What is the 'extends' keyword and how does it enable job templates?

The extends keyword lets one job inherit configuration from another job (or a hidden job used as a template). This reduces duplication and keeps the pipeline DRY (Don't Repeat Yourself).

# Define a hidden job template (starts with .)
.base-deploy:
  image: alpine:latest
  before_script:
    - apk add --no-cache curl
    - echo "Logging into registry..."
  after_script:
    - echo "Deployment complete"
  rules:
    - if: $CI_COMMIT_BRANCH == "main"

# Jobs extend the template
deploy-staging:
  extends: .base-deploy
  script:
    - ./deploy.sh staging
  environment:
    name: staging

deploy-production:
  extends: .base-deploy
  script:
    - ./deploy.sh production
  environment:
    name: production
  rules:                          # overrides the template rules
    - if: $CI_COMMIT_TAG
      when: manual

# extends with multiple parents (array form)
.logging:
  after_script:
    - send-logs.sh

my-job:
  extends:
    - .base-deploy
    - .logging          # merges config from both (later parent wins on conflict)

Merge behaviour: when a job extends a template, YAML hashes are merged deeply. Arrays (like script, rules) in the child job replace (not append to) the parent's arrays. A job can extend multiple templates by providing an array - conflicts are resolved with the last-listed parent winning, then the child overriding all.

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