terraform/website/docs/internals/resource-addressing.html.markdown

layout	page_title	sidebar_current	description
docs	Internals: Resource Address	docs-internals-resource-addressing	Resource addressing is used to target specific resources in a larger infrastructure.

Resource Addressing

A Resource Address is a string that references a specific resource in a larger infrastructure. An address is made up of two parts:

[module path][resource spec]

Module path

A module path addresses a module within the tree of modules. It takes the form:

module.module_name[module index]

• module - Module keyword indicating a child module (non-root). Multiple module keywords in a path indicate nesting.
• module_name - User-defined name of the module.
• [module index] - (Optional) Index into a module with multiple instances, surrounded by square brace characters ([ and ]).

An address without a resource spec, i.e. module.foo applies to every resource within the module if a single module, or all instances of a module if a module has multiple instances. To address all resources of a particular module instance, include the module index in the address, such as module.foo[0].

If the module path is omitted, the address applies to the root module.

An example of the module keyword delineating between two modules that have multiple instances:

module.foo[0].module.bar["a"]

-> Module index only applies to modules in Terraform v0.13 or later, as in earlier versions of Terraform, a module could not have multiple instances.

Resource spec

A resource spec addresses a specific resource in the config. It takes the form:

resource_type.resource_name[resource index]

• resource_type - Type of the resource being addressed.
• resource_name - User-defined name of the resource.
• [resource index] - (Optional) Index into a resource with multiple instances, surrounded by square brace characters ([ and ]).

-> In Terraform v0.12 and later, a resource spec without a module path prefix matches only resources in the root module. In earlier versions, a resource spec without a module path prefix will match resources with the same type and name in any descendent module.

Index values for Modules and Resources

The following specifications apply to index values on modules and resources with multiple instances:

• [N] where N is a 0-based numerical index into a resource with multiple instances specified by the count meta-argument. Omitting an index when addressing a resource where count > 1 means that the address references all instances.
• ["INDEX"] where INDEX is a alphanumerical key index into a resource with multiple instances specified by the for_each meta-argument.

Examples

count Example

Given a Terraform config that includes:

resource "aws_instance" "web" {
  # ...
  count = 4
}

An address like this:

aws_instance.web[3]

Refers to only the last instance in the config, and an address like this:

aws_instance.web

Refers to all four "web" instances.

for_each Example

Given a Terraform config that includes:

resource "aws_instance" "web" {
  # ...
  for_each = {
    "terraform": "value1",
    "resource":  "value2",
    "indexing":  "value3",
    "example":   "value4",
  }
}

An address like this:

aws_instance.web["example"]

Refers to only the "example" instance in the config.