What is difference between extends and anchor tag (<<: *anchor) in yaml (Gitlab CICD)?

Question

When we use extends and when we use anchor tag ? Please refer below CI CD pipeline

  stages:
    - stage1
  .random-variables:
    variables:
      ABC: ${XYZ}
    
  .hidden-job: &hidden-job
    stage: stage1
    image: docker:latest
    services:
      - docker:dind
    script:
      #  My Scripts

  hidden-job:dev:
    extends:
      - .random-variables
    <<: *hidden-job
    only:
      - dev

Thanks in advance for clarifying my doubt.

As of now I understand how pipeline is working like anchor tag is use with <<: *alias to pull in the other block of code in current block.

Same extends is use to pull in variables in current block

Answer 1

They are essentially the same except you can use a yaml anchor from another file using the extends field. For example:

include:
 - 'https://example.com/some-file.yaml'

# this will work
my_job:
  extends: .some-anchor-from-the-included-file

# this will fail
my_other_job:
  <<: *some-anchor-from-the-included-file

You can also use the !reference tag to pull yaml anchors from other files.

https://docs.gitlab.com/ee/ci/yaml/yaml_optimization.html#reference-tags