What are YAML anchors and aliases in GitLab CI and how do they reduce duplication?

YAML anchors (&name) and aliases (*name) are standard YAML features that let you define a block once and reuse it multiple times. In GitLab CI, they reduce duplication for shared configuration like rules, before_script, or variables - without requiring the extends keyword.

# Define a reusable block with an anchor
.common_rules: &common_rules
  rules:
    - if: $CI_COMMIT_BRANCH == "main"
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"

.common_setup: &common_setup
  image: node:20-alpine
  before_script:
    - npm ci

# Reuse with alias (*) or merge key (<<: *)
build-job:
  <<: *common_setup        # merge the anchor content
  <<: *common_rules        # YAML 1.1 syntax
  script:
    - npm run build

test-job:
  <<: *common_setup
  <<: *common_rules
  script:
    - npm test

# Anchors for variables
.env_vars: &prod_vars
  variables:
    ENVIRONMENT: production
    LOG_LEVEL: error

deploy-prod:
  <<: *prod_vars
  script: ./deploy.sh production

Anchors vs extends:

• YAML anchors are processed before GitLab reads the YAML - they are pure YAML features and work at the text level
• extends is a GitLab-specific feature processed after YAML parsing, offering deep merging, multiple parents, and better tooling visibility
• GitLab recommends using extends over YAML anchors for job configuration since it is more explicit and readable

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