An IoT system consists of hundreds of thousands of sensors, connectivity media, cloud analytics, and users. Building such a large and complex IoT system such as smart-city requires careful planning, selection of the right set of hardware and software technology and development of scalable computing infrastructure and applications which can handle millions of sensors feeding terabytes of data. Building applications for such large scale systems is really challenging and validating the functionality of such applications for scalability, reliability and business logic is even more complex. Research questions are: How can we achieve ultra-high messaging and message processing scalability across a fully distributed system and stack? Which technologies are suitable (zero-copy mechanisms in the OS, OS bypassing through direct hardware access, cloud functions, bulk transfers, GPU compute instances) and to which degree can they be implemented with existing commercial cloud vendors at which economic price tag?