Depends On

What Is Depends On in Terraform?

depends_on is a meta-argument you add inside a resource or module block to tell Terraform "create this only after that other thing is fully created" — even when nothing in the configuration's actual arguments points to that other thing.

Here is the part most tutorials skip: Terraform almost never needs you to specify order manually. If resource_b uses resource_a.id anywhere inside its arguments, Terraform automatically knows resource_a must exist first and builds that into its dependency graph behind the scenes. depends_on exists only for the rare cases where a dependency is real but invisible — nothing in the code references the other resource directly, yet the order genuinely matters.

At Hotstar, an engineer once created an EC2 instance with an IAM instance profile attached, right after creating the IAM role and policy in the same apply. The apply succeeded — Terraform created the role, then the instance. But the instance failed to read from S3 for the first 90 seconds, because AWS IAM permissions take a few seconds to propagate globally after creation, even though Terraform considers the role "done." Terraform had finished — AWS had not. depends_on cannot fix propagation delay by itself, but the same lesson — that "Terraform finished" and "AWS is ready" are not always the same moment — is exactly the class of invisible-dependency problem depends_on is meant for.

Implicit Dependency — What Terraform Figures Out on Its Own

Whenever you reference one resource's attribute inside another resource's arguments, Terraform builds the dependency automatically. No depends_on needed:

Explicit Dependency — When You Need depends_on

Sometimes two resources have a real ordering requirement but no argument actually references the other resource. This is when you reach for depends_on:

Without the depends_on line, Terraform might create the role and the Lambda function in parallel — and the Lambda could attempt its first invocation before the policy is attached, even though the role itself already exists.

Depends On with Modules

depends_on works on entire module blocks too — useful when a module's internal resources have no direct reference to another module's resources, but still need to wait for them:

The Performance Cost of Depends On

INFORMATION

Tip from a senior engineer: `depends_on` is not free. Terraform normally creates independent resources in parallel — speeding up applies significantly on large configs. Every `depends_on` you add forces those resources into a strict sequential order, even resources that have nothing to do with each other. Adding it "just to be safe" on ten resources can turn a two-minute apply into a fifteen-minute apply.

Common Over-Use of Depends On

COMMON MISTAKE / WARNING

Common mistake: reaching for `depends_on` to fix a plan error before checking whether a normal reference would solve it instead. Nine times out of ten, the real fix is referencing an attribute — like `aws_iam_role.lambda.arn` — which gives Terraform both the correct value AND the correct ordering for free. Only use `depends_on` when there is truly no attribute to reference.

Quick Reference