# DRY Configuration with Locals

_Atmos Design Pattern_

Locals are file-scoped variables that reduce repetition within a single stack file. Use them to define values once and reference them throughout the file.

## Problem

You have repeated values in a stack file that you want to define once and reuse:

```yaml
components:
  terraform:
    vpc:
      vars:
        name: acme-prod-vpc
        tags:
          Project: acme-prod
    cluster:
      vars:
        name: acme-prod-cluster
        tags:
          Project: acme-prod
```

## Solution

Use locals to define the value once:

```yaml
locals:
  project: acme-prod

components:
  terraform:
    vpc:
      vars:
        name: "{{ .locals.project }}-vpc"
        tags:
          Project: "{{ .locals.project }}"
    cluster:
      vars:
        name: "{{ .locals.project }}-cluster"
        tags:
          Project: "{{ .locals.project }}"
```

## Building on Locals

Locals can reference other locals:

```yaml
locals:
  namespace: acme
  environment: prod
  prefix: "{{ .locals.namespace }}-{{ .locals.environment }}"

components:
  terraform:
    vpc:
      vars:
        name: "{{ .locals.prefix }}-vpc"
```

## Accessing Settings and Vars

Locals can also access `settings` and `vars` from the same file:

```yaml
settings:
  version: v1

vars:
  stage: prod

locals:
  label: "{{ .vars.stage }}-{{ .settings.version }}"
```

## Key Points

- **File-scoped**: Locals only exist within the file where they're defined.
- **Not inherited**: Locals from imported files are not accessible.
- **Use for DRY**: Extract repeated values to reduce duplication.

For cross-file values, use `vars` or `settings` instead.

## References

- [Locals Documentation](/stacks/locals)