Online experimentation is an agile software development practice, which plays a central role in enabling rapid innovation. It helps shorten code delivery cycles, which is critical for companies to survive in a competitive software-driven market. Recent advances in cloud computing, including the maturity of container-based technologies and cloud infrastructure, as well as the advent of service meshes, have created an opportunity to broaden the scope of online experimentation and further increase developers' agility. In this paper, we propose a novel formulation for online experimentation of cloud applications which generalizes traditional approaches applied to web and mobile applications by incorporating the unique challenges posed by cloud environments. To enable practitioners to apply our formulation, we develop and present JACKPOT, a system for online cloud experimentation in the presence of multiple interacting microservices. We discuss an initial prototype of JACKPOT along with a preliminary evaluation of this prototype based on experiments on a public container cloud.