Decentralized self-adaptation for elastic Data Stream Processing

Data Stream Processing (DSP) applications are widely used to develop new pervasive services, which require to seamlessly process huge amounts of data in a near real-time fashion. To keep up with the high volume of daily produced data, these applications need to dynamically scale their execution on multiple computing nodes, so to process the incoming data flow in parallel. In this paper, we present a hierarchical distributed architecture for the autonomous control of elastic DSP applications. It consists of a two-layered hierarchical solution, where a centralized per-application component coordinates the run-time adaptation of subordinated distributed components, which, in turn, locally control the adaptation of single DSP operators. Thanks to its features, the proposed solution can efficiently run in large-scale Fog computing environments. Exploiting this framework, we design several distributed self-adaptation policies, including a popular threshold-based approach and two reinforcement learning solutions. We integrate the hierarchical architecture and the devised self-adaptation policies in Apache Storm, a popular open-source DSP framework. Relying on the DEBS 2015 Grand Challenge as a benchmark application, we show the benefits of the presented self-adaptation policies, and discuss the strengths of reinforcement learning based approaches, which autonomously learn from experience how to optimize the application performance.