Dynamic plan migration is concerned with the on-the-fly transition from one continuous query plan to a semantically equivalent yet more efficient plan. Migration is important for stream monitoring systems where long-running queries may have to withstand fluctuations in stream workloads and data characteristics. Existing migration methods generally adopt a pause-drain-resume strategy that pauses the processing of new data, purges all old data in the existing plan, until finally the new plan can be plugged into the system. However, these existing strategies do not address the problem of migrating query plans that contain stateful operators, such as joins. We now develop solutions for online plan migration for continuous stateful plans. In particular, in this paper, we propose two alternative strategies, called the moving state strategy and the parallel track strategy, one exploiting reusability and the second employs parallelism to seamlessly migrate between continuous join plans without affecting the results of the query. We develop cost models for both migration strategies to analytically compare them. We embed these migration strategies into the CAPE [7], a prototype system of a stream query engine, and conduct a comparative experimental study to evaluate these two strategies for window-based join plans. Our experimental results illustrate that the two strategies can vary significantly in terms of output rates and intermediate storage spaces given distinct system configurations and stream workloads.
Main memory is a critical resource when processing longrunning queries over data streams with state intensive operators. In this work, we investigate state spill strategies that handle run-time memory shortage when processing such complex queries by selectively pushing operator states into disks. Unlike previous solutions which all focus on one single operator only, we instead target queries with multiple state intensive operators. We observe an interdependency among multiple operators in the query plan when spilling operator states. We illustrate that existing strategies, which do not take account of this interdependency, become largely ineffective in this query context. Clearly, a consolidated plan level spill strategy must be devised to address this problem. Several data spill strategies are proposed in this paper to maximize the run-time query throughput in memory constrained environments. The bottom-up state spill strategy is an operator-level strategy that treats all data in one operator state equally. More sophisticated partition-level data spill strategies are then proposed to take different characteristics of the input data into account, including the local output, the global output and the global output with penalty strategies. All proposed state spill strategies have been implemented in the D-CAPE query system. The experimental results confirm the effectiveness of our proposed strategies. In particular, the global output strategy and the global output with penalty strategy have shown favorable results as compared to the other two more localized strategies.
Container shipping gives a rise of international trade since the 1960s. Based on navigation data start from the mid-1990s to 2016, this paper empirically analyses the spatial pattern of China's international maritime linkages along the "twenty-first-century Maritime Silk Road". We interpret such evolutionary dynamics in terms of growth, hierarchical diffusion and networking phases. Networking is a new stage of the evolution of the port system, which is approached based on the graph theory, complex network methods and geomatics, the paper discusses the networking's basic characteristics: multi-hub spatial agglomeration, the connection of the network develops across space, functional differentiation and a division of labour appear among ports. Our results show that, while the scope of China's maritime linkages had expanded overtime, more foreign ports become connected to the "Maritime Silk Road". In addition, the external linkages of domestic ports tend to be dispersed, reflecting upon the decline of Pearl River Delta ports and the rise of Yangtze River Delta ports, with mixed evidence for the Bohai Rim region. Lastly, the analysis underlines the emergence of a polycentric shipping system, from the Hong Kong dominance to the more diversified Shanghai/Ningbo/Shenzhen configuration. Academic and managerial implications are included.
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