Abstract: For modern data center switches, the ability to—with minimum latency and maximum flexibility— react to current network conditions is important for managing increasingly dynamic networks. The traditional approach to implementing this type of behavior is through a control plane that is orders of magnitude slower than the speed at which typical data center congestion events occur. More recent alternatives like programmable switches can remember statistics about passing traffic and adjust behavior accordingly, but unfortunately, their capabilities severely limit what can be done.In this paper, we present Mantis, a framework for implementing fine-grained reactive behavior on today’s programmable switches with the help of a specialized reactive control plane architecture. Mantis is, thus, a combination of language for specifying dynamic components of packet processing and an optimized, general, and safe control loop for implementing them. Mantis provides a simple-to-reason-about set of abstractions for users, and the Mantis control plane can react to changes in the network in 10s of μs.