Joy Clark talks with Chad Fowler about legacy software and immutable infrastructure. They begin by discussing legacy software and why the description 'legacy' shouldn't actually be a bad thing. Then they contrast how reusable libraries differ from a system of services which is flexible and can change over time. Chad then describes how they developed Wunderlist using a microservices architecture and answers questions about how to practically develop and test such a system and how to decrease coupling between components. They wrap up by discussing how the principles of immutable infrastructure can be applied to software development in general.