The Terraform Knows What It Should Provision At All Times.

the terraform knows what it should provision at all times. it knows this because it knows what it hasn’t provisioned.

by subtracting what is from what ought to be (or what ought to be from what is, depending on provider drift), it obtains a plan, or diff.

the planning subsystem uses diffs to generate imperative operations to drive the infrastructure from a state that is, to a state that isn’t, and in arriving at a state that wasn’t, it now is.

consequently, the state that now is, is not the state that was, and therefore the state that was is no longer what is. this is called idempotence.

in the event that the state that is does not match the state that should be, the system has acquired drift. drift is the discrepancy between the actual state and the desired state, and if drift is nontrivial, the terraform will attempt reconciliation.

however, terraform must also know what it last applied.

the terraform plan execution proceeds as follows: because drift may have invalidated some resource attributes, terraform is not exactly sure what is, but it is sure what should be, and within tolerances, what was.

it now subtracts what should be from what is not yet, or vice versa, and by interpolating this against the algebraic projection of what must not be, it obtains the execution plan.

this plan, when applied, corrects the infra such that what should be becomes, what was is forgotten, and what is not yet is now managed.

Happy Canada Day.