J. R. González de Mendívil

Managing Transaction Conflicts in Middleware-based Database Replication Architectures

Database replication protocols need to detect, block or abort part of conflicting transactions. A possible solution is to check their writesets (and also their readsets in case a serialisable isolation level is requested), which however burdens the consumption of CPU time. This gets even worse when the replication support is provided by a middleware, since there is no direct DBMS support in that layer. We propose and discuss the use of the concurrency control support of the local DBMS for detecting conflicts between local transactions and writesets of remote transactions. This allows to simplify many database replication protocols and to enhance their performance

Fuzzy location and tracking on wireless networks

This paper describes user location and tracking on indoor scenarios through a wireless network. We propose a fuzzy location algorithm, using fuzzy inference systems, in order to deal with imprecise location based on radio-frequency trilateration estimations, providing high location rates near to 90. This indoor positioning approach is based on the pattern recognition of IEEE 802.11 (WiFi) signal strength measurements using fuzzy logic to deal with the vagueness and uncertainty of the trilateration based on signal strength. User location and tracking are considered in order to provide complete intelligent location based services. Fuzzy location techniques allow increasing location ratios even when the user trilateration can not be as precise as desired. Fuzzy tracking is performed by means of a fuzzy automaton.

SIPRe: a partial database replication protocol with SI replicas

Database replication has been researched as a solution to overcome the problems of performance and availability of distributed systems. Full database replication, based on group communication systems, is an attempt to enhance performance that works well for a reduced number of sites. If application locality is taken into consideration, partial replication, i.e. not all sites store the full database, also enhances scalability. On the other hand, it is needed to keep all copies consistent. If each DBMS provides SI, the execution of transactions has to be coordinated so as to obtain Generalized-SI (GSI). In this paper, a partial replication protocol providing GSI is introduced that gives a consistent view of the database, providing an adaptive replication technique and supporting the failure and recovery of replicas.

Fuzzy automata with ε-moves compute fuzzy measures between strings

This paper introduces fuzzy automata with transitions by empty string (@?-moves), and shows their relationship with other classes of classical fuzzy automata. The @?-move represents a state change of the automaton without consuming any symbol of the input string. In approximate string matching, @?-moves allow to model the effect of the insertion of a symbol (one of the possible edit operations). We provide a fuzzy measure between strings based on the concepts of string alignments and fuzzy edit operations. The main contribution of this paper is to prove that a particular class of fuzzy automata with @?-moves computes those fuzzy measures without restricting the number of errors between the strings. Given a fuzzy measure, a building method for constructing the fuzzy automaton with @?-moves that computes it, is also proposed.

A protocol for reconciling recovery and high-availability in replicated databases

We describe a recovery protocol which boosts availability, fault tolerance and performance by enabling failed network nodes to resume an active role immediately after they start recovering. The protocol is designed to work in tandem with middleware-based eager update-everywhere strategies and related group communication systems. The latter provide view synchrony, i.e., knowledge about currently reachable nodes and about the status of messages delivered by faulty and alive nodes. That enables a fast replay of missed updates which defines dynamic database recovery partition. Thus, speeding up the recovery of failed nodes which, together with the rest of the network, may seamlessly continue to process transactions even before their recovery has completed. We specify the protocol in terms of the procedures executed with every message and event of interest and outline a correctness proof.

A Lock Based Algorithm for Concurrency Control and Recovery in a Middleware Replication Software Architecture

Data replication among different sites is viewed as a way to increase application performance and its data availability. In this paper, we propose an algorithm design for concurrency control and recovery in a middleware architecture called COPLA (Common Object Programmer Library Access). This architecture provides persistent object state replication. The algorithm is based on locks, it is an adaptation of the Optimistic Two Phase Locking (O2PL) protocol to this architecture. The recovery process of this algorithm allows applications to continue (or start) executing transactions at all nodes, even in the node being recovered.

Revisiting certification-based replicated database recovery

Certification-based database replication protocols are a good means for supporting transactions with the snapshot isolation level. Such kind of replication protocol does not demand readset propagation and allows the usage of a symmetric algorithm for terminating transactions, thus eliminating the need of a final voting phase. Recovery mechanisms especially adapted for certification-based replication protocols have not been thoroughly studied in previous works. In this paper we propose two recovery techniques for this kind of replication protocols and analyze their performance. The first technique consists in dividing the recovery in two stages, reducing the certification load and the amount of information to be recovered in the second stage. The second technique scans and compacts the set of items to transfer, sending only the latest version of each item. We show that these techniques can be easily combined, reducing thus the recovery time.

A deterministic database replication protocol where multicastwritesets never get aborted

Several approaches for the full replication of data in distributed databases [1] have been studied. One of the preferred techniques is the eager update everywhere based on the total-order multicast delivery service [2], where the most outstanding varieties are: certification-based and weak-voting [1]. Under this approach, the execution flow of a transaction can be split into two different main phases: the first one, all operations are entirely executed at the delegate replica of the transaction; and followed by the second phase, started when the transaction requests its commit, all updates are collected and grouped (denoted as writeset) at the delegate replica and sent to all replicas. The commitment or abortion of a transaction is decided upon the delivery of the message. In the case of certification-based ones, each replica holds an ordered log of already committed transactions and the writeset is certified [3], against the log, to commit or abort the transaction. On the other hand, weak-voting ones atomically apply the delivered writeset at remote replicas whilst the delegate, if it is still active, reliably multicasts [2] a commit message. Thus, the certification-based presents a better behavior in terms of performance, only one message is multicast per transaction, but with higher abortion rates [1]. Recently, due to the use of DBMS providing SI, we have found several certification-based protocols to achieve, actually a weaker form called GSI [3], this isolation level in a replicated setting [3] while quite a few weak-voting ones [4].

A formal characterization of SI-based ROWA replication protocols

Snapshot isolation (SI) is commonly used in some commercial DBMSs with a multiversion concurrency control mechanism since it never blocks read-only transactions. Recent database replication protocols have been designed using SI replicas where transactions are firstly executed in a delegate replica and their updates (if any) are propagated to the rest of the replicas at commit time; i.e. they follow the Read One Write All (ROWA) approach. This paper provides a formalization that shows the correctness of abstract protocols which cover these replication proposals. These abstract protocols differ in the properties demanded for achieving a global SI level and those needed for its generalized SI (GSI) variant - allowing reads from old snapshots. Additionally, we propose two more relaxed properties that also ensure a global GSI level. Thus, some applications can further optimize their performance in a replicated system while obtaining GSI.

A closer look at database replication middleware architectures for enterprise applications

Middleware-supported database replication is away to increase performance and tolerate failures of enterprise applications. Middleware architectures distinguish themselves by their performance, scalability and their application interface, on one hand, and the degree to which they guarantee replication consistency, on the other. Both groups of features may conflict since the latter comes with an overhead that bears on the former. We review different techniques proposed to achieve and measure improvements of the performance, scalability and overhead introduced by different degrees of data consistency. We do so with a particular emphasis on the requirements of enterprise applications.