Terraform does not fail when input is missing. It replaces it.
The coalesce function selects the first value that is not null and not an empty string. This creates a fallback chain that keeps your configuration valid even when inputs are incomplete.
That behavior looks safe. It is not always safe.
If a variable is blank due to a pipeline error or misconfiguration, coalesce silently replaces it with a default. The plan succeeds. The resource deploys. The error signal disappears.
This lesson breaks down the exact mechanism behind coalesce, how Terraform resolves types before evaluation, and why this function only controls values, not whether resources exist.
The key takeaway is simple.coalesce is not just a helper. It is a decision to tolerate failure instead of exposing it.