arXiv subjects

Carla Ferreira

Publications and source records attributed to Carla Ferreira.

23 records · Page 2Linked to original sources

arXiv2015

Extending Eventually Consistent Cloud Databases for Enforcing Numeric Invariants

Geo-replicated databases often operate under the principle of eventual consistency to offer high-availability with low latency on a simple key/value store abstraction. Recently, some have adopted commutative data types to provide seamless reconciliation for special purpose data types, such as counters. Despite this, the inability to enforce numeric invariants across all replicas still remains a key shortcoming of relying on the limited guarantees of eventual consistency storage. We present a new replicated data type, called bounded counter, which adds support for numeric invariants to eventually consistent geo-replicated databases. We describe how this can be implemented on top of existing cloud stores without modifying them, using Riak as an example. Our approach adapts ideas from escrow transactions to devise a solution that is decentralized, fault-tolerant and fast. Our evaluation shows much lower latency and better scalability than the traditional approach of using strong consistency to enforce numeric invariants, thus alleviating the tension between consistency and availability.

cs.DC↗

arXiv2012

Identification of the Risk Related to a Process on Hospital Emergency Service: a Case Study

This paper, framed in a vast investigation, describes the application of techniques and methodologies in Organizational Engineering connected to the associated risk to the processes developed in an Emergency Service of an important Portuguese Hospital. The transactions performed in an emergency service and the consequent risk identification (negative behaviour associated to those transactions) is done based on static and dynamic models, developed during the business modelling. Any non-trivial system is better portrayed trough a small number of reasonably independent models. From this point of view it is important to look at the systems from a "micro" perspective, which allows us to analyse the system at the transaction level. All processes have some associated risk (inherent risk). Its identification will be decisive for future analysis and for the consequent decision over the need, or not, to study internal control mechanisms. This decision will depend on the risk level that the organization considers acceptable.

cs.SE↗

arXiv2012

Urgency/Emergency Health Processes' Modelling: A Case Study

The growing complexity and sophistication of the organizational information systems, and hospital ones particularly, render difficult their comprehension and, consequently, the implementation of control mechanisms that may assure, at all times, the auditability of the above mentioned systems, without having to use models. This paper, framed in a wider investigation, aims to describe the application of techniques and methodologies, in the sphere of action of Organizational Engineering, in the modelling of business processes developed in the main Operating Theatre of the Coimbra's University Hospital Emergency Service, as a support for the implementation of an information system architecture, using for that purpose the CEO framework, developed and suggested by the Centre for Organizational Engineering (CEO), based on the UML language.

cs.SE↗

arXiv2012

Structure-preserving Schur methods for computing square roots of real skew-Hamiltonian matrices

Our contribution is two-folded. First, starting from the known fact that every real skew-Hamiltonian matrix has a real Hamiltonian square root, we give a complete characterization of the square roots of a real skew-Hamiltonian matrix W. Second, we propose a structure exploiting method for computing square roots of W. Compared to the standard real Schur method, which ignores the structure, our method requires significantly less arithmetic.

math.NA↗

arXiv2012

Real dqds for the nonsymmetric tridiagonal eigenvalue problem

We present a new transform, triple dqds, to help to compute the eigenvalues of a real tridiagonal matrix C using real arithmetic. The algorithm uses the real dqds transform to shift by a real number and triple dqds to shift by a complex conjugate pair. We present what seems to be a new criteria for splitting the current pair L,U. The algorithm rejects any transform which suffers from excessive element growth and then tries a new transform. Our numerical tests show that the algorithm is about 100 times faster than the Ehrlich-Aberth method of D. A. Bini, L. Gemignani and F. Tisseur. Our code is comparable in performance to a complex dqds code and is sometimes 3 times faster.

math.NA↗