“…Indeed, by letting multiple instances process the input stream in parallel, operators can efficiently handle load peaks, while avoiding resource wastage in low-load periods. However, operator scaling is particularly challenging, especially in presence of stateful operators, as each parallelism adaptation requires the execution of a reconfiguration protocol to preserve stream and state integrity, often causing significant overhead (see, e.g., [18]). As surveyed in [15], a variety of different techniques have been used to define operator scaling policies, including threshold-based heuristics [6], control theory [4], queueing theory [9], reinforcement learning [7].…”