On stacks and russian dolls: mobile objects in configurable communication protocols

This paper introduces Groupz, a novel development framework for group communication protocol. Groupz merges advantages of traditional communication protocol support environments with object mobility, proposing multiple nested mobile objects as the natural evolution of layered protocols. By shifting the focus of protocol development from data messages to mobile objects, it makes possible to build configurable and adaptable system software, suited for problematic environments such as world-wide networks and mobile computers, without overlooking efficiency

A mutable protocol for consensus in large groups

In this paper we propose the mutable con- sensus protocol, a pragmatic and theoretically appealing approach to enhance the performance of distributed con- sensus with a large number of participants. First, an apparently inefficient consensus protocol is developed using the very simple stubborn channel abstraction for unreliable message passing. Then, the introduction of judiciously chosen finite delays in the implementation of channels makes it likely that the transmission of some messages is avoided. Although this does not affect correctness, which rests on an asynchronous system model, the message exchange pattern at the network level changes noticeably and can be made to resemble several different protocols. A particularly appealing instantiation, called the permutation gossip, allows the protocol to scale gracefully to a large number of processes

The mutable consensus protocol

In this paper we propose the mutable consensus protocol, a pragmatic and theoretically appealing approach to enhance the performance of distributed consensus. First, an apparently inefficient protocol is developed using the simple stubborn channel abstraction for unreliable message passing. Then, performance is improved by introducing judiciously chosen finite delays in the implementation of channels. Although this does not compromise correctness, which rests on an asynchronous system model, it makes it likely that the transmission of some messages is avoided and thus the message exchange pattern at the network level changes noticeably. By choosing different delays in the underlying stubborn channels, the mutable consensus protocol can actually be made to resemble several different protocols. Besides presenting the mutable consensus protocol and four different mutations, we evaluate in detail the particularly interesting permutation gossip mutation, which allows the protocol to scale gracefully to a large number of processes by balancing the number of messages to be handled by each process with the number of communication steps required to decide. The evaluation is performed using a realistic simulation model which accurately reproduces resource consumption in real systems.FCT , project STRONGREP (POSI/CHS/41285/2001)

Integration of concurrency control in a language with subtyping and subclassing

This paper describes the integration of concurrency control in BALLOON, an object-oriented language that separates the concepts of type and class as well of subtyping and subclassing. Types are interface specifications enriched with concurrency control annotations. Classes are used to implement the operational functionality of types as well as concurrency control mechanisms. Types, classes and concurrency control annotations are independently reusable and derivable. The language takes advantage of this separation to solve the typical problems of the inheritance anomaly

Reducing the cost of group communication with semantic view synchrony

View Synchrony (VS) is a powerful abstraction in the design and implementation of de- pendable distributed systems. By ensuring that processes deliver the same set of messages in each view, it allows them to maintain consistency across membership changes. However, experience indicates that it is hard to combine strong reliability guarantees as offered by VS with stable high performance. In this paper we propose a novel abstraction, Semantic View Synchrony (SVS), that exploits the application's semantics to cope with high throughput applications. This is achieved by allowing some messages to be dropped while still preserving consistency when new views are installed. Thus, SVS inherits the elegance of view synchronous communi- cation. The paper describes how SVS can be implemented and illustrates its usefulness in the context of distributed multi-player games

Semantically reliable multicast protocols

Reliable multicast protocols can strongly simplify the design of distributed applications. However it is hard to sustain a high multicast throughput when groups are large and heterogeneous. In an attempt to overcome this limitation, previous work has focused on weakening reliability properties. The authors introduce a novel reliability model that exploits semantic knowledge to decide in which specific conditions messages can be purged without compromising application correctness. This model is based on the concept of message obsolescence: a message becomes obsolete when its content or purpose is overwritten by a subsequent message. We show that message obsolescence can be expressed in a generic way and can be used to configure the system to achieve higher multicast throughput.Supported by the 234/J4 Franco/Portuguese Grant and by Praxis/ C/ EEI/ 12202/ 1998, TOPCO

Conflict classes for replicated databases: a case-study

The major challenge in fault-tolerant replicated transactional databases is providing efficient distributed concurrency control that allows non-conflicting transactions to execute concurrently. A common approach is to partition the data according to the data access patterns of the workload, assuming that this will allow operations in each partition to be scheduled independently and run in parallel. The effectiveness of this approach hinges on the characteristics of the workload: (i) the ability to identify such partitions and (ii) the actual number of such partitions that arises. Performance results that have been presented to support such proposals are thus tightly linked to the simplistic synthetic benchmarks that have been used. This is worrisome, since these benchmarks have not been conceived for this purpose and the resulting definition of partitions might not be representative of real applications. In this paper we contrast a more complex synthetic benchmark (TPC-E) with a real application in the same area (financial brokerage), concluding that the real setting makes it much harder to determine a correct partition of the data and that sub-optimal partitioning severely constrains the performance of replication

AKARA: A flexible clustering protocol for demanding transactional workloads

Shared-nothing clusters are a well known and cost-effective approach to database server scalability, in particular, with highly intensive read-only workloads typical of many 3-tier web-based applications. The common reliance on a centralized component and a simplistic propagation strategy employed by mainstream solutions however conduct to poor scalability with traditional on-line transaction processing (OLTP), where the update ratio is high. Such approaches also pose an additional obstacle to high availability while introducing a single point of failure. More recently, database replication protocols based on group communication have been shown to overcome such limitations, expanding the applicability of shared-nothing clusters to more demanding transactional workloads. These take simultaneous advantage of total order multicast and transactional semantics to improve on mainstream solutions. However, none has already been widely deployed in a general purpose database management system. In this paper, we argue that a major hurdle for their acceptance is that these proposals have disappointing performance with specific subsets of real-world workloads. Such limitations are deep-rooted and working around them requires in-depth understanding of protocols and changes to applications. We address this issue with a novel protocol that combines multiple transaction execution mechanisms and replication techniques and then show how it avoids the identified pitfalls. Experimental results are obtained with a workload based on the industry standard TPC-C benchmark

Self tuning with self confidence

Recent research on managing complex computing systems has focused on the autonomic computing vision: Systems should manage themselves according to an high level administrator’s goal [5]. As an example, system components should monitor the enviroment and self-tune to meet quality of service expectations, without requiring manual intervention in the selection of concrete configuration options or in coordinating the reconfiguration process

Semantically reliable multicast: current status and future work

In multicast communication systems, a single perturbed recipient can drastically affect the performance of a complete group of processes. One way to alleviate this problem is to weakeri reliability requirements by aliowing some messages to be omitted. We propose a multicast service that exploits semantic knowledge to select which messges can be omitted without compromising the application's correctness. This service is basecf on the concept of message obsolescence: A message becomes obsolete when its content is overwritten or implicitly conveyed by a subsequent message. Besides summarizing initial research results [10] showing that message obsolescence can be expressed in a generic way imd can be used to achieve a higher stable throughput, this text advances a definition of the service and outlines our current research directions