Advisor:

ii Big data is characterized by volume and velocity [24], and recently several real-time stream processing systems have emerged to combat this challenge. These systems process streams of data in real time and computational results. However, current popular data stream processing systems lack the ability to scale out and scale in (i.e., increase or decrease the number of machines or VMs allocated to the application) efficiently and unintrusively when requested by the user on demand. In order to scale out/in, a critical problem that needs to be solved is to determine which operator(s) of the stream processing application need to be given more resources or taken resources away from, in order to maximize the application throughput. We do so by presenting a novel metric called "Expected Throughput Percentage " (ETP). ETP takes into account not only congested elements of the stream processing application but also their effect on downstream elements and on the overall application throughput. Next, we show how our new system, called Stela (STream processin