideology. The theory argues that informal institutions explain thought styles. Well-known cases from the Cuban missile crisis, and the Wilson and Heath governments illustrate the argument. The article rebuts criticisms offered of the neo-Durkheimian institutional framework in the literature. Finally, it identifies recent developments and innovations in the approach that make it especially suited to explaining political judgement in government decision-makingThis work was supported by the Leverhulme Trust (grant number: F01374I

A Communication-Efficient Causal Broadcast Protocol

International audienceA causal broadcast ensures that messages are delivered to all nodes (processes) preserving causal relation of the messages. In this paper, we propose a causal broadcast protocol for distributed systems whose nodes are logically organized in a virtual hypercube-like topology called VCube. Messages are broadcast by dynamically building spanning trees rooted in the message's source node. By using multiple trees, the contention bottleneck problem of a single root spanning tree approach is avoided. Furthermore , different trees can intersect at some node. Hence, by taking advantage of both the out-of-order reception of causally related messages at a node and these paths intersections, a node can delay to one or more of its children in the tree, the forwarding of the messages whose some causal dependencies it knows that the children in question can not satisfy yet. Such a delay does not induce any overhead. Experimental evaluation conducted on top of PeerSim simulator confirms the communication effectiveness of our causal broadcast protocol in terms of latency and message traffic reduction

Essays in decision-making and organizational behaviour in public bureaucracies

Public servants, no matter how carefully they are selected or incentivized, are only imperfectly able to discharge the objectives of the organizations they work for. They may suffer from bounded rationality, or make systematically biased decisions. They may be pursuing goals at odds with the stated or implied organizational mandate. Or they may suffer from low engagement in their work, and be difficult to retain over time. Through three quantitative essays, I examine how decisions are made in public sector organizations, and in particular how the behaviour of agents who vary in their ability, motivation and the incentives they are responding to contribute to public service (dys)function. In my first paper, a survey experiment in a large UK public sector organization, I find exposure to politician preferences (orthogonal to organizational mandate) increases the rate at which junior decision-makers provide advice contrary to the evidence they are provided with, but no such effect is found for senior officials. In my second paper, an observational study drawing on a large and novel dataset on projects implemented by the same organization, I show that a peer review process induced substantial avoidance of the system around the threshold for eligibility, but available metrics of project quality show only weak evidence of an effect around this threshold. And in my third paper, I use a survey experiment with a sample of public servants from 28 countries to investigate the effect of varying the framing of the gains to organizational initiatives on likelihood of participation, finding few overall effects but some heterogeneity according to pre-existing variation in public servant characteristics. These three papers have implications for how public organizations should structure their work