Quality-driven processing of sliding window aggregates over out-of-order data streams.

ABSTRACTOne fundamental challenge in data stream processing is to cope with the ubiquity of disorder of tuples within a stream caused by network latency, operator parallelization, merging of asynchronous streams, etc. High result accuracy and low result latency are two conflicting goals in out-of-order stream processing. Different applications may prefer different extent of trade-offs between the two goals. However, existing disorder handling solutions either try to meet one goal to the extreme by sacrificing the other, or try to meet both goals but have shortcomings including unguaranteed result accuracy or increased complexity in operator implementation and application logic. To meet different application requirements on the latency versus result accuracy trade-off in out-of-order stream processing, in this paper, we propose to make this trade-off user-configurable. Particularly, focusing on sliding window aggregates, we introduce AQ-K-slack, a buffer-based quality-driven disorder handling approach. AQ-K-slack leverages techniques from the fields of sampling-based approximate query processing and control theory. It can adjust the input buffer size dynamically to minimize the result latency, while respecting user-specified threshold on relative errors in produced query results. AQ-K-slack requires no a priori knowledge of disorder characteristics of data streams, and imposes no changes to the query operator implementation or the application logic. Experiments over real-world out-of-order data streams show that, compared to the state-of-art, AQ-K-slack can reduce the average buffer size, thus the average result latency, by at least 51% while respecting user-specified requirement on the accuracy of query results.