Tuple dropping, though commonly used for load shedding in most data stream operations, is generally inadequate for multiway windowed stream joins. The join output rate can be unnecessarily reduced because tuple dropping fails to exploit the time correlations that are likely to exist among interrelated streams. In this paper, we introduce GrubJoin—an adaptive multiway windowed stream join that effectively performs time correlation-aware CPU load shedding. GrubJoin maximizes the output rate by achieving near-optimal window harvesting, which picks only the most profitable segments of individual windows for the join. Due mainly to the combinatorial explosion of possible multiway join sequences involving different window segments, GrubJoin faces unique challenges that do not exist for binary joins, such as determining the optimal window harvesting configuration in a time-efficient manner and learning the time correlations among the streams without introducing overhead. To tackle these challenges, we formalize window harvesting as an optimization problem, develop greedy heuristics to determine near-optimal window harvesting configurations, and use approximation techniques to capture the time correlations. Our experimental results show that GrubJoin is vastly superior to tuple dropping when time correlations exist and is equally effective when time correlations are nonexistent.
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