Chapter Five

Iteration and Conditionals

How Terraform makes many resources from one block, transforms data inline, and branches without an if statement — and why the choice between count and for_each decides what your state churns.

6 topics

HCL has no for loop and no if statement. What it has instead are meta-arguments that repeat a resource block, expressions that reshape a collection, and a single ternary that branches. This chapter is where you learn to make Terraform produce ten subnets, three IAM roles, or zero of a thing, all from one block that reads cleanly in a plan.

The most consequential decision in the whole course lives here: count versus for_each. Pick wrong and a one-line edit to a list destroys and recreates resources you never touched. The rest — for expressions, the ternary, splat — are the tools that build the exact data structures iteration needs and pull values back out of the results.

Topics in This Chapter

Making N copies of a resource from one block, the ? 1 : 0 conditional-creation idiom, and the renumbering trap that makes index-based identity dangerous.

One instance per map or set element, keyed by a stable identity so removing one element never disturbs the others. The production-safe way to iterate.

count vs for_each

The decision rule behind a large share of "Terraform wants to destroy things I didn't change" incidents, and how to migrate from one to the other without recreation.

for Expressions

Transforming one collection into another — mapping, filtering, grouping, and reshaping lists and maps inline to build the structures that feed for_each.

Conditional Expressions

The ternary that is HCL's only branch: toggling creation, selecting per-environment values, and supplying defaults — with the type rule that trips people up.

Splat Expressions

The [*] shorthand for pulling one attribute out of a list of objects, why it doesn't work on a for_each map, and when to reach for a full for instead.