YAML Anchors, Aliases, and Merge Keys

Reusable YAML patterns using anchors and aliases, with merge examples and commands for expanding and validating resolved config.

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## Anchors and aliases

Anchor and alias

defaults: &defaults
  retries: 3
  timeout: 30

api:
  <<: *defaults
  url: https://api.example.com

# Define once and reuse many times.

Merge shared mappings

base: &base
  image: node:20
  pullPolicy: IfNotPresent

prod:
  <<: *base
  replicas: 3

# Combine a base config with environment-specific overrides.

Merge multiple anchors

common: &common
  logging: json
security: &security
  readOnlyRootFilesystem: true

service:
  <<: [*common, *security]
  port: 8080

# Compose config from several reusable blocks.

Expand anchors with yq

yq 'explode(.)' config.yaml

# Materialize merged values for inspection.

## Real-world reuse patterns

Reuse CI step fragments

steps: &install_steps
  - uses: actions/checkout@v4
  - run: npm ci

jobs:
  test:
    steps: *install_steps
  build:
    steps: *install_steps

# Share common setup across jobs.

Reuse environment variables

env: &common_env
  NODE_ENV: production
  LOG_LEVEL: info

worker:
  env: *common_env

# Avoid copying the same environment block.

Catch duplicate mapping keys

yamllint config.yaml

# Lint for repeated keys that override silently in some parsers.

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