José M. Bernabéu-Aubán

Applying a Path-Compression technique to Obtain an Efficient Distributed Mutual Exclusion Algorithm

In this paper we present a distributed algorithm for mutual exclusion. The algorithm maintains a dynamic forest structure in which the paths between nodes are compressed as a result of requesting the Critical Section. We develop a formal model of the algorithms execution, which enables us to prove its correctnes. The formal model is also used to show that an execution history of the algorithm when concurrent requests are made (the ususal case) is equivalent to a history in which the requests are made serially. Based on this fact we are able to prove a logarithmic upper bound on the average number of messages needed per critical section grant.

Flexible Management of Consistency and Availability of Networked Data Replications

We describe a family of three replication protocols, each of which can operate in three different modes of consistency. The protocols are tailored to satisfy the availability demands of interconnected databases that have a high degree of data locality. The protocols accomplish a grade of transaction completion which does not compromise availability, and ensure the consistency of replicas also if a transaction needs to be aborted. Flexibility of query answering is understood as optimizing the tradeoff between consistency and availability, i.e., between correctness and timeliness of query answering. This is achieved by choosing an appropriate protocol alternative, and changing the consistency mode of operation during the session, as appropriate for a given transaction.

ROI: an invocation mechanism for replicated objects

The reliable object invocation mechanism provided by HIDRA for the coordinator-cohort and the passive replication models offers support to ensure that all the replicas of the object being invoked are correctly updated before such an invocation is terminated. This mechanism also ensures that if a primary or coordinator replica crashes, the client is able to reconnect to the previously initiated invocations, collecting their results without requiring their reexecution. All this support is provided transparently to the client of the replicated objects, which does not notice any difference in respect to the invocations made to non-replicated objects. Moreover, the protocols described in the paper deal also with the failure of any of the objects involved in this kind of invocations.

HMM: A Cluster Membership Service

The Hidra Membership Monitor (HMM) is a distributed service that maintains the current set of active nodes in a cluster of machines. This protocol allows the detection of multiple machine joins or failures in a unique reconfiguration, using a low amount of messages (with a cost that is linear on the number of nodes). These membership services are needed to detect cluster changes as soon as possible, initiating then the reconfiguration of the cluster state, where support for replicated objects has been included. The HMM also manages and synchronises the reconfiguration steps needed by the kernel and Hidra components of each node, ensuring that all of them take the same steps at once. Thus, our system does not need an atomic multicast protocol to deliver the messages in these reconfiguration steps. All these services provide the basis to develop reliable intracluster transport protocols and to reduce the reconfiguration time of replicated objects and services.

A survey on elasticity management in PaaS systems

Elasticity is a goal of cloud computing. An elastic system should manage in an autonomic way its resources, being adaptive to dynamic workloads, allocating additional resources when workload is increased and deallocating resources when workload decreases. PaaS providers should manage resources of customer applications with the aim of converting those applications into elastic services. This survey identifies the requirements that such management imposes on a PaaS provider: autonomy, scalability, adaptivity, SLA awareness, composability and upgradeability. This document delves into the variety of mechanisms that have been proposed to deal with all those requirements. Although there are multiple approaches to address those concerns, providers’ main goal is maximisation of profits. This compels providers to look for balancing two opposed goals: maximising quality of service and minimising costs. Because of this, there are still several aspects that deserve additional research for finding optimal adaptability strategies. Those open issues are also discussed.

Scalability approaches for causal multicast: a survey

Many distributed services need to be scalable: internet search, electronic commerce, e-government

Constructive and adaptable distributed shared memory

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Correctness Proof for a Distributed Memory System

… In order to achieve scalability those applications rely on replicated components. Because of the dynamics of growth and volatility of customer markets, applications need to be hosted by adaptive systems. In particular, the scalability of the reliable multicast mechanisms used for supporting the consistency of replicas is of crucial importance. Reliable multicast may propagate updates in a pre-defined order (e.g., FIFO, total or causal). Since total order needs more communication rounds than causal order, the latter appears to be the preferable candidate for achieving multicast scalability, although the consistency guarantees based on causal order are weaker than those of total order. This paper provides a historical survey of different scalability approaches for reliable causal multicast protocols.