Strymon (pronounced "stream-on") is a system for carrying out online analyses and simulations of large datacenters. Strymon leverages existing logging and monitoring pipelines of modern production datacenters to ingest cross-layer events in a streaming fashion and predict possible effects of such events in what-if scenarios by simulating the hypothetical datacenter state alongside the real one. Strymon's ultimate goal is to drive research and innovation in datacenter management. It advances the current practices in that the hypothetical datacenter state is fully exposed to Strymon users in a timely manner (as well-defined data streams), enabling the composition of deep analytics and cross-layer simulations in real time and within the same execution engine.