How can we approach the surviving evidence on Paliochora in order for it to become a site of cultural consciousness in a wider medieval context? Its architectural and iconographic identity is hidden in its thirty-four Late and Post Byzantine churches. This thesis constitutes the first complete interdisciplinary approach to this settlement accompanied by a detailed appendix in the second volume. The two parts of this study examine Paliochora through different perspectives which reveal different aspects of its character. Urban planning and individual architectural specificities are examined through the prism of four construction periods associated with political and economic factors. Structural variety and multiplicity raises questions concerning religious functions. The iconography, on the other hand, relies on the general Late Byzantine canons and influences, but demonstrates provincial tendencies and promotes a distinct style of fresco painting. Furthermore, the possible interference of the continuous Western presence creates new aspects for conceptual discussion in both fields. Consequently the three parameters of this comparative approach are underlined, either on an architectural or iconographic level: • Between the different examples in Paliochora • Within the vast array of Greek and Mediterranean ecclesiastical examples • In contrast to Western practices and models. Throughout the text problems of archaeological evidence and archival information are raised. However, this first effort to place and contextualise Paliochora on the map of existing late medieval cities of the Mediterranean is a call for further research in multiple disciplines. It is a survey which will be used as the basic material for any future actions related both to academic knowledge and restoration processes.

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State machine replication (SMR) is one of the most widely studied and used methodology for building highly available distributed applications and services. SMR replicates a service across a set of computing hosts, and executes client operations on the replicas in an agreed- upon total order, ensuring linearizability of the replicated shared state. The problem of determining a total order reduces to one of computing consensus. State-of-the-art consensus protocols are inadequate for newer classes of applications such as Blockchains and for geographically distributed infrastructures. The widely used Crash Fault Tolerance (CFT) fault model of consensus protocols is prone to malicious and adversarial behaviors as well as non-crash faults such as software bugs. The Byzantine fault-tolerance (BFT) model and its trust-based variant, the hybrid model, permit stronger failure adversaries. However, state-of-the-art Byzantine and hybrid consensus protocols have performance limitations in geographically distributed environments: they designate a primary replica for proposing total-orders, which becomes a bottleneck and yields sub-optimal latencies for faraway clients. Additionally, they do not scale to hundreds of replicas and provide consistent performance as the system size grows. To overcome these limitations and develop highly scalable SMR solutions, this dissertation presents two leaderless consensus protocols, namely ezBFT and Dester, for the Byzantine and hybrid models, respectively. These protocols enable every replica to receive and order client commands. Additionally, they exchange command dependencies to collectively order commands without relying on a primary. Our experimental evaluations in a 7-node geographically distributed setup reveals that ezBFT improves client-side latency by as much as 40% over state-of-the-art BFT protocols including PBFT, FaB, and Zyzzyva. Dester, for the hybrid model, reduces latency by as much as 30% over ezBFT. Next, the dissertation presents a new paradigm called DQBFT for designing consensus protocols that can scale to hundreds of nodes in geographically distributed environments. Since leaderless protocols exchange command dependencies, they do not scale to hundreds of nodes. DQBFT overcomes this scalability limitation by decentralizing only the heavy task of replicating commands and centralizing the process of ordering the commands. While DQBFT can be used to enhance existing primary-based protocols, Destiny is a hybrid instantiation of the DQBFT paradigm using linear communication for better scalability than naive instantiations. Experimental evaluations in a 193-node geographically distributed setup reveal that Destiny achieves ≈ 3× better throughput and ≈50% better latency than state-of-the-art BFT protocols including Hotstuff, SBFT, and Hybster. Lastly, the dissertation presents two techniques for designing and implementing BFT protocols with reduced development costs. The dissertation presents Bumblebee, a methodology for manually transforming CFT protocols to tolerate Byzantine faults using trusted execution environments that are increasingly available in commodity hardware. Bumblebee is based on the observation that CFT protocols are incapable of tolerating non-malicous non-crash faults, but they are nevertheless deployed in many production systems. Bumblebee provides a Generic Algorithm that can represent protocols in both CFT and hybrid fault models, thus allowing easy construction of hybrid protocols using CFT protocols as baselines. The dissertation constructs hybrid instantiations of CFT protocols including Paxos, Raft, and M2Paxos. Experimental evaluations of the hybrid variants reveal that they perform at par with native hybrid protocols, but incur a 30% overhead over their CFT counterparts. Hybrid protocols rely on the integrity of trusted execution environments, which are increasingly subject to security exploits. To withstand exploits, the dissertation presents DuoBFT, a protocol that exposes both the BFT and hybrid fault models within a single consensus protocol. This enables consensus under both fault models within the same protocol and without additional redundancy, allowing DuoBFT to achieve the performance of hybrid protocols and the security of BFT protocols. Experimental evaluations reveal that DuoBFT achieves the best of both hybrid and BFT fault models with less than 10% overhead. / Doctor of Philosophy / Computers are ubiquitous; they perform some of the most complex and safety-critical tasks such as controlling aircraft, managing the financial markets, and maintaining sensitive medical records. The undeniable fact is that computers are faulty. They are prone to crash and can behave arbitrarily. Even the most robust computers such as those that are sent to the outer space eventually fail. External phenomenon such as power outages and network disruptions affect their operation. To make computing systems reliable, researchers and practitioners have long focussed on interconnecting many individual computers and programming them to effectively be duplicates of one another. This way when one computer fails in a system, the rest of the computers still ensure that the system as a whole is operational. Duplication requires that multiple computers effectively perform the same task. In order for multiple computers to perform the same task together, they should first agree on the task. More generally, since computing systems perform multiple tasks, they should agree on the sequence of tasks that they will individually perform and follow the agreement. This is what is known as the State Machine Replication technique. State Machine Replication (SMR) is a powerful technique that is applicable to numerous computing applications. Blockchain systems, the technology behind the cryptocurrencies such as Bitcoin and Ethereum, uses the SMR technique. In the context of Blockchain, the added challenge in that some of the computers involved in SMR can be programmed by adversarial parties and could act in a way to jeopardize the integrity of the whole system. For Bitcoin and Ethereum, this could mean embezzlement of hundreds or even millions of dollars worth of cryptocurrencies. Certain SMR systems are capable of tolerating such intrusions and ensure system integrity. Such systems are deemed to be Byzantine tolerant. This dissertation presents designs, techniques, and implementations of Byzantine State Machine Replication systems. The problems addressed in this dissertation are those that plague existing Byzantine SMR systems making them suboptimal for newer applications such as Blockchains. First, when computers that participate in SMR are spread around the world, their performance is dependent on the communication latencies between any two pair of computers. Second, the number of computers required is proportional the number of adversarial computers that need to be tolerated. Consequently, certain SMR systems for Blockchains require hundreds of computers to tolerate heavy adversarial behavior. Many existing SMR technique perform poorly under these scenarios. The techniques presented in this dissertation address various permutations of these challenges.

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Les vestiges mobiliers de l'occupation byzantine de l'Afrique antique constituent un corpus important d'objets en céramique (vaisselle, amphores et carreaux de terre cuite), en verre (lampes), en pierre (décor sculpté et mosaïques) en plus des monnaies. L'ensemble de ces objets provient des fouilles des niveaux d'occupation datés entre les VIe et VIIe siècles ap. J.-C., période pour laquelle les sources historiques témoignent de la Reconquête de la région menée par les armées impériales venues de Constantinople. Pour retrouver la trace des Byzantins eux-mêmes, c'est le travail de plusieurs génération d'antiquaires, d'explorateurs et d'archéologues qui a d'abord été examiné en se posant la question de savoir quelle était la nature des vestiges retrouvés. Ceux-ci témoignaient-ils d'une communauté de culture entre Orient et Occident rendue possible grâce à la restauration de l'autorité impériale ? Ou bien l'Afrique avait-elle révélé, à travers ses traditions artisanales fortes, toute la singularité de sa place au carrefour de la Méditerranée occidentale ? / The archaeological material from the Byzantine occupation of ancient Africa forms a huge corpus of objects in ceramics (crockery, amphoras and tiles of terra cotta), glass (lamps), stones (carved decoration and mosaics) in addition to the coins. All these objects are coming from occupations dated between VIth and VIIth centuries A.D., period for which the historical sources testify to the Reconquest of the area, carried out by the imperial armies from Constantinople. Find the traces of the Byzantines themselves was the work of several generations of antiquarians, explorers and archaeologists asking the question of the true nature of the found remains. Did those vestiges testify a cultural community between Eastern and Western Mediterranean Sea made possible by the restoration of imperial authority ? Or did Africa reveal through its strong handcrafted traditions all the singularity of its place at the crossroads of the Western Mediterranean ?

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In 1973, researchers from the Institute of Nautical Archaeology (INA) were led to the site of a wrecked ship by sponge diver Mehmet A??k??n, near his hometown of Bozburun, Turkey. During further monitoring over the following 21 years by INA, the site was identified as a merchant vessel dating from the 9th century AD. The excavation of the site by INA researchers and students from Texas A&M University occurred over four summer seasons, from 1995 to 1998, and yielded approximately 900 whole or nearly-whole amphorae, personal items, palynological material, and approximately 35 percent of the vessel??s wooden hull. This dissertation is a record of the curation, cataloging, analysis and re-assembly of the preserved elements of the Bozburun vessel??s hull, as well as a theoretical reconstruction of the entire vessel. The Bozburun vessel is unique as it is the only fully-excavated shipwreck from the 9th century AD, and is, indeed, a valuable source of examples of ship construction in the Mediterranean between the 7th and the 11th centuries AD. This dissertation, after discussing the methods of excavation and cataloging methods, posits the hypothesis that the techniques used to build this vessel represent a transitional stage in shipbuilding technology, combining distinctly old and new techniques. While the builders used embedded edge joinery in the ship??s planking, a very old method, they also appear to have used a conceptual framework and standards to design the vessel as well; methods evident in modified forms in Italian shipbuilding treatises from the Renaissance.

In 1973, researchers from the Institute of Nautical Archaeology (INA) were led to the site of a wrecked ship by sponge diver Mehmet A??k??n, near his hometown of Bozburun, Turkey. During further monitoring over the following 21 years by INA, the site was identified as a merchant vessel dating from the 9th century AD. The excavation of the site by INA researchers and students from Texas A&M University occurred over four summer seasons, from 1995 to 1998, and yielded approximately 900 whole or nearly-whole amphorae, personal items, palynological material, and approximately 35 percent of the vessel??s wooden hull. This dissertation is a record of the curation, cataloging, analysis and re-assembly of the preserved elements of the Bozburun vessel??s hull, as well as a theoretical reconstruction of the entire vessel. The Bozburun vessel is unique as it is the only fully-excavated shipwreck from the 9th century AD, and is, indeed, a valuable source of examples of ship construction in the Mediterranean between the 7th and the 11th centuries AD. This dissertation, after discussing the methods of excavation and cataloging methods, posits the hypothesis that the techniques used to build this vessel represent a transitional stage in shipbuilding technology, combining distinctly old and new techniques. While the builders used embedded edge joinery in the ship??s planking, a very old method, they also appear to have used a conceptual framework and standards to design the vessel as well; methods evident in modified forms in Italian shipbuilding treatises from the Renaissance.