“…In several modern application domains (e.g., social networks, online finance, online transaction systems), data are generated in a continuous fashion, and at such a high rate that their processing requires on-the-fly computation which can afford to maintain only a small portion of the data in memory. This computational scenario is captured by the well-known streaming model, which has received everincreasing attention in the literature over the last two decades [26,30,31,35,38]. In some prominent applications, it is also important that older data in the stream (i.e., those outside a sliding window containing the N most recent data items) be considered "stale" and thus be disregarded in the computation.…”