Novel structures for counting frequent items in time decayed streams

Identifying frequently occurring items is a fundamental building block in many data stream applications. A great deal of work for efficiently identifying frequent items has been studied on the landmark and sliding window models. In this work, we revisit this problem on a new streaming model based on the time decay, where the importance of every arrival item is decreased over the time. To address the importance changes over time, we propose an innovative heap structure, named Quasi-heap, which maintains the item order using a lazy update mechanism. Two approximation algorithm, Space Saving with Quasi-heap (SSQ) and Filtered Space Saving with Quasi-heap (FSSQ), are proposed to find the frequently occurring items based on the Quasi-heap structure. To achieve better accuracy of frequency estimation for all the items in the stream, we introduce a new count-min-min (CMM) sketch structure, which can estimate the count of an item with almost error free. Extensive experiments conducted on both real-world and synthetic data demonstrate the superiority of proposed methods in terms of both efficiency (i.e., response time) and effectiveness (i.e., accuracy).