Conference Proceedings

Shoker A, Almeida PS, Moreno CB.  2015.  Exactly-Once Quantity Transfer. Abstractquantity-transfer-srds-w-psds.pdfparishandoffaverage2015.pdf

Strongly consistent systems supporting distributed transactions can be prone to high latency and do not tolerate partitions. The present trend of using weaker forms of consistency, to achieve high availability, poses notable challenges in writing applications due to the lack of linearizability, e.g., to ensure global invariants, or perform mutator operations on a distributed datatype. This paper addresses a specific problem: the exactly-once transfer of a “quantity” from one node to another on an unreliable network (coping with message duplication, loss, or reordering) and without any form of global synchronization. This allows preserving a global property (the sum of quantities remains unchanged) without requiring global linearizability and only through using pairwise interactions between nodes, therefore allowing partitions in the system. We present the novel quantitytransfer algorithm while focusing on a specific use-case: a redistribution protocol to keep the quantities in a set of nodes balanced; in particular, averaging a shared real number across nodes. Since this is a work in progress, we briefly discuss the correctness of the protocol, and we leave potential extensions and empirical evaluations for future work.

Almeida PS, Shoker A, Moreno CB.  2014.  Efficient State-based CRDTs by Decomposition. EuroSys Workshop Proceedings . 1:2. Abstractdeltapapec.pdf

CRDTs are distributed data types that make eventual consistency of a distributed object possible and non ad-hoc. Specifically, state-based CRDTs achieve this by sharing local state changes through shipping the entire state, that is then merged to other replicas with an idempotent, associative, and commutative join operation, ensuring convergence. This imposes a large communication overhead as the state size becomes larger. We introduce Delta State Conflict-Free Replicated Datatypes ({\delta}-CRDT), which make use of {\delta}-mutators, defined in such a way to return a delta-state, typically, with a much smaller size than the full state. Delta-states are joined to the local state as well as to the remote states (after being shipped). This can achieve the best of both worlds: small messages with an incremental nature, as in operation-based CRDTs, disseminated over unreliable communication channels, as in traditional state-based CRDTs. We introduce the {\delta}-CRDT framework, and we explain it through establishing a correspondence to current state- based CRDTs. In addition, we present two anti-entropy algorithms: a basic one that provides eventual convergence, and another one that ensures both convergence and causal consistency. We also introduce two {\delta}-CRDT specifications of well-known replicated datatypes.

Moreno CB, Almeida PS, Shoker A.  2014.  Making Operation-based CRDTs Operation-based. EuroSys Workshop Proceedings . :2. AbstractPaper

Conflict-free Replicated Datatypes can simplify the design of predictable eventual consistency. They can be classified into state-based or operation-based. Operation-based approaches have the potential for allowing compact designs in both the sent message and the object state size, but cur- rent approaches are still far from this objective. Here we explore the design space for operation-based solutions, and we leverage the interaction with the middleware by offering a technique that delivers very compact solutions, while only broadcasting operation names and arguments.

Faria J, Aragão AP, Moreno CB, Barbosa LS, Henriques PR.  2003.  Paperwork on the Fly. EUNIS 2003 Conference: Beyond the Network, Innovative It-services : Proceedingsbook. Abstractpaperwork_on_the_fly.pdf

A particularly boring duty of every university lecturer is the filling up several academic management forms like: course: program, objectives, bibliographic lists,…;lecture notes, exercises,…; class: timetables, summaries,…; etc – some of them on a daily basis. He/she must refill these forms again and again, every semester and every year, sometimes with replication in different forms and comply with various requirements. This type of work require time borrowed from research and teaching.

With this in mind, we developed an web application to carry on such bureaucratic jobs in a quick and more effective way. This paper introduces ADIS-Academic Department Information System, a web-based application currently under testing and implementation on the Departamento de Informática of Universidade do Minho.

It also describes in more technical detail, the Fly component and the lessons learned after its use along one academic semester. This on-line web application allows the lecturer to register summaries, programs, list its own timetable and print them. Summaries can also be filled using an email message.

The server which promotes this application is a usual PC with Linux, MySQL, PHP and a web server Apache.