The principle of deterministic functions with well-defined inputs and outputs is not only good for fault tolerance (see “Idempotence”), but also simplifies reasoning about the dataflows in an organization [7]. No matter whether the derived data is a search index, a statistical model, or a cache, it is helpful to think in terms of data pipelines that derive one thing from another, pushing state changes in one system through functional application code and applying the effects to derived systems.
