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Publicações

Publicações por Paulo Sérgio Almeida

1999

Aliasing in object oriented systems

Autores
Noble, J; Vitek, J; Lea, D; Almeida, PS;

Publicação
OBJECT-ORIENTED TECHNOLOGY

Abstract
This chapter contains summaries of the presentations given at the Intercontinental Workshop on Aliasjng in Object-Oriented Systems (IWAOOS'99) at the European Conference on Object-Oriented Programming (ECOOP'99) which was held in Lisbon, Portugal on June 15, 1999.

1999

Type-checking balloon types

Autores
Almeida, PS;

Publicação
Electronic Notes in Theoretical Computer Science

Abstract
Current data abstraction mechanisms are not adequate to control sharing of state in the general case involving objects in linked structures. The pervading possibility of sharing is a source of errors and an obstacle to language implementation techniques. Balloon types, which we have introduced in [2], are a general extension to programming languages. They make the ability to share state a first class property of a data type. The balloon invariant expresses a strong form of encapsulation: no state reachable (directly or transitively) by a balloon object is referenced by any external object. In this paper we describe the checking mechanism for balloon types. It relies on a non-trivial static analysis, described as an abstract interpretation. Here we focus in particular on the design of the abstract domain which allows the checking mechanism to work under realistic assumptions regarding possible object aliasing. ©1999 Published by Elsevier Science B. V.

2009

Probabilistic Estimation of Network Size and Diameter

Autores
Cardoso, JCS; Baquero, C; Almeida, PS;

Publicação
LADC: 2009 4TH LATIN-AMERICAN SYMPOSIUM ON DEPENDABLE COMPUTING

Abstract
Determining the size of a network and its diameter are important functions in distributed systems, as there are a number of algorithms which rely on such parameters, or at least on estimates of those values. The Extrema Propagation technique allows the estimation of the size of a network in a fast, distributed and fault tolerant manner. The technique was previously studied in a simulation setting where rounds advance synchronously and where there is no message loss. This work presents two main contributions. The first, is the study of the Extrema Propagation technique under asynchronous rounds and integrated in the Network Friendly Epidemic Multicast (NeEM) framework. The second, is the evaluation of a diameter estimation technique associated with the Extrema Propagation. This study also presents a small enhancement to the Extrema Propagation in terms of communication cost and points out some other possible enhancements. Results show that there is a clear trade-off between time and communication that must be considered when configuring the protocol-a faster convergence time implies a higher communication cost Results also show that its possible to reduce the total communication cost by more than 18% using a simple approach. The diameter estimation technique is shown to have a relative error of less than 10% even when using a small sample of nodes.

2009

Fast Estimation of Aggregates in Unstructured Networks

Autores
Baquero, C; Almeida, PS; Menezes, R;

Publicação
ICAS: 2009 FIFTH INTERNATIONAL CONFERENCE ON AUTONOMIC AND AUTONOMOUS SYSTEMS

Abstract
Aggregation of data values plays an important role on distributed computations, in particular over peer-to-peer and sensor networks, as it can provide a summary of some global system property and direct the actions of self-adaptive distributed algorithms. Examples include using estimates of the network Size to dimension distributed hash tables or estimates of the average system load to direct load-balancing. Distributed aggregation using non-idempotent functions, like sums, is not trivial as it is not easy to prevent a given value from being accounted for multiple times; this is especially the case if no centralized algorithms or global identifiers can be used. This paper introduces Extrema Propagation, a probabilistic technique for distributed estimation of the sum of positive real numbers. The technique relies on the exchange of duplicate insensitive messages and can be applied in flood and/or epidemic settings, where multi-path routing occurs; it is tolerant of message loss; it is fast, as the number of message exchange steps equals the diameter; and it is fully, distributed, with no single point of failure and the result produced at every node.

2010

Fault-Tolerant Aggregation for Dynamic Networks

Autores
Jesus, P; Baquero, C; Almeida, PS;

Publicação
2010 29TH IEEE INTERNATIONAL SYMPOSIUM ON RELIABLE DISTRIBUTED SYSTEMS SRDS 2010

Abstract
Data aggregation is a fundamental building block of modern distributed systems. Averaging based approaches, commonly designated gossip-based, are an important class of aggregation algorithms as they allow all nodes to produce a result, converge to any required accuracy, and work independently from the network topology. However, existing approaches exhibit many dependability issues when used in faulty and dynamic environments. This paper extends our own technique, Flow Updating, which is immune to message loss, to operate in dynamic networks, improving its fault tolerance characteristics. Experimental results show that the novel version of Flow Updating vastly outperforms previous averaging algorithms; it self adapts to churn without requiring any periodic restart, supporting node crashes and high levels of message loss.

2012

Extrema Propagation: Fast Distributed Estimation of Sums and Network Sizes

Autores
Baquero, C; Almeida, PS; Menezes, R; Jesus, P;

Publicação
IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS

Abstract
Aggregation of data values plays an important role on distributed computations, in particular, over peer-to-peer and sensor networks, as it can provide a summary of some global system property and direct the actions of self-adaptive distributed algorithms. Examples include using estimates of the network size to dimension distributed hash tables or estimates of the average system load to direct load balancing. Distributed aggregation using nonidempotent functions, like sums, is not trivial as it is not easy to prevent a given value from being accounted for multiple times; this is especially the case if no centralized algorithms or global identifiers can be used. This paper introduces Extrema Propagation, a probabilistic technique for distributed estimation of the sum of positive real numbers. The technique relies on the exchange of duplicate insensitive messages and can be applied in flood and/or epidemic settings, where multipath routing occurs; it is tolerant of message loss; it is fast, as the number of message exchange steps can be made just slightly above the theoretical minimum; and it is fully distributed, with no single point of failure and the result produced at every node.

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