83 lines
2.5 KiB
Markdown
83 lines
2.5 KiB
Markdown
---
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layout: "docs"
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page_title: "Local Values - Configuration Language"
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sidebar_current: "docs-config-locals"
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description: |-
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Local values assign a name to an expression that can then be used multiple times
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within a module.
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---
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# Local Values
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-> **Note:** This page is about Terraform 0.12 and later. For Terraform 0.11 and
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earlier, see
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[0.11 Configuration Language: Local Values](../configuration-0-11/locals.html).
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A local value assigns a name to an [expression](./expressions.html),
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allowing it to be used multiple times within a module without repeating
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it.
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Comparing modules to functions in a traditional programming language:
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if [input variables](./variables.html) are analogous to function arguments and
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[outputs values](./outputs.html) are analogous to function return values, then
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_local values_ are comparable to a function's local temporary symbols.
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-> **Note:** For brevity, local values are often referred to as just "locals"
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when the meaning is clear from context.
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## Declaring a Local Value
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A set of related local values can be declared together in a single `locals`
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block:
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```hcl
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locals {
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service_name = "forum"
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owner = "Community Team"
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}
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```
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The expressions assigned to local value names can either be simple constants
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like the above, allowing these values to be defined only once but used many
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times, or they can be more complex expressions that transform or combine
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values from elsewhere in the module:
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```hcl
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locals {
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# Ids for multiple sets of EC2 instances, merged together
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instance_ids = concat(aws_instance.blue.*.id, aws_instance.green.*.id)
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}
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locals {
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# Common tags to be assigned to all resources
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common_tags = {
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Service = local.service_name
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Owner = local.owner
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}
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}
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```
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As shown above, local values can be referenced from elsewhere in the module
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with an expression like `local.common_tags`, and locals can reference
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each other in order to build more complex values from simpler ones.
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```
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resource "aws_instance" "example" {
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# ...
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tags = local.common_tags
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}
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```
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## When To Use Local Values
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Local values can be helpful to avoid repeating the same values or expressions
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multiple times in a configuration, but if overused they can also make a
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configuration hard to read by future maintainers by hiding the actual values
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used.
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Use local values only in moderation, in situations where a single value or
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result is used in many places _and_ that value is likely to be changed in
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future. The ability to easily change the value in a central place is the key
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advantage of local values.
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