In a distributed processing environment, the static placement of query operators may result in unsatisfactory system performance due to unpredictable factors such as changes of servers' load, data arrival rates, etc. The problem is exacerbated for continuous (and long running) monitoring queries over data streams as any suboptimal placement will affect the system for a very long time. In this paper, we formalize and analyze the operator placement problem in the context of a locally distributed continuous query system. We also propose a solution, that is asynchronous and local, to dynamically manage the load across the system nodes. Essentially, during runtime, we migrate query operators/fragments from overloaded nodes to lightly loaded ones to achieve better performance. Heuristics are also proposed to maintain good data flow locality. Results of a performance study shows the effectiveness of our technique.
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