`!append` YAML Function

The !append YAML function concatenates lists during stack inheritance instead of replacing them. This provides fine-grained control over list merging behavior on a per-field basis.

Use-cases

• Dependency Management: Append additional depends_on items without losing base dependencies
• Security Groups: Add environment-specific security rules to a base configuration
• IAM Policies: Extend base policies with additional statements
• Tags and Labels: Add extra tags while preserving organizational defaults
• Node Groups: Extend base cluster configurations with additional node pools

Example

stacks/base.yaml

components:
  terraform:
    eks:
      vars:
        cluster_name: main
      settings:
        depends_on:
          - vpc
          - iam-role

stacks/prod.yaml

import:
  - base

components:
  terraform:
    eks:
      settings:
        # Without !append: [rds, elasticache] replaces [vpc, iam-role]
        # With !append: results in [vpc, iam-role, rds, elasticache]
        depends_on: !append
          - rds
          - elasticache

Behavior

The !append function:

• Applies to lists only - Can only be used on YAML sequences
• Works per-field - Does not affect other lists in the configuration
• Operates during merging - Executes when stack configurations are merged
• Preserves order - Base items appear first, appended items follow

Interaction with Global Settings

The !append tag works independently of the global list_merge_strategy setting:

Global Setting	Field without !append	Field with !append
replace (default)	Replaces entire list	Appends to base list
append	Appends to base list	Appends to base list
merge	Deep merges list items	Appends to base list

Advanced Examples

Multiple Inheritance Levels

stacks/base.yaml

components:
  terraform:
    app:
      settings:
        security_groups:
          - sg-base

stacks/staging-base.yaml

import:
  - base

components:
  terraform:
    app:
      settings:
        security_groups: !append
          - sg-staging

stacks/staging-east.yaml

import:
  - staging-base

components:
  terraform:
    app:
      settings:
        security_groups: !append
          - sg-east-region
        # Result: [sg-base, sg-staging, sg-east-region]

Mixed List Behaviors

stacks/config.yaml

components:
  terraform:
    database:
      vars:
        # This list uses normal replacement
        availability_zones:
          - us-east-1c
          - us-east-1d
      settings:
        # This list uses append
        backup_retention_policies: !append
          - daily-snapshots
          - weekly-archive

Notes

• The !append function only affects the specific list it's applied to
• Cannot be used on non-list values (maps, strings, numbers)
• Appended items maintain their original structure and type
• Works with all list item types: strings, numbers, maps, or nested lists