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e41883d972
This adopts a more guide-like writing style, similar to what prior commits have done to some other subsections of this section. In the process of writing this, I identified some unclear statements in the "Resources" subsection, and so these are also revised here for consistency with the output values documentation.
123 lines
4.5 KiB
Markdown
123 lines
4.5 KiB
Markdown
---
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layout: "docs"
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page_title: "Configuring Output Values"
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sidebar_current: "docs-config-outputs"
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description: |-
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Output values are the return values of a Terraform module.
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---
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# Output Values
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Output values are like the return values of a Terraform module, allowing
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a subset of the resource attributes within a child module to be exposed to
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a parent module, or making certain values from a root module visible in the
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CLI output after running `terraform apply`.
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Resource instances managed by Terraform each export attributes whose values
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can be used elsewhere in configuration. Output values are a way to expose some
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of that information to the user of your module.
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## Declaring an Output Value
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Each output value exported by a module must be declared using an `output`
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block:
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```hcl
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output "instance_ip_addr" {
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value = aws_instance.server.private_ip
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}
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```
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The label immediately after the `output` keyword is the name that can be used
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to access this output in the parent module, if any, or the name that will be
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displayed to the user for output values in the root module.
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For brevity, output values are often referred to simply as "outputs", where
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the meaning is clear from context.
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The `value` argument takes an [expression](/docs/configuration/expressions.html)
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whose result is to be returned to the user. In this example, the expression
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refers to the `private_ip` attribute exposed by an `aws_instance` resource
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defined elsewhere in this module (not shown). Any valid expression is allowed
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as an output value.
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Several other optional arguments are allowed within `output` blocks. These
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will be described in the following sections.
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## Output Value Documentation
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Because the output values of a module are part of the user interface of
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the module, you may specify a short description of the purpose of each
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value using the optional `description` argument:
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```hcl
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output "instance_ip_addr" {
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value = aws_instance.server.private_ip
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description = "The private IP address of the main server instance."
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}
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```
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The description for an output value should be a concise description of the
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purpose of the variable and what kind of value is expected. This description
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string may be included in documentation about the module, and so it should be
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written from the perspective of the user of the module rather than its
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maintainer. For commentary for module maintainers, use comments.
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## Sensitive Output Values
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An output can be marked as containing sensitive material using the optional
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`sensitive` argument:
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```hcl
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output "db_password" {
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value = aws_db_instance.db.password
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description = "The password for logging in to the database."
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sensitive = true
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}
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```
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Setting an output value in the root module as sensitive prevents Terraform
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from showing its value at the end of `terraform apply`. It may still be shown
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in the CLI output for other reasons, such as if the value is referenced in
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an expression for a resource argument.
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Sensitive output values are still recorded in the
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[state](/docs/state/index.html), and so will be visible to anyone who is able
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to access the state data. For more information, see
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[_Sensitive Data in State_](/docs/state/sensitive-data.html).
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## Output Dependencies
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Since output values are just a means for passing data out of a module, it is
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usually not necessary to worry about their relationships with other nodes in
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the dependency graph.
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However, when a parent module accesses an output value exported by one of its
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child modules, the dependencies of that output value allow Terraform to
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correctly determine the dependencies between resources defined in different
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modules.
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Just as with
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[resource dependencies](/docs/configuration/resources.html#resource-dependencies),
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Terraform analyzes the `value` expression for an output value and autmatically
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determines a set of dependencies, but in less-common cases there are
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dependencies that cannot be recognized implicitly. In these rare cases, the
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`depends_on` argument can be used to create additional explicit dependencies:
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```hcl
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output "instance_ip_addr" {
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value = aws_instance.server.private_ip
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description = "The private IP address of the main server instance."
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depends_on = [
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# Security group rule must be created before this IP address could
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# actually be used, otherwise the services will be unreachable.
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aws_security_group_rule.local_access,
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]
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}
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```
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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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