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website: Revise the "Output Values" configuration docs section 

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.
This commit is contained in:
Martin Atkins 2018-05-05 19:53:38 -07:00
parent f8f2ff2202
commit e41883d972
2 changed files with 97 additions and 77 deletions
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website/docs/configuration/outputs.html.md
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@ -1,106 +1,122 @@
---
layout: "docs"
page_title: "Configuring Outputs"
page_title: "Configuring Output Values"
sidebar_current: "docs-config-outputs"
description: |-
Outputs define values that will be highlighted to the user when Terraform applies, and can be queried easily using the output command. Output usage is covered in more detail in the getting started guide. This page covers configuration syntax for outputs.
Output values are the return values of a Terraform module.
---
# Output Configuration
# Output Values
Outputs define values that will be highlighted to the user
when Terraform applies, and can be queried easily using the
[output command](/docs/commands/output.html). Output usage
is covered in more detail in the
[getting started guide](/intro/getting-started/outputs.html).
This page covers configuration syntax for outputs.
Output values are like the return values of a Terraform module, allowing
a subset of the resource attributes within a child module to be exposed to
a parent module, or making certain values from a root module visible in the
CLI output after running `terraform apply`.
Terraform knows a lot about the infrastructure it manages.
Most resources have attributes associated with them, and
outputs are a way to easily extract and query that information.
Resource instances managed by Terraform each export attributes whose values
can be used elsewhere in configuration. Output values are a way to expose some
of that information to the user of your module.
This page assumes you are familiar with the
[configuration syntax](/docs/configuration/syntax.html)
already.
## Declaring an Output Value
## Example
A simple output configuration looks like the following:
Each output value exported by a module must be declared using an `output`
block:
```hcl
output "address" {
value = "${aws_instance.db.public_dns}"
output "instance_ip_addr" {
value = aws_instance.server.private_ip
}
```
This will output a string value corresponding to the public
DNS address of the Terraform-defined AWS instance named "db". It
is possible to export complex data types like maps and lists as
well:
The label immediately after the `output` keyword is the name that can be used
to access this output in the parent module, if any, or the name that will be
displayed to the user for output values in the root module.
For brevity, output values are often referred to simply as "outputs", where
the meaning is clear from context.
The `value` argument takes an [expression](/docs/configuration/expressions.html)
whose result is to be returned to the user. In this example, the expression
refers to the `private_ip` attribute exposed by an `aws_instance` resource
defined elsewhere in this module (not shown). Any valid expression is allowed
as an output value.
Several other optional arguments are allowed within `output` blocks. These
will be described in the following sections.
## Output Value Documentation
Because the output values of a module are part of the user interface of
the module, you may specify a short description of the purpose of each
value using the optional `description` argument:
```hcl
output "addresses" {
value = ["${aws_instance.web.*.public_dns}"]
output "instance_ip_addr" {
value = aws_instance.server.private_ip
description = "The private IP address of the main server instance."
}
```
## Description
The description for an output value should be a concise description of the
purpose of the variable and what kind of value is expected. This description
string may be included in documentation about the module, and so it should be
written from the perspective of the user of the module rather than its
maintainer. For commentary for module maintainers, use comments.
The `output` block configures a single output variable. Multiple
output variables can be configured with multiple output blocks.
The `NAME` given to the output block is the name used to reference
the output variable. It must conform to Terraform variable naming
conventions if it is to be used as an input to other modules.
## Sensitive Output Values
Within the block (the `{ }`) is configuration for the output.
These are the parameters that can be set:
- `value` (required) - The value of the output. This can be a string, list, or
map. This usually includes an interpolation since outputs that are static
aren't usually useful.
- `description` (optional) - A human-friendly description for the output. This
is primarily for documentation for users using your Terraform configuration. A
future version of Terraform will expose these descriptions as part of some
Terraform CLI command.
- `depends_on` (list of strings) - Explicit dependencies that this output has.
These dependencies will be created before this output value is processed. The
dependencies are in the format of `TYPE.NAME`, for example `aws_instance.web`.
- `sensitive` (optional, boolean) - See below.
## Syntax
The full syntax is:
```text
output NAME {
value = VALUE
}
```
## Sensitive Outputs
Outputs can be marked as containing sensitive material by setting the
`sensitive` attribute to `true`, like this:
An output can be marked as containing sensitive material using the optional
`sensitive` argument:
```hcl
output "sensitive" {
sensitive = true
value = VALUE
output "db_password" {
value = aws_db_instance.db.password
description = "The password for logging in to the database."
sensitive = true
}
```
When outputs are displayed on-screen following a `terraform apply` or
`terraform refresh`, sensitive outputs are redacted, with `<sensitive>`
displayed in place of their value.
Setting an output value in the root module as sensitive prevents Terraform
from showing its value at the end of `terraform apply`. It may still be shown
in the CLI output for other reasons, such as if the value is referenced in
an expression for a resource argument.
### Limitations of Sensitive Outputs
Sensitive output values are still recorded in the
[state](/docs/state/index.html), and so will be visible to anyone who is able
to access the state data. For more information, see
[_Sensitive Data in State_](/docs/state/sensitive-data.html).
- The values of sensitive outputs are still stored in the Terraform state, and
available using the `terraform output` command, so cannot be relied on as a
sole means of protecting values.
## Output Dependencies
- Sensitivity is not tracked internally, so if the output is interpolated in
another module into a resource, the value will be displayed.
Since output values are just a means for passing data out of a module, it is
usually not necessary to worry about their relationships with other nodes in
the dependency graph.
However, when a parent module accesses an output value exported by one of its
child modules, the dependencies of that output value allow Terraform to
correctly determine the dependencies between resources defined in different
modules.
Just as with
[resource dependencies](/docs/configuration/resources.html#resource-dependencies),
Terraform analyzes the `value` expression for an output value and autmatically
determines a set of dependencies, but in less-common cases there are
dependencies that cannot be recognized implicitly. In these rare cases, the
`depends_on` argument can be used to create additional explicit dependencies:
```hcl
output "instance_ip_addr" {
value = aws_instance.server.private_ip
description = "The private IP address of the main server instance."
depends_on = [
# Security group rule must be created before this IP address could
# actually be used, otherwise the services will be unreachable.
aws_security_group_rule.local_access,
]
}
```
The `depends_on` argument should be used only as a last resort. When using it,
always include a comment explaining why it is being used, to help future
maintainers understand the purpose of the additional dependency.

6
website/docs/configuration/resources.html.md
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@ -100,7 +100,7 @@ Because of this, in most cases it is not necessary to mention explicitly
any dependencies between resources.
However, in some less-common situations there are dependencies between
resources that cannot be expressed directly in configuration. For example,
resources that cannot be recognized implicitly in configuration. For example,
if Terraform is being used to both manage access control policies _and_ take
actions that require those policies to be present, there may be a hidden
dependency between the access policy and a resource whose creation depends
@ -162,6 +162,10 @@ in the `depends_on` argument value, because its value must be known before
Terraform knows resource relationships and thus before it can safely
evaluate expressions.
The `depends_on` argument should be used only as a last resort. When using it,
always include a comment explaining why it is being used, to help future
maintainers understand the purpose of the additional dependency.
## Multiple Resource Instances
By default, a single `resource` block corresponds to only one real