Existem várias linguagens e plataformas que permitem a programação baseada no modelo de atores, uma solução elegante para a programação concorrente proposta há algumas décadas. Segundo esse modelo, implementa-se o programa na forma de uma série de agentes que são executados em paralelo e se comunicam entre si somente por meio da troca de mensagens, sem a necessidade de memória compartilhada ou estruturas tradicionais de sincronização como semáforos e mutexes. Uma das áreas nas quais esse modelo seria particularmente adequado é a programação de aplicações web, isto é, aplicações cujas lógicas de negócios e de dados residem num servidor e que são acessadas pelo usuário por intermédio de um navegador. Porém, existem muitos obstáculos ao desenvolvimento de aplicações desse tipo, entre eles a falta de linguagens e ferramentas que permitam integrar tanto o servidor quanto o cliente (navegador) no modelo de atores, as dificuldades de conversões de dados que se fazem necessárias quando o servidor e o cliente são desenvolvidos em linguagens diferentes, e a necessidade de contornar as dificuldades inerentes aos detalhes do protocolo de comunicação (HTTP). O PAWEB é uma proposta de uma plataforma para o desenvolvimento e execução de aplicações web que fornece a infraestrutura necessária para que tanto o lado cliente quanto o lado servidor do aplicativo hospedado possam ser escritos numa mesma linguagem (Python), e possam criar e gerenciar atores que trocam mensagens entre si,tanto local quanto remotamente, de maneira transparente e sem a necessidade de implementar conversões de dados ou outros detalhes de baixo nível. / There are several programming languages and platforms that allow the development of systems based on the actor model, an elegant solution for concurrent programming proposed a few decades ago. According to this model, the program is implemented in the form of several agents that run concurrently and only communicate amongst themselves through the exchange of messages, without the need for shared memory or traditional synchronization structures such as semaphores and mutexes. One of the areas where this model would be particularly appropriate would be the development of web applications, that is, applications whose business and database logic reside on the server and are accessed by the user by means of a web browser. However, there are several obstacles to the development of this type of application, amongst them the lack of languages and tools that allow for the integration of both the server and the client (browser) into the actor model, data conversion difficulties arising from using different programming languages on the server and the client, and the need to circumvent the inherent difficulties posed by the details of the communications protocol (HTTP). PAWEB is a proposal for an application development and execution platform that supplies the infrastructure needed so that both the server and client sides of the hosted application can be written in the same language (Python) and so that they may create and manage actors that exchange messages with one another, both locally and remotely, in a transparent manner and without the need to implement data conversions or other low-level mechanisms.

O modelo de programação baseado em atores é frequentemente utilizado para o desenvolvimento de grandes aplicações e sistemas. Podemos citar como exemplo o serviço de bate-papo do Facebook ou ainda o WhatsApp. Estes sistemas dão suporte a milhares de usuários conectados simultaneamente levando em conta estritas restrições de desempenho e interatividade. Tais sistemas normalmente são amparados por infraestruturas de hardware com processadores de múltiplos núcleos. Normalmente, máquinas deste porte são baseadas em uma estrutura de memória compartilhada hierarquicamente (NUMA - Non-Uniform Memory Access). Nossa análise dos atuais ambientes de execução para atores e a pesquisa na literatura mostram que poucos estudos sobre a adequação deste ambientes a essas plataformas hierárquicas foram conduzidos. Estes ambientes de execução normalmente assumem que o espaço de memória é uniforme o que pode causar sérios problemas de desempenho. Nesta tese nós estudamos os desafios enfrentados por um ambiente de execução para atores quando da sua execução nestas plataformas. Estudamos particularmente os problemas de gerenciamento de memória, de escalonamento e de balanceamento de carga. Neste documento nós também analisamos e caracterizamos as aplicações baseadas no modelo de atores. Tal análise nos permitiu evidenciar o fato de que a execução de benchmarks e aplicações criam estruturas de comunicação peculiares entre os atores. Tais peculiaridades podem, então, ser utilizadas pelos ambientes de execução para otimizar o seu desempenho. A avaliação dos grafos de comunicação e a implementação da prova de conceito foram feitas utilizando um ambiente de execução real, a máquina virtual da linguagem Erlang. A linguagem Erlang utiliza o modelo de atores para concorrência com uma sintaxe clara e consistente. As modificações que nós efetuamos nesta máquina virtual permitiram uma melhora significativa no desempenho de certas aplicações através de uma melhor afinidade de comunicação entre os atores. O escalonamento e o balanceamento de carga também foram melhorados graças à utilização do conhecimento sobre o comportamento da aplicação e sobre a plataforma de hardware. / The actor model is present in several mission-critical systems, such as those supporting WhatsApp and Facebook Chat. These systems serve thousands of clients simultaneously, therefore demanding substantial computing resources usually provided by multi-processor and multi-core platforms. Non-Uniform Memory Access (NUMA) architectures account for an important share of these platforms. Yet, research on the suitability of the current actor runtime environments for these machines is very limited. Current runtime environments, in general, assume a flat memory space, thus not performing as well as they could. In this thesis we study the challenges hierarchical shared memory multi-core platforms present to actor runtime environments. In particular, we investigate aspects related to memory management, scheduling, and load-balancing. In this document, we analyze and characterize actor based applications to, in light of the above, propose improvements to actor runtime environments. This analysis highlighted the existence of peculiar communication structures. We argue that the comprehension of these structures and the knowledge about the underlying hardware architecture can be used in tandem to improve application performance. As a proof of concept, we implemented our proposal using a real actor runtime environment, the Erlang Virtual Machine (VM). Concurrency in Erlang is based on the actor model and the language has a consistent syntax for actor handling. Our modifications to the Erlang VM significantly improved the performance of some applications thanks to better informed decisions on scheduling and on load-balancing.

The Swift programming language recently introduced the actor model alongside Swift concurrency as a first-class citizen. Previously, concurrent programming in Swift required importing modules, e.g., Grand Central Dispatch. Grand Central Dispatch is a concurrency framework, often used for implementing shared memory concurrency in Swift. In contrast to Grand Central Dispatch, the Swift actor provides implicit mutual exclusion via message passing concurrency and the notion of actor isolation. This thesis evaluates the performance of the Swift actor and Swift concurrency in comparison to thread-safe objects with Grand Central Dispatch. The study assembled a benchmark suite to measure the performance over central bottlenecks (Chameneos), large counts of independent objects (Dining Philosophers), long-living concurrent objects (Dining Philosophers) and massively parallel messaging (Big). The results indicate the lack of a catch-all solution to concurrent programming in Swift. In comparison to Grand Central Dispatch, a system of actors tends to require more memory to finish execution. By upper bounding concurrency in actors, memory use is effectively limited at the cost of possibly longer execution times. In particular, the results were gathered on a macOS computer. With unique implementations of threads in different operating systems, further studies are needed to evaluate Swift concurrency and the Swift actor on other systems and hardware. In addition, further studies may evaluate the risk of congestion at large numbers of concurrent actors. / Programmeringsspråket Swift introducerade nyligen actor-modellen med Swift concurrency som en förstklassig medborgare. Samtidig programmering i Swift krävde importmoduler innan dess, t.ex. Grand Central Dispatch. Grand Central Dispatch är ett ramverk för samtidighet och används ofta för att implementera samtidighet med delat minne i Swift. I kontrast till Grand Central Dispatch tillhandahåller Swift actors implicit ömsesidig uteslutning via message passing och actor-isolering. Detta examensarbete utvärderar prestandan av Swift actors och Swift concurrency i jämförelse med trådsäkra objekt via Grand Central Dispatch. Studien sammanställde en benchmark suite för att mäta prestandan över centrala flaskhalsar (Chameneos), stora antal oberoende objekt (Dining Philosophers), samtidiga objekt med långa liv (Dining Philosophers) och massivt parallella meddelanden (Big). Resultaten indikerar en avsaknad av en fullständig lösning för samtidig programmering i Swift. I jämförelse med Grand Central Dispatch tenderar ett system av Swift actors att använda mer minne för att slutföra exekvering. Med en övre gräns för samtidighet hos actors begränsas minnesanvändningen effektivt med eventuellt längre exekveringstider. Denna studie samlade resultat med en macOS-dator. Med unika implementeringar av trådar i olika operativsystem behövs ytterligare studier för att utvärdera Swift concurrency och Swift actors på andra system och hårdvaror. Dessutom kan ytterligare studier utvärdera risken av trängsel vid stora antal samtidiga actors.

本研究針對物聯網與雲端大數據分析之不同程度的通訊需求，以微服務架構為基礎，設計一個四層式物聯網與大數據平台之通訊架構。面對物聯網之即時通訊需求，本研究採用MQTT通訊協定做為解決方案，而雲端大數據分析之通訊需求則採用Apache Kafka。 本研究基於參與者模型（Actor Model）所提出之「裝置代理人」，全面的解決了物聯網中異質通訊協定所產生的複雜性，同時解決了集中式物聯網閘道器所造成的系統複雜性與效能瓶頸，使物聯網閘道器能進行分散式部署，並且共享運算資源。

O modelo de atores tem sido visto como uma abordagem alternativa à programação concorrente convencional, baseada em travas e variáveis de condição. Atores são agentes computacionais que se comunicam por troca de mensagens e que possuem uma caixa de correio e um comportamento. As mensagens destinadas a um ator são armazenadas na caixa de correio do ator e processadas de maneira assíncrona. Sistemas de middleware orientados a mensagens trabalham com troca assíncrona de mensagens e formam uma base que simplifica o desenvolvimento de aplicações distribuídas. Tais sistemas permitem interoperabilidade com baixo acoplamento e provêm suporte para tratamento robusto de erros em caso de falhas. Message brokers são frequentemente apresentados como uma tecnologia que pode mudar a maneira com que sistemas distribuídos são construídos. A especificação AMQP é uma proposta recente de padronização de um protocolo para message brokers. Neste trabalho exploramos a potencial sinergia entre um message broker e uma implementação do modelo de atores. Criamos uma versão modificada da implementação do modelo de atores do projeto Akka que utiliza um message broker AMQP como mecanismo de transporte de mensagens para atores remotos. / The actor model has been seen as an alternative for conventional concurrent programming based on locks and condition variables. Actors are computational agents that communicate by sending messages and have a mailbox and a behavior. The messages sent to an actor are stored in its mailbox and are asynchronously processed. Message oriented middleware systems work with asynchronous message exchange and create a base that simplifies the development of distributed applications. These systems have interoperability with low coupling and provide support for robust error handling in case of failures. Message brokers are often presented as a technology that can change the way distributed systems are built. The AMQP specification is a recent proposal of a standard protocol for message brokers. In this document we explore the potential synergy between a message broker and an implementation of the actor model. We created a modified version of the actor model implementation provided by the Akka project. Our modified implementation uses an AMQP message broker as the transport engine for messages to remote actors.

<p>Sve češćom upotrebom cloud okruženja dolazi do potrebe da se postojeća softverska rešenja migriraju. Metodologija za migraciju na cloud postoji više, gde se u finalnim fazama planiraju modifikacije nad arhitekturom softvera tako da se iskoriste prednosti cloud sistema. Za ekonomsku efikasnost usled uštede resursa je neophodna višeorganizacijska osobina. Svrha ovog istraživanja je da pojasni višeorganizacijsko svojstvo i da predloži rešenje za migraciju postojećih softvera na višeorganizacijski SaaS ali sa što manje neophodnih modifikacija ciljnog softvera. S toga je predloženo rešenje platforma koja omogućuje lakšu migraciju. Nakon faze istraživanja i sagledavanja domena pametnih elektroenergetskih mreža, kreiran je prototip predloženog rešenja kao i niz eksperimenata u skladu sa definisanim naučnim pitanjima. Eksperimenti su izvršeni u privatnom cloud okruženju. Hipoteze su adresirane kroz viziju primene rešenja na NDMS (Napredni distributivni menadžment sistem) u slučaju šest organizacija, a doneti su sledeći zaključci: višeorganizacijskim modelom se ostvaruju uštede u resursima od 32%, za tri reda veličine veća visoka dostupnost, ali uz usporenja do 20 milisekundi po svakom servisnom zahtevu. Takođe, aplikativni model učesnika modernih PaaS usluga nije primeren gde je sinhronizam zahtevan, niti u slučajevima gde se očekuju odgovori nad skupom učesnika u realnom vremenu. Istraživanje ukazuje na mogućnost primene višeorganizacijskog modela čak i u slučaju kompleksnih rešenja kakva se sreću u domenu pametnih elektroenergetskih mreža, a akademskom validacijom potvrđuju način implementacije važnog finalnog koraka u procesu migracije softvera na cloud bazirani SaaS.</p> / <p>Progressive cloud adoption requires migration of existing software solutions. Today, many cloud adoption methodologies exist. Usually, the final phase in cloud adoption include software architecture modifications to make the most of the benefits of cloud computing, like multi-tenancy which enables economic efficiency. The aim of this research is to explain the multi-tenancy and to provide a solution for migration of existing software to multi-tenant SaaS while modifying the target software as little as possible. Therefore, the research proposes a platform that enables easier cloud adoption. After the research phase focused on a smart grid domain, the prototype was created with experiments targeting formulated research questions. The experiments were conducted in a private cloud environment. Research hypotheses were analyzed using hypothetical multi-tenant ADMS (Advanced Distribution Management System) in case of six tenants, with the following conclusions: multi-tenancy saves 32% of resources, it provides three orders of magnitude higher availability, but affects performances by introducing a delay of up to 20 milliseconds per service request. Also, reliable actors programing model used in modern PaaS services is not suitable in use cases with needs for synchronous behavior, nor in in use cases where querying a set of actors is needed in real time. The research demonstrates feasibility of applying multi-tenancy even in cases of complex software solutions like the ones in the smart grid domain. The proposed solution is academically validated and it can be used as a final important step in migration of existing software to cloud based multi-tenant SaaS.</p>

O modelo de atores tem sido visto como uma abordagem alternativa à programação concorrente convencional, baseada em travas e variáveis de condição. Atores são agentes computacionais que se comunicam por troca de mensagens e que possuem uma caixa de correio e um comportamento. As mensagens destinadas a um ator são armazenadas na caixa de correio do ator e processadas de maneira assíncrona. Sistemas de middleware orientados a mensagens trabalham com troca assíncrona de mensagens e formam uma base que simplifica o desenvolvimento de aplicações distribuídas. Tais sistemas permitem interoperabilidade com baixo acoplamento e provêm suporte para tratamento robusto de erros em caso de falhas. Message brokers são frequentemente apresentados como uma tecnologia que pode mudar a maneira com que sistemas distribuídos são construídos. A especificação AMQP é uma proposta recente de padronização de um protocolo para message brokers. Neste trabalho exploramos a potencial sinergia entre um message broker e uma implementação do modelo de atores. Criamos uma versão modificada da implementação do modelo de atores do projeto Akka que utiliza um message broker AMQP como mecanismo de transporte de mensagens para atores remotos. / The actor model has been seen as an alternative for conventional concurrent programming based on locks and condition variables. Actors are computational agents that communicate by sending messages and have a mailbox and a behavior. The messages sent to an actor are stored in its mailbox and are asynchronously processed. Message oriented middleware systems work with asynchronous message exchange and create a base that simplifies the development of distributed applications. These systems have interoperability with low coupling and provide support for robust error handling in case of failures. Message brokers are often presented as a technology that can change the way distributed systems are built. The AMQP specification is a recent proposal of a standard protocol for message brokers. In this document we explore the potential synergy between a message broker and an implementation of the actor model. We created a modified version of the actor model implementation provided by the Akka project. Our modified implementation uses an AMQP message broker as the transport engine for messages to remote actors.

Denna undersökning kommer analysera Kinas handelspolitiska strategier med finanskrisen och handelskriget som tidpunkt samt även påpeka hur dess strategier förändrades underrespektive tillfälle. Undersökningen redogör för vilka likheter och skillnader det finns i handelspolitiken mellan perioderna med syfte att förklara förändringen i handelspolitiska preferenser. Den handelspolitiska förändringen som Kina genomgick mellan perioderna är viktig att undersöka då Kina fortsatt växer sig starka på den globala marknaden. Metoden i studien utgörs av en teorikonsumerande fallstudie där rational actor model används för att undersöka och förklara den rationella aktörens beslutsfattande i respektive situation. Spelteori används som en komplimenterande teori för att förklara Kinas vägval under handelskriget. Slutsatsen i undersökningen är att Kinas främsta mål och rationella val i krissituationer är att främja sin egen ekonomiska tillväxt oavsett vilka långsiktiga konsekvenserna detta kan innebära. Mot den bakgrunden återfinns även den främsta skillnaden mellan kriserna som var hanteringen av protektionism. Kina förebyggde mycket protektionism under finanskrisen men utövade det själva under handelskriget. Förändringen av agerandet mellan kriserna berodde likaväl på USA som på Kina och kan förklaras som det mest rationella valet för Kina för att främja sitt främsta mål med handelspolitiken, ekonomisk tillväxt. Förändringen förklaras som en tillfällig avvikelse från deras handelspolitik givet att det var det mest rationella beslutet givet omständigheterna och rådande situation.

This study focused on policy agenda-setting models for school sport and physical education in South Africa. The primary objective was to assess and propose options for improved agenda-setting by focussing on the use of agenda-setting models and by applying it to physical education and school sport and the policy agenda of the national government. The study has shown that pertinent school sport and physical education policy issues, as supported by key role-players and principal actors, were initially not placed on the formal policy agenda of government during the research investigation period (2005-2009). However, during 2010 and 2011 the issue of school sport and physical education received prominent attention by authorities and these developments were subsequently included in the study. The study aimed at contributing to existing policy agenda-setting models and by recommending changes to the Generic Process Model.The study also made a contribution by informing various role-players and stakeholders in education and school sport on the opportunities in policy agenda-setting. The study showed that policy agenda-setting is a vital step in the Generic Policy Process Model. Policy agendasetting in South Africa is critical, as it is important to place new and emerging policy issues on the policy agenda and as a participative public policy process is relatively new in this young democracy. The reader should not confuse the study as one dealing with school sport and physical education primarily, but rather as a research investigation dealing with policy agenda-setting models as applied to school sport and physical education.The secondary objectives of the study included the development of a historical perspective on trends and tendencies in education and sport in South Africa. A second objective was to provide theoretical perspectives on public policy and specifically on policy agenda-setting. From these theoretical perspectives, the Generic Policy Process Model was selected to use as a model that provided guidance on the overall policy process normally followed in South Africa. The Issue Attention Cycle and Principal Actor Models on Agenda-Setting were selected to apply to the case study to specifically ascertain important factors related to policy agenda-setting such as the identification of key role players as well as key policy issues. The Generic Policy Process Model provided for both a comprehensive set of phases as well as specific requirements and key issues to be addressed during each phase of the policy process.In terms of findings the study found that a number of specific agenda-setting elements or phases needed to be added to the Generic Policy Process Model, which includes a problem stage, triggers, initiator, issue creation and actors or policy stakeholders.The Principal Actor Model to agenda-setting was selected for application to the case as different actors have different levels of success at each policy stage. In the South African experience it is important to look at who sets the policy agenda and why, who can initiate agenda-setting and the role played by these principal actors in the agenda-setting process.Issue emergence often places policy issues on the policy agenda. The public is initially involved in issues, but in the long term public interest declines. The government realizes the significant costs involved in placing policy issues back on the agenda. This leads to a decline in issue attention by policy-makers and the public. The Issue Attention Cycle Model of agenda setting was used to analyse this phenomenon in South African Education policy.The study provides a case assessment of the South African experience. From the research findings, a set of conclusions and recommendations were developed for improved policy agenda-setting models and implications for school sport and physical education, as well as tools to place it on the national policy agenda were identified. The research findings suggest that pertinent school sport and physical education policy issues, as supported by key roleplayers,stakeholders and principal actors were not placed on the formal policy agenda of the government as a vital step in the policy process between 2005 and 2009. Ever since, principal policy actors, civil society NGOs, and government officials placed sufficient pressure on the Minister of Basic Education to place Physical Education on the agenda. Subsequently,Minister Angie Motshega has placed physical education in the school Curriculum under the subject Life Orientation and Lifeskills. It has become evident from the research that agendasetting is both necessary to, and a complex phase in, the policy-making process.This study has shown that major policy issues such as physical education and school sport were neglected during the period 2005 and 2009 despite reformed and advanced policy cycles in government. It has also shown that the role of policy agenda-setting in the overall policymaking process was revisited by government in the subsequent period 2010/2011 and placed on the policy agenda. Specific lessons of experience emanated from this process.The study recommends that the triggers of the agenda-setting phases be added to the Generic Policy Process Model, which should include the problem stage, triggers, initiators, issue creation, actors and policy stakeholders. Principal actors in the agenda-setting model in South Africa want the issue of physical education and school sport to be part of the school curriculum, and therefore be placed back on the policy agenda by the Government on its institutional agenda. Furthermore, the study showed that actors wanted it to be compulsory in all phases of the school (Foundation, Intermediate, Senior, GET, FET) and that it should have the same legal status as other subjects.The important findings include that: Comprehensive policy process models such as that of Dunn, Wissink and the Generic Process model may need to be reviewed to incorporate more fully the policy-agenda setting stages of the overall process; Current policy agenda setting models in use are relevant and valuable in identifying key role players as well as key issues and considerations regarding the policy process; Institutional arrangements to strengthen the role of NGOs and lower level institutions,such as schools to participate in policy agenda setting are important; and the study has shown that a number of key factors have been identified that had a key influence on policy agenda-setting in the case of physical education and school sport in South Africa. These included the influence of changing political leadership, the competency of policy capacities in government, the profile of issues in the media etc. The key findings of the study have shown that further potential exists to improve monitoring and evaluation and policy analysis.The study made a set of recommendations to principal actors such as the Minister of Education, Minister of Sport and Recreation, non-governmental organisations, interest groups,department officials and pressure groups. A set of research topics was also identified for future research. / Philosophiae Doctor - PhD

This paper examines causal mechanisms of the process leading up to the Russian military intervention in Syria that began in September 2015. It aims to concretize the causal processes of three different hypotheses that are based on commonplace assumptions of Russian foreign policy on Syria. It thoroughly explores three different causal paths, mapping events that may have had implications to the apparent change of heart within the Russian leadership. The paper analyses the relevance of these processes through a rational choice theory framework.