At Pinterest, we recently completed a rewrite our iOS app (http://www.wired.com/2016/04/pinterest-reinvents-prove-really-worth-billions/). A lot of the work in an app rewrite goes unnoticed by the general public. Besides the sleek design and cool animations on the surface, one of the goals of our re-architecture was to move to a completely immutable model layer. This talk will focus on how we keep data consistent within our app with immutable models. We'll walk through updating models, loading and caching server data, and the architecture of our consistency framework.