Protocols for Integrity Constraint Checking in Federated Databases

A federated database is comprised of multiple interconnected database systems that primarily operate independently but cooperate to a certain extent. Global integrity constraints can be very useful in federated databases, but the lack of global queries, global transaction mechanisms, and global concurrency control renders traditional constraint management techniques inapplicable. This paper presents a threefold contribution to integrity constraint checking in federated databases: (1) The problem of constraint checking in a federated database environment is clearly formulated. (2) A family of protocols for constraint checking is presented. (3) The differences across protocols in the family are analyzed with respect to system requirements, properties guaranteed by the protocols, and processing and communication costs. Thus, our work yields a suite of options from which a protocol can be chosen to suit the system capabilities and integrity requirements of a particular federated database environment

MDCC: Multi-Data Center Consistency

Replicating data across multiple data centers not only allows moving the data closer to the user and, thus, reduces latency for applications, but also increases the availability in the event of a data center failure. Therefore, it is not surprising that companies like Google, Yahoo, and Netflix already replicate user data across geographically different regions. However, replication across data centers is expensive. Inter-data center network delays are in the hundreds of milliseconds and vary significantly. Synchronous wide-area replication is therefore considered to be unfeasible with strong consistency and current solutions either settle for asynchronous replication which implies the risk of losing data in the event of failures, restrict consistency to small partitions, or give up consistency entirely. With MDCC (Multi-Data Center Consistency), we describe the first optimistic commit protocol, that does not require a master or partitioning, and is strongly consistent at a cost similar to eventually consistent protocols. MDCC can commit transactions in a single round-trip across data centers in the normal operational case. We further propose a new programming model which empowers the application developer to handle longer and unpredictable latencies caused by inter-data center communication. Our evaluation using the TPC-W benchmark with MDCC deployed across 5 geographically diverse data centers shows that MDCC is able to achieve throughput and latency similar to eventually consistent quorum protocols and that MDCC is able to sustain a data center outage without a significant impact on response times while guaranteeing strong consistency

Developing a distributed electronic health-record store for India

The DIGHT project is addressing the problem of building a scalable and highly available information store for the Electronic Health Records (EHRs) of the over one billion citizens of India

Verifying and Enforcing Application Constraints in Antidote SQL

Geo-replicated storage systems are currently a fundamental piece in the development of large-scale applications where users are distributed across the world. To meet the high requirements regarding la- tency and availability of these applications, these database systems are forced to use weak consistency mechanisms. However, under these consistency models, there is no guarantee that the invariants are preserved, which can jeopardise the correctness of applications. The most obvious alternative to solve this problem would be to use strong consistency, but this would place a large burden on the system. Since neither of these options was feasible, many systems have been developed to preserve the invariants of the applications without sacrificing low latency and high availability. These systems, based on the analysis of operations, make it possible to increase the guarantees of weak consistency by introducing consistency at the level of operations that are potentially dangerous to the invariant. Antidote SQL is a database system that, by combining strong with weak consistency mechanisms, attempts to guarantee the preservation of invariants at the data level. In this way, and after defining the concurrency semantics for the application, any operation can be performed without coordination and without the risk of violating the invariant. However, this approach has some limitations, namely the fact that it is not trivial for developers to define appropriate concurrency semantics. In this document, we propose a methodology for the verification and validation of defined prop- erties, such as invariants, for applications using Antidote SQL. The proposed methodology uses a high-level programming language with automatic verification features called VeriFx and provides guidelines for programmers who wish to implement and verify their own systems and specifications using this tool.Os sistemas de armazenamento geo-replicados são atualmente uma peça fundamental no desenvolvi- mento de aplicações de grande escala em que os utilizadores se encontram espalhados pelo mundo. Com o objetivo de satisfazer os elevados requisitos em relação à latência e à disponibilidade destas aplicações, estes sistemas de bases de dados vêem-se obrigados a recorrer a mecanismos de consistên- cia fracos. No entanto, sob estes modelos de consistência não existe qualquer tipo de garantia de que os invariantes são preservados, o que pode colocar em causa a correção das aplicações. A alternativa mais óbvia para resolver este problema passaria por utilizar consistência forte, no entanto esta incutiria uma grande sobrecarga no sistema. Sendo que nenhuma destas opções é viável, muitos sistemas foram desenvolvidos no sentido de preservar os invariantes das aplicações, sem contudo, abdicar de baixas latências e alta disponibilidade. Estes sistemas, baseados na análise das operações, permitem aumentar as garantias de consistência fraca com a introdução de consistência ao nível das operações potencialmente perigosas para o invari- ante. O Antidote SQL é um sistema de base de dados que através da combinação de mecanismos de consistência fortes com mecanismos de consistência fracos tenta garantir a preservação dos invariantes ao nível dos dados. Desta forma, e depois de definidas as semânticas de concorrência para a aplicação, qualquer operação pode ser executada sem coordenação e sem perigo de quebra do invariante. No entanto esta abordagem apresenta algumas limitações nomeadamente o facto de não ser trivial para os programadores definirem as semânticas de concorrência adequadas. Neste documento propomos uma metodologia para a verificação e validação de propriedades defi- nidas, como os invariantes, para aplicações que usam o Antidote SQL. A metodologia proposta utiliza uma linguagem de programação de alto nível com capacidade de verificação automática designada por VeriFx, e fornece as diretrizes a seguir para que o programador consiga implementar e verificar os seus próprios sistemas e especificações, utilizando a ferramenta

State-of-the-art on evolution and reactivity

This report starts by, in Chapter 1, outlining aspects of querying and updating resources on the Web and on the Semantic Web, including the development of query and update languages to be carried out within the Rewerse project. From this outline, it becomes clear that several existing research areas and topics are of interest for this work in Rewerse. In the remainder of this report we further present state of the art surveys in a selection of such areas and topics. More precisely: in Chapter 2 we give an overview of logics for reasoning about state change and updates; Chapter 3 is devoted to briefly describing existing update languages for the Web, and also for updating logic programs; in Chapter 4 event-condition-action rules, both in the context of active database systems and in the context of semistructured data, are surveyed; in Chapter 5 we give an overview of some relevant rule-based agents frameworks

Integrated testing and verification system for research flight software design document

The NASA Langley Research Center is developing the MUST (Multipurpose User-oriented Software Technology) program to cut the cost of producing research flight software through a system of software support tools. The HAL/S language is the primary subject of the design. Boeing Computer Services Company (BCS) has designed an integrated verification and testing capability as part of MUST. Documentation, verification and test options are provided with special attention on real time, multiprocessing issues. The needs of the entire software production cycle have been considered, with effective management and reduced lifecycle costs as foremost goals. Capabilities have been included in the design for static detection of data flow anomalies involving communicating concurrent processes. Some types of ill formed process synchronization and deadlock also are detected statically

Next generation software environments : principles, problems, and research directions

The past decade has seen a burgeoning of research and development in software environments. Conferences have been devoted to the topic of practical environments, journal papers produced, and commercial systems sold. Given all the activity, one might expect a great deal of consensus on issues, approaches, and techniques. This is not the case, however. Indeed, the term "environment" is still used in a variety of conflicting ways. Nevertheless substantial progress has been made and we are at least nearing consensus on many critical issues.The purpose of this paper is to characterize environments, describe several important principles that have emerged in the last decade or so, note current open problems, and describe some approaches to these problems, with particular emphasis on the activities of one large-scale research program, the Arcadia project. Consideration is also given to two related topics: empirical evaluation and technology transition. That is, how can environments and their constituents be evaluated, and how can new developments be moved effectively into the production sector