Atmos YAML Functions

Atmos YAML Functions

Overview

YAML functions are the recommended way to add dynamic behavior to Atmos stack configurations. They use YAML explicit tags (the ! prefix) and operate on structured data after YAML parsing. They cannot break YAML syntax, are type-safe, and produce clear error messages.

All YAML functions support Go template expressions in their arguments. Atmos processes templates first, then executes the YAML functions.

Available YAML Functions

Supported Sections

YAML functions work in all Atmos stack manifest sections:

• vars, settings, env, metadata, command, component
• providers, overrides, backend, backend_type
• remote_state_backend, remote_state_backend_type

!terraform.state -- Fast State Backend Access (Recommended)

Reads outputs directly from the Terraform state backend without initialization. Supports S3, local, GCS, and azurerm backends. 10-100x faster than !terraform.output.

vars:
  # Two-parameter form: component + output (current stack)
  vpc_id: !terraform.state vpc vpc_id

  # Three-parameter form: component + stack + output
  vpc_id: !terraform.state vpc plat-ue2-prod vpc_id

  # Using Go templates for dynamic stack references
  vpc_id: !terraform.state vpc {{ .stack }} vpc_id

  # YQ expressions for complex outputs
  first_subnet: !terraform.state vpc .private_subnet_ids[0]
  db_host: !terraform.state config .config_map.username

  # Default values for unprovisioned components
  vpc_id: !terraform.state vpc .vpc_id // "default-vpc"

  # YQ string concatenation
  url: !terraform.state 'aurora-postgres .master_hostname | "jdbc:postgresql://" + . + ":5432"'

  # Bracket notation for keys with special characters
  key: !terraform.state security '.users["github-dependabot"].access_key_id'

Cold State and terraform plan --all

!terraform.state resolves configuration before Terraform plans a component. On a first aggregate plan, an upstream component may therefore have no state yet. Use a YQ // default for values that must exist at plan time, with a deterministic, provider-valid mock value:

vars:
  kms_key_arn: !terraform.state kms-key '.key_arn // "arn:aws:kms:us-east-2:000000000000:key/00000000-0000-0000-0000-000000000000"'

The deployed upstream output supersedes the fallback automatically. Dependency metadata controls deployment order; it does not create state before an aggregate plan.

Reusable Mocks vs. a One-Off // Default

The // default above is a per-expression fallback. For a producer component's mock outputs to be declared once and resolved consistently by every consumer, use the component's mocks: stack-config section together with --use-mocks (supported by atmos terraform plan and atmos describe component) instead of repeating a // default in every consuming expression:

components:
  terraform:
    vpc:
      mocks:
        vpc_id: vpc-mock1234
        private_subnet_ids: [subnet-a, subnet-b]
    app:
      vars:
        vpc_id: !terraform.state vpc vpc_id

atmos terraform plan app -s dev --use-mocks

mocks: is Terraform-only, never templated or YAML-function-processed, and (unlike a // default) requires the referenced component to declare mocks: -- with one exception: a // default in the caller's expression is still honored even when the referenced component declares no mocks: section at all, mirroring how a // default rescues a component with no real state. Do not conflate this feature with the //-default idiom above; they're separate mechanisms.

!terraform.output -- Remote State Access

Reads Terraform outputs by running terraform output. Requires Terraform initialization (downloading providers), which is significantly slower than !terraform.state. Use !terraform.state instead when your backend is supported.

vars:
  vpc_id: !terraform.output vpc vpc_id
  vpc_id: !terraform.output vpc plat-ue2-prod vpc_id
  vpc_id: !terraform.output vpc {{ .stack }} vpc_id
  first_subnet: !terraform.output vpc .private_subnet_ids[0]

!store -- Component-Aware Store Access

Reads values from configured stores (SSM Parameter Store, Redis, Artifactory, etc.) following the Atmos stack/component/key naming convention:

vars:
  vpc_id: !store prod/ssm vpc vpc_id
  vpc_id: !store prod/ssm plat-ue2-prod vpc vpc_id
  vpc_id: !store prod/ssm {{ .stack }} vpc vpc_id
  api_key: !store prod/ssm config api_key | default "not-set"
  db_host: !store prod/ssm config connection | query .host

!store.get -- Arbitrary Key Store Access

Reads arbitrary keys from stores without following the component/stack naming convention:

vars:
  db_password: !store.get ssm /myapp/prod/db/password
  feature_flag: !store.get ssm /features/new-feature | default "disabled"
  api_key: !store.get redis app-config | query .api.key
  config: !store.get redis "config-{{ .vars.region }}"

!secret -- Declared Secret Access

Use !secret for sensitive values declared under secrets.vars. Do not use raw !store calls for values that should be masked and lifecycle-managed as secrets.

components:
  terraform:
    app:
      secrets:
        vars:
          DATADOG_API_KEY:
            store: prod/ssm
            required: true
      vars:
        datadog_api_key: !secret DATADOG_API_KEY

!append and !unset -- Merge Control

Use !append when a child stack should add to an inherited list instead of replacing it. Use !unset when a child stack should remove inherited config.

vars:
  security_groups: !append
    - sg-extra
  deprecated_setting: !unset

!env -- Environment Variables

Reads from stack manifest env: sections (merged via inheritance) or OS environment variables:

vars:
  api_key: !env API_KEY
  app_name: !env APP_NAME my-app
  description: !env 'APP_DESC "my application"'

Resolution order: stack manifest env: sections -> OS environment variables -> default value.

!exec -- Shell Script Execution

Executes shell scripts and assigns the output:

vars:
  timestamp: !exec date +%s

  # Multi-line script
  result: |
    !exec
      foo=0
      for i in 1 2 3; do
        foo+=$i
      done
      echo $foo

  # Complex types must be returned as JSON
  config: !exec get-config.sh --format json

!include -- File Inclusion

Includes local or remote files, parsing them based on extension:

vars:
  config: !include ./config.yaml
  vpc_defaults: !include stacks/catalog/vpc/defaults.yaml
  region_config: !include https://raw.githubusercontent.com/org/repo/main/config.yaml
  cidr: !include ./vpc_config.yaml .vars.ipv4_primary_cidr_block
  vars: !include config/prod.tfvars
  description: !include ./description.md

Supported protocols: local files, HTTP/HTTPS, GitHub (github://), S3 (s3::), GCS (gcs::), SCP/SFTP, OCI.

!template -- Go Template Evaluation

Evaluates Go template expressions and converts JSON output to proper YAML types. Essential for handling complex outputs (maps, lists) from atmos.Component:

vars:
  subnet_ids: !template '{{ toJson (atmos.Component "vpc" .stack).outputs.private_subnet_ids }}'
  config: !template '{{ toJson (atmos.Component "config" .stack).outputs.config_map }}'
  cidrs: !template '{{ toJson .settings.allowed_ingress_cidrs }}'

!literal -- Bypass Template Processing

Preserves values exactly as written, preventing Atmos from evaluating template-like syntax:

vars:
  annotation: !literal "{{ .Values.ingress.class }}"
  user_data: !literal "#!/bin/bash\necho ${hostname}"
  config_url: !literal "{{external.config_url}}"

!random -- Random Number Generation

Generates cryptographically secure random integers:

vars:
  port: !random 1024 65535
  id: !random 1000 9999
  default_random: !random

AWS Identity Functions

vars:
  account_id: !aws.account_id
  org_id: !aws.organization_id
  caller_arn: !aws.caller_identity_arn
  caller_user_id: !aws.caller_identity_user_id
  region: !aws.region

Utility Functions

vars:
  working_dir: !cwd
  repo_root: !repo-root
  repo: !git.repository

When to Use YAML Functions vs. Go Templates

Performance Best Practices

1. Prefer !terraform.state over !terraform.output -- 10-100x faster (no Terraform init)
2. Prefer !store over atmos.Component for outputs -- Avoids Terraform initialization
3. All YAML functions cache results per execution for repeated calls
4. Cold-start errors -- !terraform.output and !store fail if the referenced component is not yet provisioned. Use YQ defaults (//) or | default to handle this.

Additional Resources

• For the full YAML functions reference with detailed syntax and examples, see references/yaml-functions.md