An efficient distributed mutual exclusion algorithm based on relative consensus voting

2003-2004 > Academic research: refereed > Refereed conference paperVersion of RecordPublishe

A Holistic Approach to Lowering Latency in Geo-distributed Web Applications

User perceived end-to-end latency of web applications have a huge impact on the revenue for many businesses. The end-to-end latency of web applications is impacted by: (i) User to Application server (front-end) latency which includes downloading and parsing web pages, retrieving further objects requested by javascript executions; and (ii) Application and storage server(back-end) latency which includes retrieving meta-data required for an initial rendering, and subsequent content based on user actions. Improving the user-perceived performance of web applications is challenging, given their complex operating environments involving user-facing web servers, content distribution network (CDN) servers, multi-tiered application servers, and storage servers. Further, the application and storage servers are often deployed on multi-tenant cloud platforms that show high performance variability. While many novel approaches like SPDY and geo-replicated datastores have been developed to improve their performance, many of these solutions are specific to certain layers, and may have different impact on user-perceived performance. The primary goal of this thesis is to address the above challenges in a holistic manner, focusing specifically on improving the end-to-end latency of geo-distributed multi-tiered web applications. This thesis makes the following contributions: (i) First, it reduces user-facing latency by helping CDNs identify and map objects that are more critical for page-load latency to the faster CDN cache layers. Through controlled experiments on real-world web pages, we show the potential of our approach to reduce hundreds of milliseconds in latency without affecting overall CDN miss rates. (ii) Next, it reduces back-end latency by optimally adapting the datastore replication policies (including number and location of replicas) to the heterogeneity in workloads. We show the benefits of our replication models using real-world traces of Twitter, Wikipedia and Gowalla on a 8 datacenter Cassandra cluster deployed on EC2. (iii) Finally, it makes multi-tier applications resilient to the inherent performance variability in the cloud through fine-grained request redirection. We highlight the benefits of our approach by deploying three real-world applications on commercial cloud platforms

Efficient Passive Clustering and Gateways selection MANETs

Passive clustering does not employ control packets to collect topological information in ad hoc networks. In our proposal, we avoid making frequent changes in cluster architecture due to repeated election and re-election of cluster heads and gateways. Our primary objective has been to make Passive Clustering more practical by employing optimal number of gateways and reduce the number of rebroadcast packets

Un protocole contrôle de réplique d'une structure d'arborescence arbitraire. An arbitrary tree-structured replica control protocol

La réplication des données, qui est un problème de calcul distribué, est utilisée dans les grands systèmes distribués, en vue de parvenir à la tolérance aux pannes ainsi que d'améliorer les performances du système. Cependant, des sousjacents protocoles de synchronisation, également connus sous le nom de protocoles contrôle de réplique, sont nécessaires afin de maintenir la cohérence des données entre les répliques. De nombreux protocoles contrôle de réplique existent, chacun ayant ses avantages et inconvénients. Ceux-ci sont mesurés par le coût de communication, la disponibilité ainsi que la charge de système induite par les opérations de lecture ou d'écriture de ces protocoles. En général, ces protocoles contrôle de réplique sont répartis en deux familles: ceux qui supposent que les répliques du système sont organisées logiquement dans une structure et ceux qui ne nécessitent pas d'imposer une structure spécifique aux répliques. Dans cette thèse, à l'équipe ASTRE de l'IRIT (Institut de Recherche en Informatique de Toulouse) et sous la direction du Professeur Jean Paul Bahsoun, nous nous intéressons à l'étudier les protocoles de réplication qui organisent logiquement les répliques dans une structure d'un arbre et étudier la façon de contourner les inconvénients de la racine que ces protocoles en arbre souffrent de son goulot d'étranglement.In large distributed systems, replication is the most widely used approach to offer high data availability, low bandwidth consumption, increased faulttolerance and improved scalability of the overall system. Replication-based systems implement replica control (consistency) protocols that enforce a specified semantics of accessing data. Also, the performance depends on a host of factors chief of which is the protocol used to maintain consistency among the replicas. Several replica control protocols have been described in the literature. They differ according to various parameters such as their communication costs, their ability to tolerate replica failures (also termed as their availability), as well as the load they impose on the system when performing read and write operations. Moreover these replica control protocols can be classified into two families: some protocols assume that replicas of the system are arranged logically into a specific structure (Finite Projective Plane, Grid or Tree) whereas others do not require any specific structure to be imposed on the replicas. In this thesis, at group ASTRE of IRIT and under the supervision of professor Jean-Paul Bashoun, we are interested in studying the replication protocols that arrange logically the replicas into a tree structure and investigate how to circumvent the drawbacks of the root replica as the existing treestructured protocols suffer from the root replica's bottleneck

Distributed virtual environment scalability and security

Distributed virtual environments (DVEs) have been an active area of research and engineering for more than 20 years. The most widely deployed DVEs are network games such as Quake, Halo, and World of Warcraft (WoW), with millions of users and billions of dollars in annual revenue. Deployed DVEs remain expensive centralized implementations despite significant research outlining ways to distribute DVE workloads. This dissertation shows previous DVE research evaluations are inconsistent with deployed DVE needs. Assumptions about avatar movement and proximity - fundamental scale factors - do not match WoW’s workload, and likely the workload of other deployed DVEs. Alternate workload models are explored and preliminary conclusions presented. Using realistic workloads it is shown that a fully decentralized DVE cannot be deployed to today’s consumers, regardless of its overhead. Residential broadband speeds are improving, and this limitation will eventually disappear. When it does, appropriate security mechanisms will be a fundamental requirement for technology adoption. A trusted auditing system (“Carbon”) is presented which has good security, scalability, and resource characteristics for decentralized DVEs. When performing exhaustive auditing, Carbon adds 27% network overhead to a decentralized DVE with a WoW-like workload. This resource consumption can be reduced significantly, depending upon the DVE’s risk tolerance. Finally, the Pairwise Random Protocol (PRP) is described. PRP enables adversaries to fairly resolve probabilistic activities, an ability missing from most decentralized DVE security proposals. Thus, this dissertations contribution is to address two of the obstacles for deploying research on decentralized DVE architectures. First, lack of evidence that research results apply to existing DVEs. Second, the lack of security systems combining appropriate security guarantees with acceptable overhead