A Systems Approach to Minimize Wasted Work in Blockchains

Blockchain systems and distributed ledgers are getting increasing attention since the release of Bitcoin. Everyday they make headlines in the news involving economists, scientists, and technologists. The technology invented by Satoshi Nakamoto gave to the world a quantum leap in the fields of distributed systems and digital currencies. Even so, there are still some problems regarding the architecture in most existing blockchain systems. One of the main challenges in these systems is the structure of the network topology and how peers disseminate messages between them, this leads to problems regarding the system scalability and the efficiency of the transaction and blocks propagation, wasting computational power, energy and network resources. In this work we propose a novel solution to tackle these limitations. We propose the design of membership and message dissemination protocols, based on the state-ofart, that will boost the efficiency of the overlay network that support the interactions between miners, reducing the number of exchanged messages and the used bandwidth. This solution also reduces the computational power and energy consumed across all nodes in the network, since the nodes avoid to process redundant network messages, and, becoming aware of mined blocks faster, avoid to perform computations over an outdated chain configuration

Discreet - Pub/Sub for Edge Systems

The number of devices connected to the Internet has been growing exponentially over the last few years. Today, the amount of information available to users has reached a point that makes it impossible to consume it all, showing that we need better ways to filter what kind of information is sent our way. At the same time, while users are online and access all this information, their actions are also being collected, scrutinized and commercialized with little regard for privacy. This thesis addresses those issues in the context of a decentralized Publish/Subscribe solution for edge systems. Working at the edge of the Internet aims to prevent centralized control from a single entity and lessen the chance of abuse. Our goal was to devise a solution that achieves efficient message delivery, with good load-balancing properties, without revealing its participants subscription interests to preserve user privacy. Our solution uses cryptography and probabilistic data sets as a way to obfuscate event topics and user subscriptions. We modeled a cooperative solution, where publisher and subscriber nodes work in concert to route events among themselves, by leveraging a onehop structured overlay. By using an experimental evaluation, we attest the scalability and general performance of the proposed algorithms, including latency, false negative and false positive rates, and other useful metrics.O número de aparelhos ligados a Internet têm vindo a crescer exponencialmente ao longo dos últimos anos. Hoje em dia, a quantidade de informação que os utilizadores têm disponível, chegou a um ponto que torna impossível o seu total consumo. Isto leva a que seja necessário encontrarmos melhores formas de filtrar a informação que recebemos. Ao mesmo tempo, as ações do utilizadores estão a ser recolhidas, examinadas e comercializadas, sem qualquer respeito pela privacidade. Esta tese trata destes assuntos no contexto de um sistema Publish/Subscribe descentralizado, para sistemas na periferia. O objectivo de operar na preferia da Internet está em prevenir o controlo centralizado por uma única entidade e diminuir a oportunidade para abusos. O nosso objectivo foi conceber uma solução que realiza entrega de mensagens eficientemente, com boas propriedades na distribuição de carga e sem revelar on interesses dos participantes, de forma a preservar a sua privacidade. A nossa solução usa criptografia e estruturas de dados probabilísticas, como uma forma de ofuscar os tópicos dos eventos e as subscrições dos utilizadores. Modelamos o sistema com o objectivo de ser uma solução cooperativa, onde ambos os tipos de nós Editores e Assinantes trabalham em concertadamente para encaminhar eventos entre eles, ao fazerem uso de uma estrutura de rede sobreposta com um salto. Fazendo uma avaliação experimental testámos a escalabilidade e o desempenho geral dos algoritmos propostos, incluindo a latência, falsos negativos, falsos positivos e outras métricas úteis

Non-uniform replication for replicated objects

A large number of web applications/services are supported by applications running in cloud computing infrastructures. Many of these application store their data in georeplicated key-value stores, that maintain replicas of the data in several data centers located across the globe. Data management in these settings is challenging, with solutions needing to balance availability and consistency. Solutions that provide high-availability, by allowing operations to execute locally in a single data center, have to cope with a weaker consistency model. In such cases, replicas may be updated concurrently and a mechanism to reconcile divergent replicas is needed. Using the semantics of data types (and operations) helps in providing a solution that meets the requirements of applications, as shown by conflict-free replicated data types. As information grows it becomes difficult or even impossible to store all information at every replica. A common approach to deal with this problem is to rely on partial replication, where each replica maintains only part of the total system information. As a consequence, each partial replica can only reply to a subset of the possible queries. In this thesis, we introduce the concept of non-uniform replication where each replica stores only part of the information, but where all replicas store enough information to answer every query. We apply this concept to eventual consistency and conflict-free replicated data types and propose a set of useful data type designs where replicas synchronize by exchanging operations. Furthermore, we implement support for non-uniform replication in AntidoteDB, a geo-distributed key-value store, and evaluate the space efficiency, bandwidth overhead, and scalability of the solution

Integrating XQuery and P2P in MonetDB/XQuery*

MonetDB/XQuery* is a fully functional publicly available XML DBMS that has been extended with distributed and P2P data management functionality. Our (minimal) XQuery language extension XRPC adds the concept of RPC to XQuery, and exploits the set-at-a-time database processing model to optimize the networking cost through a technique called Bulk RPC. We describe our approach to include the services offered by diverse P2P network structures (such as DHTs), in a way that avoids any further intrusion in the XQuery language and semantics, and show how this, similarly to Bulk RPC, will lead to further query optimization opportunities where the XDBMS interacts with the underlying P2P network. We also discuss some P2P data management applications were MonetDB/XQuery* is being used (an in-home small scenario and a wide-area collaborative application). As this research is work-in-progress, we outline some research questions on our path towards defining and realizing P2P XDBMS technology

Scaling up publish/subscribe overlays using interest correlation for link sharing

Topic-based publish/subscribe is at the core of many distributed systems, ranging from application integration middleware to news dissemination. Therefore, much research was dedicated to publish/subscribe architectures and protocols, and in particular to the design of overlay networks for decentralized topic-based routing and efficient message dissemination. Nonetheless, existing systems fail to take full advantage of shared interests when disseminating information, hence suffering from high maintenance and traffic costs, or construct overlays that cope poorly with the scale and dynamism of large networks. In this paper we present StaN, a decentralized protocol that optimizes the properties of gossip-based overlay networks for topicbased publish/subscribe by sharing a large number of physical connections without disrupting its logical properties. StaN relies only on local knowledge and operates by leveraging common interests among participants to improve global resource usage and promote topic and event scalability. The experimental evaluation under two real workloads, both via a real deployment and through simulation shows that StaN provides an attractive infrastructure for scalable topic-based publish/subscribe

Considering Complex Search Techniques in DHTs Under Churn

Abstract-Traditionally complex queries have been performed over unstructured P2P networks by means of flooding, which is inherently inefficient due to the large number of redundant messages generated. While Distributed Hash Tables (DHTs) can provide very efficient look-up operations, they traditionally do not provide any methods for complex queries. By exploiting the structure inherent in DHTs we can perform complex querying over structured P2P networks by means of efficiently broadcasting the search query. This allows every node in the network to process the query locally, and hence is as powerful and flexible as flooding in unstructured networks, but without the inefficiency of redundant messages. While there have been various approaches proposed for broadcasting search queries over DHTs, the focus has not been on validation under churn. Comparing blind search methods for DHTs through simulation we see that churn, in particular nodes leaving the network, has a large impact on query success rate. In this paper we present novel results comparing blind search over Chord and Pastry while under varying levels of churn. We further consider how different data replication strategies can be used to enhance the query success rate

Optimising Structured P2P Networks for Complex Queries

With network enabled consumer devices becoming increasingly popular, the number of connected devices and available services is growing considerably - with the number of connected devices es- timated to surpass 15 billion devices by 2015. In this increasingly large and dynamic environment it is important that users have a comprehensive, yet efficient, mechanism to discover services. Many existing wide-area service discovery mechanisms are centralised and do not scale to large numbers of users. Additionally, centralised services suffer from issues such as a single point of failure, high maintenance costs, and difficulty of management. As such, this Thesis seeks a Peer to Peer (P2P) approach. Distributed Hash Tables (DHTs) are well known for their high scalability, financially low barrier of entry, and ability to self manage. They can be used to provide not just a platform on which peers can offer and consume services, but also as a means for users to discover such services. Traditionally DHTs provide a distributed key-value store, with no search functionality. In recent years many P2P systems have been proposed providing support for a sub-set of complex query types, such as keyword search, range queries, and semantic search. This Thesis presents a novel algorithm for performing any type of complex query, from keyword search, to complex regular expressions, to full-text search, over any structured P2P overlay. This is achieved by efficiently broadcasting the search query, allowing each peer to process the query locally, and then efficiently routing responses back to the originating peer. Through experimentation, this technique is shown to be successful when the network is stable, however performance degrades under high levels of network churn. To address the issue of network churn, this Thesis proposes a number of enhancements which can be made to existing P2P overlays in order to improve the performance of both the existing DHT and the proposed algorithm. Through two case studies these enhancements are shown to improve not only the performance of the proposed algorithm under churn, but also the performance of traditional lookup operations in these networks

The ViP2P Platform: XML Views in P2P

The growing volumes of XML data sources on the Web or produced by enterprises, organizations etc. raise many performance challenges for data management applications. In this work, we are concerned with the distributed, peer-to-peer management of large corpora of XML documents, based on distributed hash table (or DHT, in short) overlay networks. We present ViP2P (standing for Views in Peer-to-Peer), a distributed platform for sharing XML documents based on a structured P2P network infrastructure (DHT). At the core of ViP2P stand distributed materialized XML views, defined by arbitrary XML queries, filled in with data published anywhere in the network, and exploited to efficiently answer queries issued by any network peer. ViP2P allows user queries to be evaluated over XML documents published by peers in two modes. First, a long-running subscription mode, when a query can be registered in the system and receive answers incrementally when and if published data matches the query. Second, queries can also be asked in an ad-hoc, snapshot mode, where results are required immediately and must be computed based on the results of other long-running, subscription queries. ViP2P innovates over other similar DHT-based XML sharing platforms by using a very expressive structured XML query language. This expressivity leads to a very flexible distribution of XML content in the ViP2P network, and to efficient snapshot query execution. ViP2P has been tested in real deployments of hundreds of computers. We present the platform architecture, its internal algorithms, and demonstrate its efficiency and scalability through a set of experiments. Our experimental results outgrow by orders of magnitude similar competitor systems in terms of data volumes, network size and data dissemination throughput.Comment: RR-7812 (2011