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Rewrite of the module usage page 

Tidied up the page and fixed some formatting, style and naming issues.
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@ -16,17 +16,11 @@ module "consul" {
}
```
You can view the full documentation for the syntax of configuring
modules [here](/docs/configuration/modules.html).
You can view the full documentation for configuring modules in the [Module Configuration](/docs/configuration/modules.html) section.
As you can see, it is very similar to defining resources, with the exception
that we don't specify a type, and just a name. This name can be used elsewhere
in the configuration to reference the module and its variables.
As you can see, configuring modules is very similar to defining resources, with the exception that we only specify a name rather than a name and a type. This name can be used elsewhere in the configuration to reference the module and its variables.
The existence of the above configuration will tell Terraform to create
the resources in the "consul" module which can be found on GitHub with the
given URL. Just like a resource, the module configuration can be deleted
to remove the module.
The existence of the above configuration will tell Terraform to create the resources in the `consul` module which can be found on GitHub at the given URL. Just like a resource, the module configuration can be deleted to remove the module.
## Multiple instances of a module
@ -58,33 +52,18 @@ resource "aws_iam_user" "deploy_user" {
}
```
In this example you can provide module implementation in the `./publish_bucket`
subfolder - define there, how to create a bucket resource, set access and
caching rules, create e.g. a CloudFront resource, which wraps the bucket and
all the other implementation details, which are common to your project.
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.
In the snippet above, you now use your module definition twice. The string
after the `module` keyword is a name of the instance of the module.
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`.
Note: the resource names in your implementation get prefixed by the
`module.<module-instance-name>` when instantiated. Example: your `publish_bucket`
implementation 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`. So beware, if you
extract your implementation to a module. The resource names will change and
this will lead to destroying s3 buckets and creating new ones - so always
check with `tf plan` before running `tf apply`.
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`.
## Source
The only required configuration key 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. These
are documented on a [separate page](/docs/modules/sources.html).
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. These
are documented in the [Module sources documentation](/docs/modules/sources.html).
Prior to running any command such as `plan` 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).
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).
```
$ terraform get
@ -92,25 +71,19 @@ $ 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.
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 very easily.
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 supported by variables, including maps and lists.
Additionally, because these map directly to variables, module configuration can have any data type available for variables, including maps and lists.
## Outputs
Modules can also specify their own [outputs](/docs/configuration/outputs.html).
These outputs can be referenced in other places in your configuration.
For example:
Modules can also specify their own [outputs](/docs/configuration/outputs.html). These outputs can be referenced in other places in your configuration, for example:
```
resource "aws_instance" "client" {
@ -120,44 +93,34 @@ resource "aws_instance" "client" {
}
```
This purposely is very similar to accessing resource attributes. But instead
of mapping to a resource, the variable in this case maps to an output of
a module.
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.
Just like resources, this will create a dependency from the `aws_instance.client` resource to the module, so the module will be built first.
## Plans and Graphs
With modules, 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.
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, here is what the graph output looks like by default:
<div class="center">
![Terraform Expanded Module Graph](docs/module_graph_expand.png)
</div>
But if we set `-module-depth=0`, the graph will look like this:
If instead we set `-module-depth=0`, the graph will look like this:
<div class="center">
![Terraform Module Graph](docs/module_graph.png)
</div>
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 the `-module-depth` flag is purely a formatting flag; it doesn't affect what modules are created or not.
## 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:
```
terraform taint -module=salt_master aws_instance.salt_master
```
It is not (yet) possible to taint an entire module.
It is currently not possible to taint an entire module.