Recognizing the explosion of large-scale real-time analytics needs, a plethora of stream processing systems, such as Apache Storm and Flink, have been developed to support such applications. Under these systems, a stream processing application is realized as a directed acyclic graph (DAG) of operators, where the resource configuration of each operator has a significant impact on its overall throughput and latency performance. However, there is a lack of dynamic resource allocation schemes, which are theoretically sound and practically implementable, especially under the drastically changing offered load. To address this challenge, we present Dragster1, an online-optimization-based dynamic resource allocation scheme for elastic stream processing. By combining the online optimization framework with upper confidence bound (UCB) techniques, Dragster can guarantee, in expectation, a sub-linear increase in the throughput regret w.r.t. time. To demonstrate the efficacy, we implement Dragster to improve the throughput of Flink applications over Kubernetes. Compared to the state-of-the-art algorithm Dhalion, Dragster can achieve a 1.8X-2.2X speed-up in converging to the optimal configuration. It can contribute to 20.0%-25.8% gain in tuple-processing goodput and 14.6%-15.6% cost-savings.