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website: rewrite modules usage documentation for new features 

The modules mechanism has changed quite a bit for version 0.11 and so
although simple usage remains broadly compatible there are some
significant changes in the behavior of more complex modules.

Since large parts of this were rewritten anyway, I also took the
opportunity to do some copy-editing to make the prose on this page more
consistent with our usual editorial voice and to wrap the long
lines.
This commit is contained in:
Martin Atkins 2017-10-30 17:34:04 -07:00
parent 3ef86deb28
commit 91a4af9c8a
3 changed files with 296 additions and 101 deletions
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56
website/docs/configuration/modules.html.md
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@ -19,8 +19,6 @@ already.
## Example
Module configuration looks like the following:
```hcl
module "consul" {
source = "hashicorp/consul/aws"
@ -30,40 +28,30 @@ module "consul" {
## Description
The `module` block configures a module and tells Terraform to build
its resources. Multiple module blocks may be used to configure and use
multiple modules.
A `module` block instructs Terraform to create an instance of a module,
and in turn to instantiate any resources defined within it.
The NAME of the module is logical: it is used only to reference the
module in other places in the configuration. It has no effect on the
source of the module. Therefore, you may name modules whatever you'd like.
The name given in the block header is used to reference the particular module
instance from expressions within the calling module, and to refer to the
module on the command line. It has no meaning outside of a particular
Terraform configuration.
Within the block (the `{ }`) is configuration for the module.
The only required key is `source`, which tells Terraform where this module
can be downloaded from. Valid source values are covered in more detail
in the
[module section](/docs/modules/index.html).
Within the block body is the configuration for the module. All attributes
within the block must correspond to [variables](/docs/configuration/variables.html)
within the module, with the exception of the following which Terraform
treats as special:
Other configuration within the module are dependent on the module itself.
Module configuration maps directly to
[variables](/docs/configuration/variables.html) within the module, so
parameters can have any of the data types that variables support, including
lists and maps.
* `source` - (Required) A [module source](/docs/modules/sources.html) string
specifying the location of the child module source code.
## Syntax
* `version` - (Optional) A [version constraint](/docs/modules/usage.html#module-versions)
string that specifies which versions of the referenced module are acceptable.
The newest version matching the constraint will be used. `version` is supported
only for modules retrieved from module registries.
The full syntax is:
```text
module NAME {
source = SOURCE_URL
CONFIG ...
}
```
where `CONFIG` is:
```text
KEY = VALUE
```
* `providers` - (Optional) A map whose keys are provider configuration names
that are expected by child module and whose values are corresponding
provider names in the calling module. This allows
[provider configurations to be passed explicitly to child modules](/docs/modules/usage.html#providers-within-modules).
If not specified, the child module inherits all of the default (un-aliased)
provider configurations from the calling module.

6
website/docs/configuration/variables.html.md
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@ -69,6 +69,12 @@ These are the parameters that can be set:
future version of Terraform will expose these descriptions as part of some
Terraform CLI command.
The name of a variable can be any valid identifier. However, due to the
interpretation of [module configuration blocks](/docs/configuration/modules.html),
the names `source`, `version` and `providers` are reserved for Terraform's own
use and are thus not recommended for any module intended to be used as a
child module.
-> **Note**: Default values can be strings, lists, or maps. If a default is
specified, it must match the declared type of the variable.

335
website/docs/modules/usage.html.markdown
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@ -7,7 +7,7 @@ description: Using modules in Terraform is very similar to defining resources.
# Module Usage
Using modules in Terraform is very similar to defining resources:
Using child modules in Terraform is very similar to defining resources:
```shell
module "consul" {
@ -18,18 +18,237 @@ module "consul" {
You can view the full documentation for configuring modules in the [Module Configuration](/docs/configuration/modules.html) section.
In modules we only specify a name rather than a name and a type (as in resources). This name can be used elsewhere in the configuration to reference the module and its outputs.
In modules we only specify a name, rather than a name and a type as for resources.
This name is used elsewhere in the configuration to reference the module and
its outputs.
The source tells Terraform what to create. In this example, we create
The source tells Terraform what to create. In this example, we instantiate
the [Consul module for AWS](https://registry.terraform.io/modules/hashicorp/consul/aws)
from the [Terraform Registry](https://registry.terraform.io). You can learn
more about the [source configuration below](#source).
from the [Terraform Registry](https://registry.terraform.io). Other source
types are supported, as described in the following section.
Just like a resource, the module configuration can be deleted to remove the module.
Just like a resource, the a module's configuration can be deleted to destroy the
resources belonging to the module.
## Multiple instances of a module
## Source
You can instantiate a module multiple times.
The only required configuration key for a module is the `source` parameter. The
value of this tells Terraform where to download the module's source code.
Terraform comes with support for a variety of module sources.
The recommended source for external modules is a
[Terraform Registry](/docs/registry/index.html), which provides the full
capabilities of modules such as version constraints.
Registry modules are specified using a simple slash-separated path like the
`hashicorp/consul/aws` path used in the above example. The full source string
for each registry module can be found from the registry website.
Terraform also supports modules in local directories, identified by a relative
path starting with either `./` or `../`. Such local modules are useful to
organize code more complex repositories, and are described in more detail
in [_Creating Modules_](/docs/modules/create.html).
Finally, Terraform can download modules directly from various storage providers
and version control systems. These sources do not support versioning and other
registry benefits, but can be convenient for getting started when already
available within an organization. The full list of available sources
are documented in [the module sources documentation](/docs/modules/sources.html).
When a configuration uses modules, they must first be installed by running
[`terraform init`](/docs/commands/init.html):
```shell
$ terraform init
```
This command will download any modules that haven't been updated already,
as well as performing other Terraform working directory initialization such
as installing providers.
By default the command will not check for available updates to already-installed
modules, but you can use the `-update` option to check for available upgrades.
When version constraints are specified (as described in the following section)
a newer version will be used only if it is within the given constraint.
## Module Versions
It is recommended to explicitly constrain the acceptable version numbers for
each external module so that upstream changes aren't automatically adopted,
since this may result in unexpected or unwanted changes changes.
The `version` attribute within the `module` block is used for this purpose:
```shell
module "consul" {
source = "hashicorp/consul/aws"
version = "0.0.5"
servers = 3
}
```
The `version` attribute value may either be a single explicit version or
a version constraint expression. Constraint expressions use the following
syntax to specify a _range_ of versions that are acceptable:
* `>= 1.2.0`: version 1.2.0 or newer
* `<= 1.2.0`: version 1.2.0 or older
* `~> 1.2`: any non-beta patch release within the `1.2` range
* `>= 1.0.0, <= 2.0.0`: any version between 1.0.0 and 2.0.0 inclusive
When depending on third-party modules, references to specific versions are
recommended since this ensures that updates only happen when convenient to you.
For modules maintained within your organization, a version range strategy
may be appropriate if a semantic versioning methodology is used consistently
or if there is a well-defined release process that avoids unwanted updates.
Version constraints are supported only for modules installed from a module
registry, such as the [Terraform Registry](https://registry.terraform.io/).
Other module sources may provide their own versioning mechanisms within the
source string itself, or they may not support versions at all. In particular,
modules whose sources are local file paths do not support `version` because
they are constrained to share the same version as their caller by being
obtained by the same source repository.
## Configuration
The arguments used in a `module` block, such as the `servers` parameter above,
correspond to [variables](/docs/configuration/variables.html) within the module
itself. You can therefore discover all the available variables for a module by
inspecting the source of it.
The special arguments `source`, `version` and `providers` are exceptions. These
are used for special purposes by Terraform and should therefore not be used
as variable names within a module.
## Outputs
Modules encapsulate their resources. A resource in one module cannot directly depend on resources or attributes in other modules, unless those are exported through [outputs](/docs/configuration/outputs.html). These outputs can be referenced in other places in your configuration, for example:
```hcl
resource "aws_instance" "client" {
ami = "ami-408c7f28"
instance_type = "t1.micro"
availability_zone = "${module.consul.server_availability_zone}"
}
```
This is deliberately very similar to accessing resource attributes. Instead of
referencing a resource attribute, however, the expression in this case
references an output of the module.
Just like with resources, interpolation expressions can create implicit
dependencies on resources and other modules. Since modules encapsulate
other resources, however, the dependency is not on the module as a whole
but rather on the `server_availability_zone` output specifically, which
allows Terraform to work on resources in different modules concurrently rather
than waiting for the entire module to be complete before proceeding.
## Providers within Modules
For convenience in simple configurations, child modules by default inherit
provider configurations from their parent. This means that in most cases
only the root module needs explicit `provider` blocks, and then any defined
provider can be freely used with the same settings in child modules.
In more complex situations it may be necessary for a child module to use
different provider settings than its parent. In this situation it is
possible to define
[multiple provider instances](/docs/configuration/providers.html#multiple-provider-instances)
and pass them explicitly and selectively to a child module:
```hcl
# The default "aws" configuration is used for AWS resources in the root
# module where no explicit provider instance is selected.
provider "aws" {
region = "us-west-1"
}
# A non-default, or "aliased" configuration is also defined for a different
# region.
provider "aws" {
alias = "usw2"
region = "us-west-2"
}
# An example child module is instantiated with the _aliased_ configuration,
# so any AWS resources it defines will use the us-west-2 region.
module "example" {
source = "./example"
providers = {
aws = "aws.usw2"
}
}
```
The `providers` argument within a `module` block serves the same purpose as
the `provider` argument within a resource as described for
[multiple provider instances](/docs/configuration/providers.html#multiple-provider-instances),
but is a map rather than a single string because a module may contain resources
from many different providers.
Once the `providers` argument is used in a `module` block it overrides all of
the default inheritance behavior, so it is necessary to enumerate mappings
for _all_ of the required providers. This is to avoid confusion and surprises
when mixing both implicit and explicit provider passing.
In more complex situations it may be necessary for a child module _itself_
to have multiple instances of the same provider. For example, a module
that configures connectivity between networks in two AWS regions is likely
to need both a source and a destination region. In that case, the root module
may look something like this:
```hcl
provider "aws" {
alias = "usw1"
region = "us-west-1"
}
provider "aws" {
alias = "usw2"
region = "us-west-2"
}
module "tunnel" {
source = "./tunnel"
providers = {
"aws.src" = "aws.usw1"
"aws.dst" = "aws.usw2"
}
}
```
The subdirectory `./tunnel` should then contain configuration like the
following, to declare the two provider aliases it expects:
```
provider "aws" {
alias = "src"
}
provider "aws" {
alias = "dst"
}
```
Each resource should then have its own `provider` attribute set to either
`"aws.src"` or `"aws.dst"` to choose which of the provider instances to use.
It is recommended to use the default inheritance behavior in most cases where
only a single default instance of each provider is used, and switch to
passing providers explicitly as soon as multiple instances are needed.
In all cases it is recommended to keep explicit provider declarations only in
the root module and pass them (either implicitly or explicitly) down to
descendent modules. This avoids the provider configurations being "lost"
when descendent providers are removed from the configuration. It also allows
the user of a configuration to determine which providers require credentials
by inspecting only the root module.
## Multiple Instances of a Module
A particular module source can be instantiated multiple times:
```hcl
# my_buckets.tf
@ -50,7 +269,7 @@ module "media_bucket" {
variable "name" {} # this is the input parameter of the module
resource "aws_s3_bucket" "the_bucket" {
resource "aws_s3_bucket" "example" {
# ...
}
@ -59,82 +278,64 @@ resource "aws_iam_user" "deploy_user" {
}
```
In this example you define a module in the `./publish_bucket` subdirectory. That module has configuration to create a bucket resource, set access and caching rules. The module wraps the bucket and all the other implementation details required to configure a bucket.
This example defines a local child module in the `./publish_bucket`
subdirectory. That module has configuration to create an S3 bucket. The module
wraps the bucket and all the other implementation details required to configure
a bucket.
We can then define the module multiple times in our configuration by naming each instantiation of the module uniquely, here `module "assets_bucket"` and `module "media_bucket"`, whilst specifying the same module `source`.
We can then instantiate the module multiple times in our configuration by
giving each instance a unique name -- here `module "assets_bucket"` and
`module "media_bucket"` -- whilst specifying the same `source` value.
The resource names in your module get prefixed by `module.<module-instance-name>` when instantiated, for example the `publish_bucket` module creates `aws_s3_bucket.the_bucket` and `aws_iam_access_key.deploy_user`. The full name of the resulting resources will be `module.assets_bucket.aws_s3_bucket.the_bucket` and `module.assets_bucket.aws_iam_access_key.deploy_user`. Be cautious of this when extracting configuration from your files into a module, the name of your resources will change and Terraform will potentially destroy and recreate them. Always check your configuration with `terraform plan` before running `terraform apply`.
Resources from child modules are prefixed with `module.<module-instance-name>`
when displayed in plan output and elsewhere in the UI. For example, the
`./publish_bucket` module contains `aws_s3_bucket.example`, and so the two
instances of this module produce S3 bucket resources with [_resource addresses_](/docs/internals/resource-addressing.html)
`module.assets_bucket.aws_s3_bucket.example` and `module.media_bucket.aws_s3_bucket.example`
respectively. These full addresses are used within the UI and on the command
line, but are not valid within interpolation expressions due to the
encapsulation behavior described above.
## Source
When refactoring an existing configuration to introduce modules, moving
resource blocks between modules causes Terraform to see the new location
as an entirely separate resource to the old. Always check the execution plan
after performing such actions to ensure that no resources are surprisingly
deleted.
The only required configuration key for a module is the `source` parameter. The value of this tells Terraform where the module can be downloaded, updated, etc. Terraform comes with support for a variety of module sources. The recommended source for modules is a
[Terraform Registry](/docs/registry/index.html) since this enables additional
features for modules such as versioning.
Each instance of a module may optionally have different providers passed to it
using the `providers` argument described above. This can be useful in situations
where, for example, a duplicated set of resources must be created across
several regions or datacenters.
Terraform can also download modules directly from various storage providers
and version control systems. These sources do not support versioning and other
registry benefits. The full list of available sources
are documented in the [module sources documentation](/docs/modules/sources.html).
## Summarizing Modules in the UI
Prior to running any Terraform command with a configuration that uses modules, you'll have to [get](/docs/commands/get.html) the modules. This is done using the [get command](/docs/commands/get.html).
By default, commands such as the [plan command](/docs/commands/plan.html) and
[graph command](/docs/commands/graph.html) will show each resource in a nested
module to represent the full scope of the configuration. For more complex
configurations, the `-module-depth` option may be useful to summarize some or all
of the modules as single objects.
```shell
$ terraform get
```
This command will download the modules if they haven't been already.
By default, the command will not check for updates, so it is safe (and fast) to run multiple times. You can use the `-update` flag to check and download updates.
## Configuration
The parameters used to configure modules, such as the `servers` parameter above, map directly to [variables](/docs/configuration/variables.html) within the module itself. Therefore, you can quickly discover all the configuration
for a module by inspecting the source of it.
Additionally, because these map directly to variables, module configuration can have any data type available for variables, including maps and lists.
## Outputs
Modules encapsulate their resources. A resource in one module cannot directly depend on resources or attributes in other modules, unless those are exported through [outputs](/docs/configuration/outputs.html). These outputs can be referenced in other places in your configuration, for example:
```hcl
resource "aws_instance" "client" {
ami = "ami-408c7f28"
instance_type = "t1.micro"
availability_zone = "${module.consul.server_availability_zone}"
}
```
This purposely is very similar to accessing resource attributes. Instead of mapping to a resource, however, the variable in this case maps to an output of a module.
Just like resources, this will create a dependency from the `aws_instance.client` resource to the module, so the module will be built first.
To use module outputs via command line you have to specify the module name before the variable, for example:
```shell
$ terraform output -module=consul server_availability_zone
```
## Plans and Graphs
Commands such as the [plan command](/docs/commands/plan.html) and [graph command](/docs/commands/graph.html) will expand modules by default. You can use the `-module-depth` parameter to limit the graph.
For example, with a configuration similar to what we've built above, here is what the graph output looks like by default:
For example, with a configuration similar to what we've built above, the default
graph output looks like the following:
![Terraform Expanded Module Graph](docs/module_graph_expand.png)
If instead we set `-module-depth=0`, the graph will look like this:
If we instead set `-module-depth=0`, the graph will look like this:
![Terraform Module Graph](docs/module_graph.png)
Other commands work similarly with modules. Note that the `-module-depth` flag is purely a formatting flag; it doesn't affect what modules are created or not.
Other commands work similarly with modules. Note that `-module-depth` only
affects how modules are presented in the UI; it does not affect how modules
and their contained resources are processed by Terraform operations.
## Tainting resources within a module
The [taint command](/docs/commands/taint.html) can be used to _taint_ specific resources within a module:
The [taint command](/docs/commands/taint.html) can be used to _taint_ specific
resources within a module:
```shell
$ terraform taint -module=salt_master aws_instance.salt_master
```
It is currently not possible to taint an entire module.
It is not possible to taint an entire module. Instead, each resource within
the module must be tainted separately.