Desde la aparición de las primeras bases de datos distribuidas hasta los actuales sistemas de replicación modernos, la comunidad de investigación ha propuesto múltiples protocolos para administrar la distribución y replicación de datos, junto con algoritmos de control de concurrencia para manejar las transacciones en ejecución en todos los nodos del sistema. Muchos protocolos están disponibles, por tanto, cada uno con diferentes características y rendimiento, y garantizando diferentes niveles de coherencia. Para saber qué protocolo de replicación es el más adecuado, dos aspectos deben ser considerados: el nivel necesario de coherencia y aislamiento (es decir, el criterio de corrección), y las propiedades del sistema (es decir, el escenario), que determinará el rendimiento alcanzable. Con relación a los criterios de corrección, la serialización de una copia es ampliamente aceptada como el más alto nivel de corrección. Sin embargo, su definición permite diferentes interpretaciones en cuanto a la coherencia de réplicas. En esta tesis se establece una correspondencia entre los modelos de coherencia de memoria, tal como se definen en el ámbito de la memoria compartida distribuida, y los posibles niveles de coherencia de réplicas, definiendo así nuevos criterios de corrección que corresponden a las diferentes interpretaciones identificadas sobre la serialización de una copia. Una vez seleccionado el criterio de corrección, el rendimiento alcanzable por un sistema depende en gran medida del escenario, es decir, de la suma del entorno del sistema y de las aplicaciones que se ejecutan en él. Para que el administrador pueda seleccionar un protocolo de replicación apropiado, los protocolos disponibles deben conocerse plena y profundamente. Una buena descripción de cada candidato es fundamental, pero un marco común es imperativo para comparar las diferentes opciones y estimar su rendimiento en un escenario dado. Los resultados presentados en esta tesis cumplen los objetivos establecidos y constituyen una contribución al estado del arte de la replicación de bases de datos en el momento en que se iniciaron los trabajos respectivos. Estos resultados son relevantes, además, porque abren la puerta a posibles contribuciones futuras. / Ruiz Fuertes, MI. (2011). On the Consistency, Characterization, Adaptability and Integrity of Database Replication Systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/11800 / Palancia

Introduction and objectives In Vietnam, the forest resources have been declining and degrading severely in recent years. The degradation has decreased the natural forest area, changed the forest structure seriously and reduced timber volume and biodiversity. From 1999 to 2005, the rich forest area has decreased 10.2%, whereas the poor secondary forest has increased dramatically by 20.7%. Forest structure plays an important role in forestry research. Understanding forest structure will unlock an understanding of the history, function and future of a forest ecosystem (Spies, 1998). The forest structure is an excellent basis for restoration measures. Therefore, this research is necessary to contribute to improving forest area and quality, reducing difficulties in forest management. The study also enhances the grasp of forest structure, structure changes after harvesting and fills serious gaps in knowledge. In addition, the research results will contribute to improving and rescuing the poor secondary forest and restoring it, approaching the old-growth forest in Vietnam. Material and methods The study was conducted in Kon Ka Kinh national park. The park is located in the Northeastern region of Gia Lai province, 50 km from Pleiku city center to the Northeast. The park is distributed over seven different communes in three districts: K’Bang, Mang Yang and Đăk Đoa. Data were collected from 10 plots of secondary forests (Type IIb) and 10 plots of primeval forests (Type IV). Stratified random sampling was applied to select plot locations. 1 ha plots were used to investigate gaps. 2000 m2 plots were used to measure overstorey trees such as diameter at breast height, total height, crown width and species names. 500 m2 subplots were used to record tree positions. For regeneration, 25 systematic 4 m2 subplots were established inside 1 ha plots. After data were collected in the field, data analyses were conducted by using R and Excel. Firstly, some stand information, such as density, volume and so on, was calculated, and then descriptive statistics were computed for diameter and height variables. Linear mixed effect models were applied to analyze the difference of diameter and height and to check the effect of random factor between the two forest types. Diameter and height frequency distributions were also generated and compared by using permutational analysis of variance (PERMANOVA). Non-linear regression models were analyzed for diameter and height variables. Similar analyses were implemented for gaps. Regarding spatial point patterns of overstorey trees, replicated point pattern analysis techniques were applied in this research. For biodiversity, some calculations were run such as richness and biodiversity indices, comparison of biodiversity indices by using linear mixed models and biodiversity differences between two forest types tested again by permutational analysis of variance. In terms of regeneration, some analyses were implemented such as: height frequency distribution generation, frequency difference testing, biodiversity indices for the regeneration and spatial distribution checking by using a nonrandomness index. Results and discussion After analyzing the data, some essential findings were obtained as follows: Hypothesis H1 “The overstorey structure of secondary forests is more homogeneous and uniform than old-growth forests” is accepted. In other words, the secondary forest density is about 1.8 times higher than the jungle. However, the volume is only 0.56 times as large. The average diameter and height of the secondary forest is smaller by 5.71 cm and 3.73 m than the old-growth forest, respectively. Linear mixed effect model results indicate that this difference is statistically different and the effect of the random factor (Section) is not important. Type IIb has many small trees and the diameter frequency distribution is quite homogeneous. The old-growth forest has more big trees. For both forest stages, the height frequency distribution is positively skewed. PERMANOVA results illustrate that the frequency distribution is statistically different between the two forest types. Regression functions are also more variant and diverse in the old-growth forest, because all standard deviations of the parameters are greater there. Gap analysis results indicate that the number of gaps in the young forest is slightly higher, while the average gap size is much smaller. The gap frequency distribution is statistically different between the two types. In terms of the spatial point pattern of overlayer trees, the G-test and the pair correlation function results show that trees distribute randomly in the secondary forest. In contrast, the spatial point patterns of trees are more regular and diverse in the old-growth forest. The spatial point pattern difference is not significant, and this is proved by a permutational t-test for pair correlation function (pcf). Envelope function results indicate that the variation of pcf in young forests is much lower than in the primary forests. Hypothesis H2 “The overstorey species biodiversity of the secondary forest is less than in the old-growth forest” is rejected. Results show that the number of species of the secondary forest is much greater than in the old-growth forest, especially richness. The richness of the secondary forest is 1.16 times higher. The Simpson and Shannon indices are slightly smaller in the secondary forest. The average Simpson index for both forest stages is 0.898 and 0.920, respectively. However, the difference is not significant. Species accumulation curves become relatively flatter on the right, meaning a reasonable number of plots have been observed. Estimated number of species from accumulation curves in two forest types are 105 and 95/ha. PERMANOVA results show that number of species and proportion of individuals in each species are significantly different between forest types. Hypothesis H3 “The number regenerating species of the secondary forest is less and they distribute more regularly, compared to the old-growth forest” is rejected. There are both similarities and differences between the two types. The regeneration density of the stage IIb is 22,930 seedlings/ha, greater than the old forest by 9,030 seedlings. The height frequency distribution shows a decreasing trend. Similar to overstorey, the richness of the secondary forest is 141 species, higher than the old-growth forest by 9 species. Biodiversity indices are not statistically different between two types. PERMANOVA results indicate that the number of species and the proportion of individuals for each species are also not significantly different from observed forest types. Nonrandomness index results show that the regeneration distributes regularly. Up to 95% of the plots reflect this distribution trend. Hypothesis H4 “Restoration measures (with and without human intervention) could be implemented in the regenerating forest” is accepted. The investigated results show that the secondary forest still has mother trees, and it has enough seedlings to restore. Therefore, restoration solutions with and without human intervention can be implemented. Firstly, forest protection should be applied. This measure is relevant to national park regulations in Vietnam. Rangers and other related organizations will be responsible for carrying out protection activities. These activities will protect forest resources from illegal logging, grazing and tourist activities. Environmental education and awareness-raising activities for indigenous people is also important. Another measure is additional and enrichment planting. It should focus on exclusive species of the overstorey in Type IIb or exclusive species of the primary forest. Selection of these species will lead to species biodiversity increase in the future. This also meets the purpose of the maximum biodiversity solution. Conclusion Forest resources play a very important role in human life as well as maintaining the sustainability of ecosystems. However, at present, they are under serious threat, particularly in Vietnam. Central Highland, Vietnam, where forest resources are still relatively good, is also threatened by illegal logging, lack of knowledge of people and so on. Therefore, it needs the hands of the people, especially foresters and researchers. Through research, scientists can provide the knowledge and understanding of the forest, including the structure and forest restoration. This study has obtained important findings. The secondary forest is more homogeneous and uniform, while the old-growth forest is very diverse. Biodiversity of the overstorey in the secondary forest is more than the primary. The number of regenerating species in the secondary forest is higher, but other indices are not statistically different between two types. The regeneration distribute regularly on the ground. The secondary forest still has mother trees and sufficient regeneration, so some restoration measures can be applied here. Findings of the study contribute to improve people’s understanding of the structure and the structural changes after harvesting in Kon Ka Kinh national park, Gia Lai. That is a key to have better understandings of the history and values of the forests. These findings and the proposed restoration measures address rescuing degraded forests in Central Highland in particular and Vietnam in general. And further, this is a promising basis for the management and sustainable use of forest resources in the future.:TABLE OF CONTENTS ACKNOWLEDGEMENTS I TABLE OF CONTENTS III LIST OF FIGURES VIII LIST OF TABLES XI LIST OF ABBREVIATIONS XII SUMMARY XIII CHAPTER I: INTRODUCTION 1 1.1. The decline of natural forest resources, orientation of difficulty and development in Vietnam 1 1.1.1. Decline of forest resources 1 1.1.2. Difficulties in forestry management 1 1.1.3. Management strategies 2 1.2. Forest structure role 3 1.3. Forest restoration in Vietnam 4 1.4. Importance of old-growth and secondary forests 4 1.5. Aims, scope and hypotheses 6 1.5.1. Aims 6 1.5.1.1. General objective 6 1.5.1.2. Specific objective 6 1.5.2. Scope 6 1.5.3. Hypotheses 6 CHAPTER II: LITERATURE REVIEW 8 2.1. Tropical forest structure analysis 8 2.1.1. History 8 2.1.1.1. Overstorey 8 2.1.1.2. Regeneration 12 2.1.2. Structural attributes of tropical forests 13 2.1.2.1. Overstorey 14 a. Analyzed attributes 14 b. Relevant attributes to this study 15 2.1.2.2. Regeneration 21 2.2. Secondary tropical forest restoration 22 2.2.1. Strategies for secondary forest restoration 23 2.2.1.1. Protection and natural recovery 24 2.2.1.2. Natural regeneration management 24 a. Growing conditions and yield of desirable regeneration improvement 24 b. Desirable regeneration assistance 25 2.2.1.3. Accelerated Natural Regeneration (ANR) 25 2.2.1.4. Enrichment planting 25 2.2.1.5. The framework species method 26 2.2.1.6. Maximum diversity planting method 26 CHAPTER III: MATERIAL 27 3.1. Natural conditions of the study area 27 3.1.1 Geographic location, boundaries and area of Kon Ka Kinh national park 27 3.1.2. Topography, geology and soil 28 3.1.2.1. Topography 28 3.1.2.2. Geology and soil 29 3.1.3. Climate and hydrology 30 3.1.3.1. Climate 30 3.1.3.2. Hydrology 31 3.2. Vegetation in Kon Ka Kinh national park 31 3.2.1. The area of land use types 31 3.2.2. Plant biodiversity 33 3.2.3. The flora and forest vegetation 33 3.2.3.1. Flora 33 3.2.3.2. Forest vegetation 34 3.2.3.3. History of forest exploitation in the park 35 3.3. Assessing the natural conditions and vegetation of the park 37 3.4. Population, ethnicity and labor 38 3.4.1. Population 38 3.4.2. Labor and ethnicity 39 3.4.3. Poverty status 40 3.5. Forest resources classification 40 3.5.1. The Loeschau’s classification system 40 3.5.2. The relationship between forest types with development phases 42 CHAPTER IV: METHODOLOGY 45 4.1. Plot establishment method 45 4.2. Data collection method 47 4.2.1. Data collection for overstorey stem maps 47 4.2.1.1. Tree data collection 47 4.2.1.2. Tree positions 50 4.2.1.3. Gap inventory 51 4.2.2. Data collection for regeneration 52 4.3. Data analysis method 55 4.3.1. Applied methods for the upper layer 55 4.3.1.1. Stand information 55 a. Calculation for each tree 55 b. Calculation for a stand 55 4.3.1.2. Descriptive statistics for height and diameter variables 56 a. Central tendency 56 b. Dispersion and variability 56 c. Measures of distribution shape 57 4.3.1.3. Linear mixed-effects analysis 59 a. Applications with this study and data arrangement 60 b. Homoscedasticity checking 61 c. Checking autocorrelation 63 d. Checking normal distribution of the residuals 66 e. Model selection and information summary 67 4.3.1.4. Frequency distribution 68 a. Generating frequency distributions 68 b. Frequency distribution difference testing 69 4.3.1.5. Diameter-height regression analysis 70 a. Used function forms 70 b. Theoretical calculations 71 c. Model selection 73 4.3.1.6. Gap analysis 74 a. Descriptive statistics for gaps 74 b. Calculating the gap area proportion for each forest type 74 c. Gap size frequency distribution 74 d. Gap size frequency distribution difference testing 75 4.3.1.7. Spatial point patterns of tree species 75 a. Applications 76 b. Tree density analysis 77 c. Testing for randomness 78 d. Comparing point pattern variation 83 e. Testing the difference between forest types 84 4.3.1.8. Overstorey tree species diversity analysis 85 a. Richness and species importance value index (SIVI) 85 b. Species diversity index 86 c. Species accumulation curve 88 d. Biodiversity index comparison 88 e. Tree species diversity comparison 89 4.3.2. Regenerating tree storey structure analysis 90 4.3.2.1. Frequency distribution of regeneration 90 4.3.2.2. Height frequency distribution difference testing 91 4.3.2.3. Biodiversity indices for regeneration 91 4.3.2.4. Biodiversity index comparison by using LMM 91 4.3.2.5. Regeneration species diversity comparison 91 4.3.2.6. Regeneration spatial distribution checking 91 a. Nonrandomness index 91 b. Nonrandomness index value comparison 92 CHAPTER V: RESULTS 93 5.1. Overstorey structure analysis results 93 5.1.1. Stand information 93 5.1.2. Descriptive statistics results 95 5.1.3. Linear mixed effect model results 97 5.1.3.1. Box plots for the diameter and height variables 97 5.1.3.2. Model analysis and adaptation 97 5.1.3.3. Model parameter estimation 100 5.1.4. Frequency distributions 101 5.1.4.1. Frequency distribution results for both types 101 5.1.4.2. Frequency distribution difference 107 5.1.5. Diameter-height regression results 107 5.1.5.1. Estimated parameters 107 5.1.5.2. Model selection 110 5.1.5.3. Regression charts 110 5.1.6. Gap analysis 116 5.1.6.1. Gap descriptive information 116 5.1.6.2. Gap area ratio 117 5.1.6.3. Gap size frequency distribution 117 5.1.6.4. Gap size frequency distribution difference testing results 120 5.1.7. Spatial distribution analysis 120 5.1.7.1. Density testing results 120 5.1.7.2. Randomness checking results 122 5.1.7.3. Variation difference between two types 123 5.1.7.4. Point pattern difference testing between two types 124 5.1.8. Overstorey species diversity analysis results 125 5.1.8.1. Richness, SIVI and biodiversity indices 125 5.1.8.2. Biodiversity index comparison by using LMM 127 5.1.8.3. Tree species diversity comparison 127 5.2. Regeneration storey structure analysis results 128 5.2.1. Height frequency distribution 128 5.2.2. Height frequency distribution difference testing 130 5.2.3. Biodiversity index for regeneration 131 5.2.4. Biodiversity index difference comparison 133 5.2.5. Regeneration species diversity comparison 133 5.2.6. Regeneration spatial distribution 134 5.2.6.1. Nonrandomness index results 134 5.2.6.2. Nonrandomness index value testing results 134 CHAPTER VI: DISCUSSION 135 6.1. Overstorey structure differentiation 135 6.1.1. Structure and spatial distribution difference 135 6.1.1.1. Stand information 135 6.1.1.2. Statistical descriptions for diameter and height 136 6.1.1.3. Diameter and height growth difference testing by linear mixed effect models 137 6.1.1.4. Frequency distribution dissimilarity 138 6.1.1.5. Diameter-height regression 139 6.1.1.6. Canopy gaps 140 6.1.1.7. Spatial distribution patterns 141 6.1.2. Biodiversity distinction of overstorey trees 143 6.2. Regeneration dissimilarity 145 6.2.1. Density and frequency distribution 145 6.2.2. Biodiversity indices 146 6.2.3. Spatial distribution of regeneration 147 6.3. Proposing restoration measures 147 6.4. Improved points in this research 150 CHAPTER VII: CONCLUSION AND RECOMMENDATION 152 7.1. Conclusion 152 7.2. Suggestions for further research 154 REFERENCES 156 APPENDIX 180

Some Epidemic models with fractional derivatives were proved to be well-defined, well-posed and more accurate [34, 51, 116], compared to models with the conventional derivative. An Ebola epidemic model with non-linear transmission is fully analyzed. The model is expressed with the conventional time derivative with a new parameter included, which happens to be fractional (that derivative is called the 􀀀derivative). We proved that the model is well-de ned and well-posed. Moreover, conditions for boundedness and dissipativity of the trajectories are established. Exploiting the generalized Routh-Hurwitz Criteria, existence and stability analysis of equilibrium points for the Ebola model are performed to show that they are strongly dependent on the non-linear transmission. In particular, conditions for existence and stability of a unique endemic equilibrium to the Ebola system are given. Numerical simulations are provided for particular expressions of the non-linear transmission, with model's parameters taking di erent values. The resulting simulations are in concordance with the usual threshold behavior. The results obtained here may be signi cant for the ght and prevention against Ebola haemorrhagic fever that has so far exterminated hundreds of families and is still a ecting many people in West-Africa and other parts of the world. The full comprehension and handling of the phenomenon of shattering, sometime happening during the process of polymer chain degradation [129, 142], remains unsolved when using the traditional evolution equations describing the degradation. This traditional model has been proved to be very hard to handle as it involves evolution of two intertwined quantities. Moreover, the explicit form of its solution is, in general, impossible to obtain. We explore the possibility of generalizing evolution equation modeling the polymer chain degradation and analyze the model with the conventional time derivative with a new parameter. We consider the general case where the breakup rate depends on the size of the chain breaking up. In the process, the alternative version of Sumudu integral transform is used to provide an explicit form of the general solution representing the evolution of polymer sizes distribution. In particular, we show that this evolution exhibits existence of complex periodic properties due to the presence of cosine and sine functions governing the solutions. Numerical simulations are performed for some particular cases and prove that the system describing the polymer chain degradation contains complex and simple harmonic poles whose e ects are given by these functions or a combination of them. This result may be crucial in the ongoing research to better handle and explain the phenomenon of shattering. Lastly, it has become a conjecture that power series like Mittag-Le er functions and their variants naturally govern solutions to most of generalized fractional evolution models such as kinetic, di usion or relaxation equations. The question is to say whether or not this is always true! Whence, three generalized evolution equations with an additional fractional parameter are solved analytically with conventional techniques. These are processes related to stationary state system, relaxation and di usion. In the analysis, we exploit the Sumudu transform to show that investigation on the stationary state system leads to results of invariability. However, unlike other models, the generalized di usion and relaxation models are proven not to be governed by Mittag-Le er functions or any of their variants, but rather by a parameterized exponential function, new in the literature, more accurate and easier to handle. Graphical representations are performed and also show how that parameter, called ; can be used to control the stationarity of such generalized models. / Mathematical Sciences / Ph. D. (Applied Mathematics)

Les éditeurs collaboratifs permettent à des utilisateurs éloignés de collaborer à une tâche commune qui va de l'utilisation d'un agenda partagé à la réalisation de logiciels. Ce concept est né avec SCCS en1972 et connait un engouement récent (ex: Wikipedia). L'absence de centralisation et l'asynchronisme sont des aspects essentiels de cette approche qui relève d'un modèle pair-à-pair (P2P).  D'un autre côté,le format XML est devenu une référence pour la manipulation et l'échange de documents. Notre travail vise à la réalisation d'un éditeur collaboratif P2P pour l'édition de documents semi-structurés qui sont une abstraction du format XML.  Le problème est difficile et de nombreuses propositions se sont révélées erronées ou ne passant pas à l'échelle. Nous rappelons les concepts et l'état de l'art sur l'édition collaborative, les modèles centralisés et le P2P.  Ensuite, nous explorons deux approches différentes : les transformées opérationnelles et le CRDT (Commutative Replicated Data Type) avec différentes structures de données arborescentes. L'objectif est de réaliser les opérations de base (ajout, suppression et ré-étiquetage) tout en garantissant la convergence du processus d'édition. Nous proposons un algorithme générique pour l'approche CRDT basée sur une notion d'indépendance dans la structure de données.  Nous avons étendu nos travaux afin de réaliser l'opération de déplacement d'un sous-arbre et de prendre en compte le typage XML. Peu de travaux abordent ces deux points qui sont très utiles pour l'édition de documents. Finalement, nous donnons les résultats expérimentaux obtenus avec un prototype permettant de valider notre approche. / Collaborative editors allow different users to work together on a common task. Such tasks range from using a shared calendar to realizing software programmed by users located at distant sites. This concept was invented in 1972 with SCCS. In the last years, this paradigm became popular (ex. Wikipedia). Decentralization and asynchronicity are essential in this approach, leading to peer-to-peer (P2P) models. Meanwhile, the XML format has arrived as the de facto standard for editing and exchanging documents. Our work aims at defining a collaborative editor for semi-structured documents, which provide an abstraction of the XML format. The problem is difficult since many previous approaches are flawed or not scalable. Firstly,we describe the basic concepts on collaborative edition and network models and we give the state of the art of this topic. Then, we investigate two different approaches : the operational transformation(OT) approach and the Commutative Replicated Data Type (CRDT) approach for different (tree-like) data structures. Our goal is to ensure the convergence of the editing process with the basic operations (Add, Deland rename a node. We have proposed a new generic algorithm based on semantic independence in data structure for CRDT approach. We have extended our results by dealing with the operation that moves a subtree and with XML schema compliance. Few works have been devoted to these extensions which are useful in collaborative edition. Finally, we provide experimental results obtained from our implementation that validate our approach.

[pt] Esta dissertação descreve a aplicação de Modelos Lineares Generalizados (MLGs) à análise de um experimento visando identificar a combinação dos níveis das variáveis independentes: concentração de hidróxido de sódio (A), volume de hipoclorito de sódio (B) e sua interação (AB), que minimiza a variável resposta: proporção de itens com defeitos, em um processo de beneficiamento numa indústria têxtil de pequeno porte. A variável resposta encontra-se na forma de proporção, violando os pressupostos básicos do Modelo Linear Clássico e com isso as estimativas dos coeficientes pelo método de Mínimos Quadrados Ordinários (MQO) é menos confiável. O planejamento utilizado foi o fatorial completo 22 com ponto central e replicado. Após o planejamento, a modelagem pelo MLG é aplicada, só então é possível identificar uma subdispersão dos dados, verificar que o modelo empregado está correto e que o volume de hipoclorito de sódio (B) é o único fator significativo, no processo de alvejamento industrial da empresa. Portanto, como a finalidade é minimizar a resposta, utiliza-se o nível inferior (-1) desta variável. Consequentemente, como o intuito é reduzir os custos com insumos químicos pode-se utilizar o nível mínimo da concentração de hidróxido de sódio (A) e o nível máximo da interação entre os fatores (AB), já que eles não são significativos ao modelo. / [en] This dissertation describes the application of Generalized Linear Models (GLMs) to the analysis of an experiment with the purpose identify the levels combination of independent variables: concentration of sodium hydroxide (A) volume of sodium hypochlorite (B) and their interaction (AB), that minimizes the response variable: proportion of defective items, in a process in a small plant of the textile industry. The response variable takes the form of a proportion, that violates the basic assumptions of the Classic Linear Model and, as a result, the estimates of the coefficients by Ordinary Least Squares method is less reliable. The design employed was a replicated complete 22 factorial design with central point. After doing the planning, the modeling by MLG is applied, and then it is possible to identify a underdispersion data; to verify that the model used is correct and that the volume of sodium hypochlorite (B) is the only significant factor in the industrial process of bleaching the company. Therefore, as the purpose is to minimize the response, it is used the lower level (-1) of this variable. Consequently, as the aim is to reduce costs of chemical inputs can use the minimum level of concentration of hydroxide sodium (A) and the maximum level of interaction between factors (AB), since they are not significant to the model.

With the growth of the internet, distributed systems have become increasingly important in order to provide more available and scalable applications. Con- sensus is a fundamental problem in distributed systems where multiple pro- cesses have to agree on the same proposed value in the presence of partial failures. Distributed consensus allows for building various applications such as lock services, configuration manager services or distributed databases.Two well-known consensus algorithms for building distributed logs are Multi-Paxos and Raft. Multi-Paxos was published almost three decades before Raft and gained a lot of popularity. However, critics of Multi-Paxos consider it difficult to understand. Raft was therefore published with the motivation of being an easily understood consensus algorithm. The Raft algorithm shares similar characteristics with a practical version of Multi-Paxos called Leader- based Sequence Paxos. However, the algorithms differ in important aspects such as leader election and reconfiguration.Existing work mainly compares Multi-Paxos and Raft in theory, but there is a lack of performance comparisons in practice. Hence, prototypes of Leader- based Sequence Paxos and Raft have been designed and implemented in this thesis. The prototypes were implemented using the Rust programming lan- guage and the message-passing framework Kompact and then benchmarked in real-world scenarios to compare the performance of Leader-based Sequence Paxos and Raft.The results show that Leader-based Sequence Paxos and Raft have simi- lar performance in geographically distributed deployments. However, the un- predictable leader election in Raft could greatly affect the performance if the elected leader is in an undesired location. In our experiments, the location of the Raft leader affected the average throughput by up to 35%. Furthermore, the results indicate that implementation details could have a significant impact on performance even in the parts where the algorithms are similar. By batch- ing messages more efficiently, Leader-based Sequence Paxos achieved up to 17% higher average throughput than Raft. / Med tillväxten av internet har distribuerade system blivit allt mer viktiga för att bygga mer tillgängliga och skalbara applikationer. Konsensus är ett funda- mentalt problem i distribuerade system där flera processer ska komma överens om samma föreslagna värde, samtidigt som partiella fel kan ske. Distribuerad konsensus kan appliceras till olika användningsomården som låstjänster, kon- figurationshanterare och distribuerade databaser.Två välkända konsensusalgoritmer för att bygga distribuerade loggar är Multi-Paxos och Raft. Multi-Paxos publicerades nästintill tre årtionden före Raft och blev populär. Men kritiker av Multi-Paxos anser att algoritmen är svår att förstå. Av denna anledning publicerades Raft med motivationen att vara en konsensusalgoritm som är enkel att förstå. Raft delar likheter med Leader- based Sequence Paxos, en praktisk version av Multi-Paxos. Dock skiljer sig algoritmerna i viktiga aspekter som leaderval och rekonfigurering.Befintliga arbeten jämför i huvudsak Multi-Paxos och Raft i teorin, men det saknas jämförelse av prestandan i praktiken. Av denna anledning har pro- totyper av Leader-based Sequence Paxos och Raft blivit designade och imple- menterade i denna avhandling. Dessa prototyper implementerades i program- meringsspråket Rust och message-passing ramverket Kompact, som sedan tes- tades i verkliga situationer för att jämföra Leader-based Sequence Paxos och Raft.Resultaten visar att Leader-based Sequence Paxos och Raft har liknande prestanda i geografiskt distribuerade sammanhang. Dock kan det oförutsäga- bara ledarvalet i Raft påverka prestandan avsevärt ifall den valde ledaren befin- ner sig på en oönskad plats. I våra experiment påverkade Raft ledarens plats den genomsnittliga kapaciteten med upp till 35%. Resultaten visar även att implementationsdetaljer kan ha en signifikant effekt på prestandan även i de delar där algoritmerna är liknande. Genom att sammanfoga meddelanden mer effektivt uppnådde Leader-based Sequence Paxos 17% högre genomsnittlig kapacitet än Raft.

In today's connected society, where it is common to have several connected devices per capita, it is more important than ever that the data you need is omnipresent, i.e. its available when you need it, no matter where you are. We identify one key technology and platform that could be the future—peer-to-peer communication and the Web. Unfortunately, guaranteeing consistency and availability between users in a peer-to-peer network, where network partitions are bound to happen, can be a challenging problem to solve. To solve these problems, we turned to a promising category of data types called CRDTs—Conflict Free Replicated Data Types. By following the scientific tradition of reproduction, we build upon previous research of a CRDT framework, and adjust it work in a peer-to-peer Web environment, i.e. it runs on a Web browser. CRDTs makes use of meta-data to ensure consistency, and it is imperative to remove this meta-data once it no longer has any use—if not, memory usage grows unboundedly making the CRDT impractical for real-world use. There are different garbage collection techniques that can be applied to remove this meta-data. To investigate whether the CRDT framework and the different garbage collection techniques are suitable for the Web, we try to reproduce previous findings by running our implementation through a series of benchmarks. We test whether our implementation works correctly on the Web, as well as comparing the memory efficiency between different garbage collection techniques. In doing this, we also proved the correctness of one of these techniques. The results from our experiments showed that the CRDT framework was well-adjusted to the Web environment and worked correctly. However, while we could observe similar behaviour between different garbage collection techniques as previous research, we achieved lower relative memory savings than expected. An additional insight was that for long-running systems that often reset its shared state, it might be more efficient to not apply any garbage collection technique at all. There is still much work to be done to allow for omnipresent data on the Web, but we believe that this research contains two main takeaways. The first is that the general CRDT framework is well-suited for the Web and that it in practice might be more efficient to choose different garbage collection techniques, depending on your use-case. The second take-away is that by reproducing previous research, we can still advance the current state of the field and generate novel knowledge—indeed, by combining previous ideas in a novel environment, we are now one step closer to a future with omnipresent data. / I dagens samhälle är vi mer uppkopplade än någonsin. Tack vare det faktum att vi nu ofta har fler än en uppkopplad enhet per person, så är det viktigare än någonsin att ens data är tillgänglig på alla ens enheter–oavsett vart en befinner sig. Två tekniker som kan möjliggöra denna ``allnärvaro'' av data är Webben, alltså kod som körs på en Webbläsare, tillsammans med peer-to-peer-kommunikation; men att säkerställa att distribuerad data både är tillgänglig och likadan för alla enheter är svårt, speciellt när enhetens internetanslutning kan brytas när som helst. Conflict-free replicated data-types (CRDT:er) är en lovande klass av datatyper som löser just dessa typer av problem i distribuerade system; genom att använda sig av meta-data, så kan CRDT:er fortsätta fungera trots att internetanslutningen brutits. Dessutom är de garanterade att konvergera till samma sluttillstånd när anslutningen upprättas igen. Däremot lider CRDT:er av ett speciellt problem–denna meta-data tar upp mycket minne trots att den inte har någon användning efter en stund. För att göra datatypen mer minneseffektiv så kan meta-datan rensas bort i en process som kallas för skräpsamling. Vår idé var därför att reproducera tidigare forskning om ett ramverk för CRDT:er och försöka anpassa denna till att fungera på Webben. Vi reproducerar dessutom olika metoder för skräpsamling för att undersöka om de, för det första fungerar på Webben, och för det andra är lika effektiv i denna nya miljö som den tidigare forskningen pekar på. Resultaten från våra experiment visade att CRDT-ramverket och dess olika skräpsamlingsmetoder kunde anpassas till att fungera på Webben. Däremot så noterade vi något högre relativ minnesanvändning än vad vi har förväntat oss, trots att beteendet i stort var detsamma som den tidigare forskningen. En ytterligare upptäckt vad att i vissa specifika fall så kan det vara mer effektivt att inte applicera någon skräpsamling alls. Trots att det är mycket arbete kvar för att använder CRDT:er peer-to-peer på Webben för att möjliggöra ``allnärvarande'' data, så innehåller denna uppsats två huvudsakliga punkter. För det första så fungerar det att anpassa CRDT-ramverket och dess olika skräpsamlingsmetoder till Webben, men ibland är det faktiskt bättre att inte applicera någon skräpsamling alls. För det andra så visas vikten av att reproducera tidigare forskning–inte bara visar uppsatsen att tidigare CRDT-forskning kan appliceras i andra miljöer, dessutom kan ny kunskap hämtas ur en sådan reproducering.