Object replication in a distributed system

PhD ThesisA number of techniques have been proposed for the construction of fault—tolerant applications. One of these techniques is to replicate vital system resources so that if one copy fails sufficient copies may still remain operational to allow the application to continue to function. Interactions with replicated resources are inherently more complex than non—replicated interactions, and hence some form of replication transparency is necessary. This may be achieved by employing replica consistency protocols to mask replica failures and maintain consistency of state between functioning replicas. To achieve consistency between replicas it is necessary to ensure that all replicas receive the same set of messages in the same order, despite failures at the senders and receivers. This can be accomplished by making use of order preserving reliable communication protocols. However, we shall show how it can be more efficient to use unordered reliable communication and to impose ordering at the application level, by making use of syntactic knowledge of the application. This thesis develops techniques for replicating objects: in general this is harder than replicating data, as objects (which can contain data) can contain calls on other objects. Handling replicated objects is essentially the same as handling replicated computations, and presents more problems than simply replicating data. We shall use the concept of the object to provide transparent replication to users: a user will interact with only a single object interface which hides the fact that the object is actually replicated. The main aspects of the replication scheme presented in this thesis have been fully implemented and tested. This includes the design and implementation of a replicated object invocation protocol and the algorithms which ensure that (replicated) atomic actions can manipulate replicated objects.Research Studentship, Science and Engineering Research Council. Esprit Project 2267 (Integrated Systems Architecture)

Valued Redundancy

Replicated objects increase distributed system performance and availability. An object is more valuable to the system if it contributes more to system performance (e.g. it is frequently accessed) and availability. Similarly, an object is less valuable if it is expensive to maintain (e.g. it is a large object). By replicating only the most valuable objects we use redundancy to maximize system performance and availability a t low cost. A simulation study of a distributed main-memory database shows substantial performance and availability gains with valued redundancy

A grid-based infrastructure for distributed retrieval

In large-scale distributed retrieval, challenges of latency, heterogeneity, and dynamicity emphasise the importance of infrastructural support in reducing the development costs of state-of-the-art solutions. We present a service-based infrastructure for distributed retrieval which blends middleware facilities and a design framework to ‘lift’ the resource sharing approach and the computational services of a European Grid platform into the domain of e-Science applications. In this paper, we give an overview of the DILIGENT Search Framework and illustrate its exploitation in the field of Earth Science

Unification of Transactions and Replication in Three-Tier Architectures Based on CORBA

In this paper, we describe a software infrastructure that unifies transactions and replication in three-tier architectures and provides data consistency and high availability for enterprise applications. The infrastructure uses transactions based on the CORBA object transaction service to protect the application data in databases on stable storage, using a roll-backward recovery strategy, and replication based on the fault tolerant CORBA standard to protect the middle-tier servers, using a roll-forward recovery strategy. The infrastructure replicates the middle-tier servers to protect the application business logic processing. In addition, it replicates the transaction coordinator, which renders the two-phase commit protocol nonblocking and, thus, avoids potentially long service disruptions caused by failure of the coordinator. The infrastructure handles the interactions between the replicated middle-tier servers and the database servers through replicated gateways that prevent duplicate requests from reaching the database servers. It implements automatic client-side failover mechanisms, which guarantee that clients know the outcome of the requests that they have made, and retries aborted transactions automatically on behalf of the clients

Belfast Interregnum: walls, voids and forward to new ground and porous borders

Good cities reward those who walk, looking outwards and upwards, and reflecting on the paths taken and on the sauntering asides that open an easy relationship with the city and its neighbourhoods. The Spanish paseo and the Italian passeggiata are, of course, as much about meeting friends and family as about using the connecting streets and passageways between one part of the city and another, and such act of walking is liberating, enjoyable and important in the making of place. Belfast does not, in many neighbourhoods, have that easy, social connectivity. The once easy cross-city network of minor streets is, in large part, gone. Interface walls, motorways, and other barriers and separation devices are not overtly visible in the commercial city core but continue to define key emblematic BELFAST INTERREGNUM: WALLS, VOIDS, AND FORWARD TO NEW GROUND AND POROUS BORDERS Ciaran Mackel Ulster University, Belfast, [email protected] ASTRÁGALO. Cultura de la Arquitectura y de la Ciudad, 29 (2021). ISSNe: 2469-0503 Attribution-NonCommercial-ShareAlike. Article https://dx.doi.org/10.12795/astragalo.2021.i29.04 neighbourhoods. And the growth of smallscale political tourism manifested in several bus, black-taxi trails and short neighbourhood walks to visit the interface areas and their walls, have potential result, unfortunately, that such walls, as artefacts of conflict, could tend towards permanence. As one of many post-conflict cities, Belfast is still emerging from its historical and localised condition of manifested deepseated sectarianism, where identities and allegiances are linked to a strongly held sense of ownership of territory. Hence, the reluctance to build in void spaces that could help resolve the housing crisis and be model for housingled regeneration and model for developmentpartnering for change with the public, private and community sectors. The once daily reality in the small nurture-field city-of-camouflage and the desire BELFAST INTERREGNUM: WALLS, VOIDS, AND FORWARD TO NEW GROUND AND POROUS BORDERS 76 || ASTRAGALO Nº 29 | Diciembre, December, Dezembro 2021 | Article | ISSN 2469-0503 for a more-connected city and neighbourhood does, however, have creative possibility, where conflict, friction, and collaborative opportunity –the collision and kiss of confluence– as creative act, can influence and impact one upon the other, that, as we know, sparks fly: alight. The spirit of self-help and community resilience, the confluence of history, cultural continuity, and an urgent need to act remains an opportunity for transformative change: for the making of new ground in an emerging city-formLas buenas ciudades recompensan a quienes caminan, mirando hacia fuera y hacia arriba, y reflexionando sobre los caminos recorridos y sobre los paseos que abren una relación fácil con la ciudad y sus barrios. El paseo español y la passeggiata italiana tienen que ver tanto con el encuentro con los amigos y la familia como con el uso de las calles y pasillos que conectan una parte de la ciudad con otra, y ese acto de caminar es liberador, agradable e importante en la creación del lugar. En muchos barrios de Belfast no existe esa fácil conectividad social. La red de calles secundarias que antes cruzaba la ciudad ha desaparecido en gran parte. Los muros de interconexión, las autopistas y otras barreras y dispositivos de separación no son abiertamente visibles en el núcleo comercial de la ciudad, pero siguen definiendo barrios emblemáticos clave. Y el crecimiento del turismo político a pequeña escala, que se manifiesta en varias rutas de autobuses, taxis negros y pequeños paseos por los barrios para visitar las zonas de interfaz y sus muros, tiene como resultado potencial, por desgracia, que dichos muros, como artefactos del conflicto, podrían tender a la permanencia. Como una de las muchas ciudades en situación de posconflicto, Belfast aún está saliendo de su condición histórica y localizada de sectarismo profundamente arraigado y manifiesto, en el que las identidades y las lealtades están vinculadas a un fuerte sentido de propiedad del territorio. De ahí la reticencia a construir en espacios vacíos que podrían ayudar a resolver la crisis de la vivienda y ser un modelo de regeneración dirigido por la vivienda y un modelo de asociación para el desarrollo con los sectores público, privado y comunitario. Sin embargo, la realidad antaño cotidiana en la pequeña ciudad de camuflaje y el deseo de una ciudad y un barrio más conectados tiene una posibilidad creativa, en la que el conflicto, la fricción y la oportunidad de colaboración –la colisión y el beso de la confluencia– como acto creativo, pueden influir e impactar unos sobre otros, que, como sabemos, saltan chispas: encendidas. El espíritu de autoayuda y la resistencia de la comunidad, la confluencia de la historia, la continuidad cultural y la necesidad urgente de actuar siguen siendo una oportunidad para el cambio transformador: para la creación de un nuevo terreno en una forma de ciudad emergente.As boas cidades recompensam aqueles que nela caminham, olhando para fora e para cima, e refletindo sobre os caminhos percorridos e sobre os passeios que abrem uma relação fácil com a cidade e seus bairros. O paseo espanhol e o passeggiata italiana são, é claro, tanto para encontrar amigos e familiares quanto para o uso das ruas e passagens de interligação entre uma parte da cidade e outra, e esse ato de caminhar é libertador, agradável e importante na construção do lugar. Belfast não tem, em muitos bairros, essa conectividade social fácil. A rede de ruas secundárias que cruzava a cidade, em grande parte, já não existe. Muros de interface, rodovias e outras barreiras e dispositivos de separação, não plenamente visíveis no centro comercial da cidade, continuam a definir bairros emblemáticos chave. E o crescimento do turismo político em pequena escala, manifesto em vários ônibus, trilhas de táxis pretos e passeios curtos pelos bairros para visitar as áreas de interface e seus muros, apresentam como resultado potencial, infelizmente, a tendência da permanência desses muros como artefatos de conflito. Como uma das muitas cidades pós- -conflito, Belfast ainda está emergindo de sua condição histórica e localizada de sectarismo profundamente enraizado, onde as identidades e lealdades estão ligadas a um forte senso de propriedade do território. Daí a relutância em construir espaços vazios que poderiam ajudar a resolver a crise habitacional e ser modelo para uma regeneração urbana que, liderada pela habitação, se constituísse em modelo de desenvolvimento de parcerias de a mudança das relações entre os setores público, privado e comunitário. A realidade outrora cotidiana da pequena cidade-campo-de-camuflagem e o desejo de uma cidade e bairros mais conectados tem, no entanto, possibilidades criativas onde o conflito, o atrito e a oportunidade de colaboração –a colisão e o beijo da confluência– como ato criativo, podem influenciar e impactar um sobre o outro. Como sabemos, as faíscas voam: acendem-se. O espírito de autoajuda e a resiliência da comunidade, a confluência da história, a continuidade cultural e uma necessidade urgente de agir continuam sendo uma oportunidade para uma mudança transformadora: para a construção de um novo território em uma forma emergente de cidade

Reliability Issues in Distributed Operating Systems

Distributed systems span a wide spectrum in the design space. In this paper we will look at the various kinds and discuss some of the reliability issues involved. In the first half of the paper we will concentrate on the causes of unreliability, illustrating these with some general solutions and examples. Among the issues treated are interprocess communication, machine crashes, server redundancy, and data integrity. In the second half of the paper, we will examine one distributed operating system, Amoeba, to see how reliability issues have been handled in at least one real system, and how the pieces fit together. 1. INTRODUCTION It is difficult to get two computer scientists to agree on what a distributed system is. Rather than attempt to formulate a watertight definition, which is probably impossible anyway, we will divide these systems into three broad categories: - Closely coupled systems - Loosely coupled systems - Barely coupled systems The key issue that distinguishes these syst..

Engineered environments for biomedical applications: anisotropic nanotopographies and microfluidic devices

During the last two decades micro- and nano-fabrication techniques originally developed for electronic engineering have directed their attention towards life sciences. The increase of analytical power of diagnostic devices and the creation of more biomimetic scaffolds have been strongly desired by these fields, in order to have a better insight into the complexity of physiological systems, while improving the ability to model them in vitro. Technological innovations worked to fill such a gap, but the integration of these fields of science is not progressing fast enough to satisfy the expectations. In this thesis I present novel devices which exploit the unique features of the micro- and nanoscale and, at the same time, match the requirements for successful application in biomedical research. Such biochips were used for optical detection of water-dispersed nanoparticles in microchannels, for highly controlled cell-patterning in closed microreactors, and for topography-mediated regulation of cell morphology and migration. Moreover, pilot experiments on the pre-clinical translation of micropatterned scaffolds in a rat model of peripheral nerve transaction were initiated and are ongoing. Given these results, the devices presented here have the potential to achieve clinical translation in a short/medium time, contributing to the improvement of biomedical technologies

The Petri Net Model for the Collaborative Virtual Environment on the Web

[[abstract]]This paper presents a Petri Net model to analyze the workflow of a web-based multiple participants virtual environment. The presented approach not only can conspicuously help the developer to comprehend the interaction relationship between the client-server virtual environments but also to easily construct a shared virtual world. Based on the presented Petri Net model, we propose an architecture for the collaborative virtual environment and implement a multiple user 3D web browsing system, called the SharedWeb system. Problems of providing the multi-user interaction on the Web and the solutions proposed by the Petri Net model are fully elaborated here. Some experimental results along with two demonstrated virtual world are also presented.[[notice]]補正完