Francisco Castro-Company

MADIS: a slim middleware for database replication

Data replication serves to improve the availability and performance of distributed systems. The price to be paid consists of costs caused by protocols by which a sufficient degree of consistency of replicated data is maintained. Different kinds of targeted applications require different kinds of replication protocols, each one requiring a different set of metadata. We discuss the middleware architecture used in the MADIS project for maintaining the consistency of replicated databases. Instead of reinventing wheels, MADIS makes use of basic resources provided by conventional database systems (e.g. triggers, views, etc) to achieve its purpose, to a large extent. So, the underlying databases can perform more efficiently many of the routines needed to support any consistency protocol, the implementation of which thus becomes much simpler and easier. MADIS enables the databases to simultaneously maintain different metadata needed for different replication protocols, so that the latter can be chosen, plugged in and exchanged on the fly as online-configurable modules, in order to fit the shifting needs of given applications best, at each moment.

FOBr: a version-based recovery protocol for replicated databases

Within the field of databases that are deployed in distributed environments there is a need to guarantee consistency among replicas and availability among nodes despite of network disconnections and node crashes. A recovery protocol, such as FOBr, manages the database update of a recovering node as it might have missed many transactions during its absence. FOBr does so without stopping data access, and minimizing its interference with the active nodes, their memory usage, and the network traffic. The recovery protocol we propose is very suitable when a fast recovery of the missed data is required. It balances the recovery issues among nodes very fairly and due to this, out-dated data will promptly be recovered. Thus accesses to data that is not out-dated are not interrupted or delayed in any way. These characteristics allow regular transactions to be performed during the recovery process.

CLOB: communication support for efficient replicated database recovery

Replication protocols using an eager update propagation strategy commonly need a reliable broadcast service; i.e., a broadcast primitive with atomic delivery and, in some cases, also with total order. This communication service provides some appropriate features for the recovery tasks, although in some cases this leads to partial blocking of the replica taken as the source in the updating process. CLOB is a framework for reliable broadcast protocols that log the missed update messages in case of failure, being able to automatically resend these updates when the faulty destinations recover. This behaviour is easily configurable and allows an efficient recovery mechanism in case of short-term failures, which can be combined with other version-based recovery protocols in order to manage long-term outages.

The bicompletion of fuzzy quasi-metric spaces

Extending the well-known result that every fuzzy metric space, in the sense of Kramosil and Michalek, has a completion which is unique up to isometry, we show that every KM-fuzzy quasi-metric space has a bicompletion which is unique up to isometry, and deduce that for each KM-fuzzy quasi-metric space, the completion of its induced fuzzy metric space coincides with the fuzzy metric space induced by its bicompletion.

A metaprotocol outline for database replication adaptability

Database replication tasks are accomplished with the aid of consistency protocols. Commonly, proposed solutions use a single replication protocol providing just one isolation level. The main drawback of this approach is its lack of flexibility for changing scenarios -i.e. workloads, access patterns...- or heterogeneous client application requirements. This work proposes a metaprotocol for supporting several replication protocols which use different replication techniques or provide different isolation levels. With this metaprotocol, replication protocols can either work concurrently with the same data or be sequenced for adapting to changing environments. In this line, the use of a load monitor would enable the best choice for each transaction, selecting the most appropriate protocol according to the current system characteristics. This paper is focused on outlining this metaprotocol design, establishing the metadata set needed and the required interaction between the main database replication protocol families.

A fixed point theorem for preordered complete fuzzy quasi-metric spaces and an application

We obtain a fixed point theorem for a type of generalized contractions on preordered complete fuzzy quasi-metric spaces which is applied to deduce, among other results, a procedure to show in a direct and easy fashion the existence of solution for the recurrence equations that are typically associated to Quicksort and Divide and Conquer algorithms, respectively.MSC:47H10, 54H25, 06A06, 68Q25.

Some classes of t-norms and fuzzy metric spaces

Recently, V. Gregori, S. Morillas and A. Sapena have discussed [On a class of completable fuzzy metric spaces, Fuzzy Sets and Systems 161 (2010), 2193-2205] the so-called strong fuzzy metrics when looking for a class of completable fuzzy metric spaces in the sense of A. George and P. Veeramani and state the question of finding a non-strong fuzzy metric space for a continuous t-norm different from the minimum. Later on, J. Gutierrez Garcia and S. Romaguera solved this question [Examples of non-strong fuzzy metrics, Fuzzy Sets and Systems 162 (2011) 91-93] by means of two examples for the product and the Lukasiewicz t-norm, respectively. In this direction they posed to find further examples of non-strong fuzzy metrics for continuous t-norm that are greater than the product but different from minimum. In this paper we obtain, among others facts on t-norms and fuzzy metric spaces, an example of this kind.

Experimental Results for Information Systems Based on Accesses Locality via Intuitionistic Fuzzy Metrics

The notion of an intuitionistic fuzzy metric space is a natural generalization of the fuzzy metric space concept which provides mechanisms to measure the degree of nearness and remoteness between two elements of a fuzzy set ac- cording to a parameter t. In this work we show how, interpreting t as a value representing the evolution time of an infor- mation system, we can use effective prediction tools in systems that show a strong locality component and where opera- tions require the coordination of actions over the set of elements. Furthermore we show how we can tune the fuzzy metric results in order to predict access histories working on variations of the fuzzy constructions. To this end we study the suitability of a set of continuous t-norms and t-conorms to build fuzzy constructions. We have evaluated the metrics suitability according to their computation time and to the sensitiveness for different representative cases.

Extending wide-area replication support with mobility and improved recovery

We survey the MADIS architecture. It supports high availability, fault tolerance and seamless error recovery in computer networks for distributed web services. MADIS is open for plugging in different protocols for replication, failure resilience and recuperation. We outline ongoing research which aims at improving fault tolerance and error recovery by a dynamic adaptation of protocols plugged into MADIS. Future development of MADIS envisages an extension with functionality supporting web-based services also for mobile users, possibly including an integration of voice and data traffic over a single internet link.

An Analytical Design of a Practical Replication Protocol for Distributed Systems

In replicated transactional systems, lazy update protocols have exhibited an undesirable behaviour, due to an increased abortion rate in scenarios with high degrees of access conflicts. In this paper, the abortion rate problem is studied from a statistical point of view. The resulting expressions describe the abortion problem, and were used to design a hybrid update database replication protocol, with performance similar to traditional lazy update protocols but with lower abortion rates. The protocol’s algorithm has been validated analytically. Once implemented, performance measurements have confirmed the predicted results.