Cloud-Based Control: A Multi-tenant, Horizontally Scalable Soft-PLC

In industrial automation, the control and operation of factories and plants in usually performed by Programmable Logic Controllers (PLCs). As automation tasks find their way into more and more smaller application areas where it is economically not reasonable to employ, engineer and maintain a full, hardware-based PLC, a cloud-based software PLC (Soft-PLC) can serve as a Control-as-a-Service solution. In this paper, we address the question how a Soft-PLC can be augmented with cloud-required aspects such as elastic scalability and multi-tenancy. We introduce an architecture that realizes these properties and allows for the implementation of a multi-tenant, horizontally scalable Soft-PLC. Furthermore, we evaluate the most important quality attributes, i.e. Roundtrip times and scalability of our architecture based on a prototypical implementation and a case study from the building automation domain. Finally, we present lessons learned and give an outlook to further realistic scenarios for cloud-based control in industrial applications.