Mesaac Makpangou’s research while affiliated with National Institute for Research in Computer Science and Control and other places

The Divergence Control Protocol Based on Predicted Profile allows cooperative processes deployed on a weakly- connected environment to conciliate the offering of low re- sponse time to clients with the guarantee of a bounded con- sistency, despite characteristics of the underlying commu- nication infrastructure. This paper presents how to decom- pose a global divergence bound into a set of local crite- ria that are locally checked at any invocation. We propose a protocol relying on these criteria to incrementally con- struct histories that preserve at any time, the global diver- gence bound between any replica and the "ideal state" of the replicated object. An implementation of the protocol is discussed. This paper reports a number of experiments conductedin order to evaluate the rationale and the perfor- mance of the proposed protocol. These experiments show that the protocol benefits a lot from the provision of a cor- rect profile and outperformsintuitive cooperation protocols in large-scale settings. Keywords: Distributed System, Network Service, Replication Protocol, Divergence Con- trol, Bounded Consistency.

In order to accelerate access to Web applications, content providers are increasingly relying on Content Delivery Networks. Currently, CDNs serve the dynamic content from the edge in two major ways : page assembly or edge computing. Page assembly assumes that the proportion of cacheable content is high, that the cached fragments are reusable and that they do not change very often, while edge computing generally assumes that the whole application is replicated on the edge, which is not always suitable. Besides, current CDNs do not provide a means of scaling the database component of a Web application. In this study we propose a hybrid CDN called FRACS which combines both page assembly and edge computing in order to address the needs of relatively static applications as well as more dynamic applications. FRACS automatically determines the replicable fragments of a Web application, then it modies the application's code so as to generate fragmented pages in ESI format and to enable the server to serve the fragments separately. Moreover, FRACS maintains the consistency of all the manipulated fragments. Using the TPC-W benchmark we were able to achieve up to 60% savings in bandwidth and more than an 80% reduction in response time.

This paper presents a server selection approach that relies on a response time estimator and a scalable monitoring infrastructure that provides the utilization measures for replica server hosts. The big challenge rises from the difficulty to capture, both accurately and efficiently, the dynamic state of the hosts and network resources in a large-scale replication context. The monitoring infrastructure can be tuned such as to minimize the network traffic while maintaining the estimation accuracy high.

Web services technology standards enable description, publication, discovery of and binding to services distributed towards the Internet. However, current standards do not address the service selection issue : how did a consumer select the service that matches its functional (e.g. operations’ semantics) and non-functional (e.g. price, reputation, response time) properties ? Most projects advocate automatic selection mechanism, advising adaptation or modification of the web-services model and its entities (UDDI, WSDL, Client, Provider). These proposals also do not take advantage of distributed-systems’ state of the art, mainly with respect to the collection and the dissemination of services’ QoS. This paper presents an extension of the initial model that permits automatic service selection, late binding and collection of metrics that characterize the quality of service. The extension consists on a web-service access infrastructure, made of web service proxies and a peer to peer network of QoS metrics repository (the proposal does not impose modification on UDDI registries or services). The proxies interact with common UDDI registries to find suitable services for selection and to publish descriptions. They collect QoS metrics and store them on a p2p network.

Our proposition, presented in this paper, consists in the definition of a function estimating the response time and a method for applying it to different application workloads. The function combines the application demands for various resources (such as the CPU, the disk I/O and the network bandwidth) with the resource capabilities and availabilities on the replica servers. The main benefits of our approach include: the simplicity and the transparency, from the perspective of the clients, who don’t have to specify themselves the resource requirements, the estimation accuracy, by considering the application real needs and the current degree of resource usage, determined by concurrent applications and the flexibility, with respect to the precision with which the resource-concerned parameters are specified. The experiments we conducted show two positive results. Firstly, our estimator provides a good approximation of the real response time obtained by measurements. Secondly, the ordering of the servers according to our estimation function values, matches with high accuracy the ordering determined by the real response times.

In this paper we propose a fragment-based caching system that aims at improving the performance of Web- based applications. The system fragments the dynamic pages automatically. Our approach consists in statically analyzing the programs that generate the dynamic pages rather than their output. This approach has the considerable advantage of optimizing the overhead due to fragmentation. Furthermore, we propose a mechanism that increases the reuse rate of the stored fragments, so that the site response time can be improved among other benets. We validate our approach by using TPC-W as a benchmark.

Autonomic computing has been proposed recently as a way to address the difficult management of applications whose complexity is constantly increasing. Autonomic systems will have to diagnose the problems they face themselves, devise solutions and act accordingly. In consequence, they require a very high level of flexibility and the ability to constantly monitor themselves. This work presents a framework, Pandora, which eases the construction of applications that satisfy this double goal. Pandora relies on an original application programming pattern — based on stackable layers and message passing — to obtain a minimalist model and architecture that allows control of the overhead imposed by the full reflexivity of the framework. A prototype of the framework has been implemented in C++, freely available for download on the Internet. A detailed performance study is given, together with examples of use, to assess the usability of the platform in real usage conditions.

This paper presents a flexible consistency model, aggregating a parameterized representation common for all the models along the spectrum delimited by strong consistency and eventual consistency. A specific model, required by a particular Data Object, is derived from this representation by selecting and combining the proper consistency parameters values.

L'informatique autonome a récemment été proposée comme une réponse à la difficulté de gérer au quotidien des applications dont la complexité ne cesse d'augmenter. Les applications autonomes devront être particulièrement flexibles et pouvoir se surveiller en permanence. Cette étude présente une plate-forme, Pandora, qui facilite la construction d'applications qui satisfont ce double objectif. Pandora s'appuie sur un mode de programmation original des applications - fondé sur la composition de couches et le passage de messages - pour aboutir à un modèle et une architecture minimalistes qui lui permettent de contrôler les surcoûts imposés par la complète réflexivité de la plate-forme. Un prototype fonctionnel de la plate-forme a par ailleurs été développé en C++. Une étude détaillée des performances, ainsi que des exemples d'utilisation, complètent cette présentation. ––––– Autonomic computing has been proposed recently as a way to address the difficult management of applications whose complexity is constantly increasing. Autonomous applications will have to be especially flexible and be able to monitor themselves permanently. This work presents a framework, Pandora, which eases the construction of applications that satisfy this double goal. Pandora relies on an original application programming pattern - based on stackable layers and message passing - to obtain minimalist model and architecture that allows to control the overhead imposed by the full reflexivity of the framework. Besides, a prototype of the framework has been implemented in C++. A detailed performance study, together with examples of use, complement this presentation

The efficacy of a fragment-based caching system fundamen- tally depends on the fragments' definition and the bringing into play of mechanisms that improve reuse and guarantee the consistency of the cache content (notably "purification" and invalidation mechanisms). Ex- isting caching systems assume that the administrator provides the re- quired configuration data manually, which is likely to be a heavy, time- consuming task and one that is prone to human error. This paper pro- poses a tool that helps the administrator to cope with these issues, by automating the systematic tasks and proposing a default fragmentation with the prerequisite reuse and invalidation directives, that may either be augmented or overwritten if necessary.