Infrastructure as Code Theory

• Define your current world state as a tree. Usually this is cached from a previous run of a tool or through an import process.
• Define the desired world state as a tree.
• Run a tree differencing algorithm to create a change tape from current world state to desired world state.
• Convert the change tape to a set of runnable tasks.

Policy enforcement tools can verify the desired world state or the change tape (ex. Pulumi's "guard rails".)

• Tree differencing algorithms

Database schema migration tools can work similarly. Differencing two schemas and creating a tape to migrate data from one format to another.