José Ramón González de Mendívil

Brief announcement: virtual world consistency: a new condition for STM systems

This BA presents a general consistency condition for software transactionnal memories.

Fuzzy languages with infinite range accepted by fuzzy automata: Pumping Lemma and determinization procedure

Abstract The formulation of fuzzy automata allows us to select a great variety of triangular norms. Depending on the selected triangular norm, a fuzzy automaton can accept a fuzzy language (FA-language) with infinite range. These fuzzy automata are not equivalent to the so-called deterministic fuzzy automata (deterministic automata with a fuzzy subset of final states) which only accept fuzzy languages with finite range. In this paper, we study FA-languages with infinite range and a determinization procedure in order to obtain an equivalent fuzzy deterministic automaton for a given fuzzy automaton. A fuzzy deterministic automaton is a fuzzy automaton which satisfies the deterministic condition in its state transition function. The main contributions of our paper are: (1) a Pumping Lemma of FA-languages with infinite range; (2) the formulation of fuzzy deterministic automata and a Pumping Lemma of FDA-languages; (3) the necessary conditions for the determinization of fuzzy automata under continuous triangular norms which accept fuzzy languages of infinite range; and (4) a determinization algorithm for fuzzy automata, its correctness proof and performance.

Extending Middleware Protocols for Database Replication with Integrity Support

Current middleware database replication protocols take care of read-write conflict evaluation. If there are no such conflicts, protocols sanction transactions to commit. Other conflicts may arise due to integrity violation. So, if, in addition to the consistency of transactions and replicas, also the consistency of integrity constraints is to be supported, some more care must be taken. Some classes of replication protocols are able to seamlessly deal with the integrity support of the underlying DBMS, but others are not. In this paper, we analyze the support for integrity that can be provided in various classes of replication protocols. Also, we propose extensions for those that cannot directly manage certain kinds of constraints that are usually supported in DBMSs.

Determinization of fuzzy automata via factorization of fuzzy states

Fuzzy finite-state automata over the algebra ([0,1],max,@?,0,1), in which the monoid ([0,1],@?,1) (@? denotes a continuous triangular norm) is not locally finite, can accept fuzzy languages of infinite range. For a given fuzzy finite-state automaton which accepts a fuzzy language of infinite range, we define the determinization of the fuzzy automaton via factorization of fuzzy states, i.e., the computation of an equivalent deterministic fuzzy automaton whether it is finite. This method of determinization is a generalization of the well-known accessible subset construction. Our main contribution is to determine that the representable-cycles property is the necessary and sufficient condition for determinization of a fuzzy finite-state automaton via a maximal factorization of fuzzy states. This property is more general than the twins property (adapted for fuzzy automata) which is the sufficient condition for weighted automata over the tropical semiring.

On the Cost of Database Clusters Reconfiguration

Database clusters based on share-nothing replication techniques are currently widely accepted as a practical solution to scalability and availability of the data tier. A key issue when planning such systems is the ability to meet service level agreements when load spikes occur or cluster nodes fail. This translates into the ability to provision and deploy additional nodes. Many current research efforts focus on designing autonomic controllers to perform such reconfiguration, tuned to quickly react to system changes and spawn new replicas based on resource usage and performance measurements. In contrast, we are concerned about the inherent impact of deploying an additional node to an online cluster, considering both the time required to finish such an action as well as the impact on resource usage and performance of the cluster as a whole. If noticeable, such impact hinders the practicability of self-management techniques, since it adds an additional dimension that has to be accounted for. Our approach is to systematically benchmark a number of different reconfiguration scenarios to assess the cost of bringing a new replica online. We consider factors such as: workload characteristics, incremental and parallel recovery, flow control and outdatedness of the recovering replica. As a result, we show that research should be refocused from optimizing the capture and transmition of changes to applying them, which in a realistic setting dominates the cost of the recovery operation.

A formal analysis of database replication protocols with SI replicas and crash failures

This paper provides a formal specification and proof of correctness of a basic Generalized Snapshot Isolation certification-based data replication protocol for database middleware architectures. It has been modeled using a state transition system, as well as the main system components, allowing a perfect match with the usual deployment in a middleware system. The proof encompasses both safety and liveness properties, as it is commonly done for a distributed algorithm. Furthermore, a crash failure model has been assumed for the correctness proof, although recovery analysis is not the aim of this paper. This allows an easy extension toward a crash-recovery model support in future works. The liveness proof focuses in the uniform commit: if a site has committed a transaction, the rest of sites will either commit it or it would have crashed.

Correctness proof of a database replication protocol under the perspective of the I/O automaton model

Correctness of recent database replication protocols has been justified in a rather informal way focusing only in safety properties and without using any rigorous formalism. Since a database replication protocol must ensure some degree of replica consistency and that transactions follow a given isolation level, previous proofs only focused in these two issues. This paper proposes a formalization using the I/O automaton model, identifying several components in the distributed system that are involved in the replication support (replication protocol, group communication system, database replicas) and specifying clearly their actions in the global replicated system architecture. Then, a general certification-based replication protocol guaranteeing the snapshot isolation level is proven correct. To this end, different safety and liveness properties are identified, checked and proved. Our work shows that some details of the replication protocols that were ignored in previous correctness justifications are indeed needed in order to guarantee our proposed correctness criteria.

Integrity Dangers in Certification-Based Replication Protocols

Database replication protocols check read-write and/or write-write conflicts. If there are none, protocols propagate transactions to the database, assuming they will eventually commit. But commitment may fail due to integrity constraints violations. Also, the read actions of integrity checking may give raise to new conflicts. Thus, some more care must be taken if, in addition to the consistency of transactions and replicas, also the consistency of integrity constraints is to be maintained. In this paper, we investigate how certification-based replication protocols can be adapted to correctly and transparently deal with the built-in integrity support provided by the underlying DBMS. Also, we experimentally demonstrate the negative effects that an incorrect management of integrity constraints causes in a database replication distributed system.

A comment on Construction of fuzzy automata from fuzzy regular expressions

The aim of this comment is to point out that the fuzzy language recognized by a nondeterministic finite automaton, which is associated with a fuzzy regular expression, in the context of Stamenkovic-?irics method (Stamenkovic and ?iric (2012) 11]), is recognized by a fuzzy finite automaton with e-moves. Every fuzzy automaton with e-moves is also equivalent to a fuzzy automaton and there are effective methods for removing e-moves in order to obtain the equivalent fuzzy automaton. In this way, our proposal to convert a fuzzy regular expression to a fuzzy finite automaton is based on the construction of a fuzzy automaton with e-moves for the fuzzy regular expression and the construction of an equivalent fuzzy finite automaton by means of e-removal operation.

Revising 1-Copy Equivalence in Replicated Databases with Snapshot Isolation

Multiple database replication protocols have used replicas supporting the snapshot isolation level. They have provided some kind of one-copy equivalence, but such concept was initially conceived for serializable databases. In the snapshot isolation case, due to its reliance on multi-versioned concurrency control that never blocks read accesses, such one-copy equivalence admits two different variants. The first one consists in relying on sequential replica consistency, but it does not guarantee that the snapshot used by each transaction holds the updates of the last committed transactions in the whole replicated system, but only those of the last locally committed transaction. Thus, a single user might see inconsistent results when two of her transactions have been served by different delegate replicas: the updates of the first one might not be in the snapshot of the second. The second variant avoids such problem, but demands atomic replica consistency, blocking the start (i.e., in many cases, read accesses) of new transactions. Several protocols of each kind exist nowadays, and most of them have given different names to their intended correctness criterion. We survey such previous works and propose uniform names to these criteria, justifying some of their properties.