Gustavo M. D. Vieira

Dynamic content web applications: Crash, failover, and recovery analysis

This work assesses how crashes and recoveries affect the performance of a replicated dynamic content web application. RobustStore is the result of retrofitting TPC-Ws on-line bookstore with Treplica, a middleware for building dependable applications. Implementations of Paxos and Fast Paxos are at the core of Treplicas efficient and programmer-friendly support for replication and recovery. The TPC-W benchmark, augmented with faultloads and dependability measures, is used to evaluate the behaviour of RobustStore. Experiments apply faultloads that cause sequential and concurrent replica crashes. RobustStores performance drops by less than 13% during the recovery from two simultaneous replica crashes. When subject to an identical faultload and a shopping workload, a five-replicas RobustStore maintains an accuracy of 99.999%. Our results display not only good performance, total autonomy and uninterrupted availability, they also show that it is simple to develop efficient recovery-oriented applications using Treplica.

On the coordinator's rule for Fast Paxos

Fast Paxos is an algorithm for consensus that works by a succession of rounds, where each round tries to decide a value v that is consistent with all past rounds. Rounds are started by a coordinator process and consistency is guaranteed by the rule used by this process for the selection of v and by the properties of process sets called quorums. We show a simplified version of this rule for the specific case where the quorums are defined by the cardinality of these process sets. This rule is of special interest for implementors of the algorithm.

Hardened Paxos through Consistency Validation

Due to the emergent adoption of distributed systems when building applications, demand for reliability and availability has increased. These properties can be achieved through replication techniques using middleware algorithms that must be capable of tolerating faults. Certain faults such as arbitrary faults, however, may be more difficult to tolerate, resulting in more complex and resource intensive algorithms that end up being not so practical to use. We propose and experiment with the use of consistency validation techniques to harden a benign fault-tolerant Paxos, thus being able to detect and tolerate non-malicious arbitrary faults.

Transparent Replication Using Metaprogramming in Cyan

Replication can be used to increase the availability of a service by creating many operational copies of its data called replicas. Active replication is a form of replication that has strong consistency semantics, easier to reason about and program. However, creating replicated services using active replication still demands from the programmer the knowledge of subtleties of the replication mechanism. In this paper we show how to use the metaprogramming infrastructure of the Cyan language to shield the application programmer from these details, allowing easier creation of fault-tolerant replicated applications through simple annotations.

UEFI BIOS Accessibility for the Visually Impaired

People with some kind of disability face a high level of difficulty for everyday tasks because, in many cases, accessibility was not considered necessary when the task or process was designed. An example of this scenario is a computers BIOS configuration screens, which do not consider the specific needs, such as screen readers, of visually impaired people. This paper proposes the idea that it is possible to make the pre-operating system environment accessible to visually impaired people. We report our work-in-progress in creating a screen reader prototype, accessing audio cards compatible with the High Definition Audio specification in systems running UEFI compliant firmware.

Seamless Paxos coordinators

The Paxos algorithm requires a single correct coordinator process to operate. After a failure, the replacement of the coordinator may lead to a temporary unavailability of the application implemented atop Paxos. So far, this unavailability has been addressed by reducing the coordinator replacement rate through the use of stable coordinator selection algorithms. We have observed that the cost of recovery of the newly elected coordinator’s state is at the core of this unavailability problem. In this paper we present a new technique to manage coordinator replacement that allows the recovery to occur concurrently with new consensus rounds. Experimental results show that our seamless approach effectively solves the temporary unavailability problem, its adoption entails uninterrupted execution of the application. Our solution removes the restriction that the occurrence of coordinator replacements is something to be avoided, allowing the decoupling of the application execution from the accuracy of the mechanism used to choose a coordinator. This result increases the performance of the application even in the presence of failures, it is of special importance to the autonomous operation of replicated applications that have to adapt to varying network conditions and partial failures.