Implementing database replication protocols based on O2PL in a middleware architecture

Database replication is a way to increase system performance and fault-tolerance of a given system. The price to pay is the effort needed to guarantee data consistency, and this is not an easy task. In this paper, we introduce a description of two 1-Copy-Serializable (1CS) [1] eager update everywhere replication protocols. The preliminary results of their implementation in the MADIS middleware architecture [2] are also presented. The advantage of these replication protocols is that they do not need to re-implement features that are provided by the underlying database. The first one does not rely on strong group communication primitives [3]; distributed deadlock is avoided by a deadlock prevention schema based on transaction priorities (whose information is totally local at each node). The second one manages replica consistency by the total order message delivery featured by Group Communication Systems (GCSs) [3]

J. R. González de Mendívil et al.: Non-blocking ROWA Protocols Implement GSI Using SI Replicas TR-ITI-ITE-07/10 Non-blocking ROWA Protocols Implement GSI Using SI Replicas

The concept of Generalized Snapshot Isolation (GSI) has been recently proposed as a suitable extension of conventional Snapshot Isolation (SI) for replicated databases. In GSI, transactions may use older snapshots instead of the latest snapshot required in SI, being able to provide better performance without significantly increasing the abortion rate when write/write conflicts among transactions are low. Its authors also state that GSI is needed because there is no non-blocking implementation of SI in asynchronous systems, even when databases never fail, but they do not prove such statement. We justify such property for ROWA (Read One, Write All) protocols in this paper by using the equivalence between SI-schedules. Additionally, we show and prove that if a replication protocol using SI replicas provides global atomicity and commits update transactions in the same order at all sites then it provides GSI. This sufficient condition prevents the usage of some mechanisms that exclusively order write/write conflicting transactions because they do not guarantee GSI.

A middleware database replication protocol providing different isolation levels

Database replication protocols have been usually designed in order to support a single isolation level. This paper proposes a middleware protocol able to manage three different isolation levels over multi-version DBMSs: GSI, SI, and serializable. This ensures a better support for applications that demand different isolation levels for their transactions. Additionally, this protocol is also able to merge the coordination of the replicas for each isolation level, using a weak voting approach for all of them, whilst other recent protocols need a certifying technique for GSI and SI, or a 2PC rule for serializable level

J. R. Juárez-Rodríguez et al.: A Deterministic Database Replication Protocol Where Multicast Writesets Never Got Aborted TR-ITI-ITE-07/15 A Deterministic Database Replication Protocol Where Multicast Writesets Never Got Aborted

Database replication protocols based on a certification approach are usually the best ones for achieving good performance when an eager update everywhere technique is being considered. The weak voting approach achieves a slightly longer transaction completion time, but with a lower abortion rate. So, both techniques can be considered as the best ones for eager replication when performance is a must, and both of them need atomic broadcast. We propose a new database replication strategy that shares many characteristics with such previous strategies. It is also based on totally ordering the application of writesets, using only an unordered reliable broadcast, instead of an atomic broadcast. Additionally, the writesets of transactions that are aborted in the final validation phase need not be broadcast in our strategy. Thus, this new approach always reduces the communication traffic and also achieves a good transaction response time (even shorter than those previous strategies in some system configurations).

Proof and Evaluation of a 1CS Middleware Data Replication Protocol Based on O2PL ⋆

Abstract. Middleware data replication techniques are a way to increase performance and fault tolerance without modifying the internals of a DBMS. However, they introduce overheads that may lead to poor response times. In this paper a modification of the O2PL protocol is introduced. It orders conflicting transactions by using their priority, instead of the total order obtained by an atomic multicast. Priorities are also used to avoid deadlocks. For improving its performance, it does not use the strict 2PC rule as O2PL does. We provide a formal correctness proof of its 1-Copy Serializability (1CS). This protocol has been implemented, and a comparison with other already implemented protocols is also given.

Surprising migration and population size dynamics in ancient Iberian brown bears (Ursus arctos)

The endangered brown bear populations (Ursus arctos) in Iberia have been suggested to be the last fragments of the brown bear population that served as recolonization stock for large parts of Europe during the Pleistocene. Conservation efforts are intense, and results are closely monitored. However, the efforts are based on the assumption that the Iberian bears are a unique unit that has evolved locally for an extended period. We have sequenced mitochondrial DNA (mtDNA) from ancient Iberian bear remains and analyzed them as a serial dataset, monitoring changes in diversity and occurrence of European haplogroups over time. Using these data, we show that the Iberian bear population has experienced a dynamic, recent evolutionary history. Not only has the population undergone mitochondrial gene flow from other European brown bears, but the effective population size also has fluctuated substantially. We conclude that the Iberian bear population has been a fluid evolutionary unit, developed by gene flow from other populations and population bottlenecks, far from being in genetic equilibrium or isolated from other brown bear populations. Thus, the current situation is highly unusual and the population may in fact be isolated for the first time in its history

Estimating the population size of the endangered Cantabrian brown bear through genetic sampling

Th e Cantabrian brown bear Ursus arctos population can be seen as a paradigm in conservation biology due to its endangerment status and genetic uniqueness. Th erefore, the need to obtain basic demographic data to inform management actions for conservation is imperative. Despite this, empirical data on the size and trends of the Cantabrian bear population are scarce. Here we present the fi rst estimates of population size (N c ) and eff ective population size (N e ) of the whole Cantabrian brown bear population. We genotyped 270 non-invasive samples collected during 2006 throughout the entire range of the population and subsequently identifi ed 130 individuals. Diff erent model estimators of N c based on capture – mark – recapture (CMR) procedures were compared. Th e average for the best three models (Mh Chao, Mh Darroch and CAPWIRE TIRM) yielded a total estimate of N c 223 individuals (CI 95% 183 – 278) and N e 50 (CI 95% 36 – 75) providing an N e / N c ratio of 0.22. Estimates for the two subpopulations commonly recognized in the Cantabrian range were N c 203 (CI 95% 168 – 260) and N e 47 (CI 95% 36 – 70) for the western subpopulation and N c 19 (CI 95% 12 – 40) and N e 9 (CI 95% 8 – 12) for the eastern subpopulation. Th ese data suggest that the Cantabrian brown bear population has increased recently, mainly in the western subpopulation, after a long period of decline and isolation which lead to the split of the population at the beginning of the 20th century. Population sizes in the early 1990s were thought to be only 60 individuals for the western subpopulation and 14 individuals in the eastern one. Th e eff orts to improve conservation policies made since then have probably contributed, to some extent, to the population increase during the last couple of decades.Peer reviewe