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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:

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:

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:

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:

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​