79 lines
2.6 KiB
Markdown
79 lines
2.6 KiB
Markdown
---
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layout: "language"
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page_title: "The depends_on Meta-Argument - Configuration Language"
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description: |-
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Use the Terraform language `depends_on` meta-argument to handle hidden resource or module dependencies.
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---
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# The `depends_on` Meta-Argument
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-> **Version note:** Module support for `depends_on` was added in Terraform 0.13, and
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previous versions can only use it with resources.
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Use the `depends_on` meta-argument to handle hidden resource or module dependencies that
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Terraform can't automatically infer.
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Explicitly specifying a dependency is only necessary when a resource or module relies on
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some other resource's behavior but _doesn't_ access any of that resource's data
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in its arguments.
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This argument is available in `module` blocks and in all `resource` blocks,
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regardless of resource type. For example:
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```hcl
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resource "aws_iam_role" "example" {
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name = "example"
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# assume_role_policy is omitted for brevity in this example. See the
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# documentation for aws_iam_role for a complete example.
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assume_role_policy = "..."
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}
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resource "aws_iam_instance_profile" "example" {
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# Because this expression refers to the role, Terraform can infer
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# automatically that the role must be created first.
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role = aws_iam_role.example.name
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}
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resource "aws_iam_role_policy" "example" {
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name = "example"
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role = aws_iam_role.example.name
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policy = jsonencode({
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"Statement" = [{
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# This policy allows software running on the EC2 instance to
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# access the S3 API.
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"Action" = "s3:*",
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"Effect" = "Allow",
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}],
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})
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}
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resource "aws_instance" "example" {
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ami = "ami-a1b2c3d4"
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instance_type = "t2.micro"
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# Terraform can infer from this that the instance profile must
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# be created before the EC2 instance.
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iam_instance_profile = aws_iam_instance_profile.example
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# However, if software running in this EC2 instance needs access
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# to the S3 API in order to boot properly, there is also a "hidden"
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# dependency on the aws_iam_role_policy that Terraform cannot
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# automatically infer, so it must be declared explicitly:
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depends_on = [
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aws_iam_role_policy.example,
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]
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}
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```
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The `depends_on` meta-argument, if present, must be a list of references
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to other resources or child modules in the same calling module.
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Arbitrary expressions are not allowed in the `depends_on` argument value,
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because its value must be known before Terraform knows resource relationships
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and thus before it can safely evaluate expressions.
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The `depends_on` argument should be used only as a last resort. When using it,
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always include a comment explaining why it is being used, to help future
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maintainers understand the purpose of the additional dependency.
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