As functional programmers we work hard to keep things immutable and referentially transparent. However, these noble pursuits often stop with our text editors, the resulting code flung over the wall to be built and deployed in mutable, imperative systems. In this talk we will take a look at Nelson, a deployment orchestration system that applies functional programming not only in its implementation, but also in its system behavior in managing the messy world of deployment infrastructure. Where Free algebras and streams allow the code to be easily extended to support different source repositories, schedulers, and health checkers, Nelson's strict stance on immutable deployments inform important system level decisions such as deployment workflows and service discovery. Having now been run at a couple of companies under different configurations, Nelson serves as a prime example of how functional programming is for the real world and can influence not just code, but systems as well.