How to extend and reuse configs in Gitlab CI

When I began using Gitlab CI the first thing I wanted to know was how to reuse configs. I know that we’ll have changing requirements at work and that we’ll have hundreds of repositories using basically-the-same configurations so I wanted to make sure we’d have a central place to make changes to these.

I found two ways to archive reusable configurations which I’ll explain below.

Extends

First of we have the extends keyword. This makes a config section of the CI yaml reuse another section, as illustrated below.

my-base-block:
  stage: deploy
  script:
    - echo "Hello, Gitlab CI!"

my-other-block:
  extends:
    - my-base-block

Gitlab combines this into the following YAML:

my-base-block:
  stage: deploy
  script:
  - echo "Hello, Gitlab CI!"
my-other-block:
  stage: deploy
  script:
  - echo "Hello, Gitlab CI!"
  extends:
  - my-base-block

So far so good! But there’s one downside to extends: If my-other-block has its own script, it will overwrite the original script, as shown below.

my-base-block:
  stage: deploy
  script:
    - echo "Hello, Gitlab CI!"

my-other-block:
  extends:
    - my-base-block
  script:
    - echo "Bye, Gitlab CI!"

Merged YAML:

my-base-block:
  stage: deploy
  script:
  - echo "Hello, Gitlab CI!"
my-other-block:
  stage: deploy
  script:
  - echo "Bye, Gitlab CI!"
  extends:
  - my-base-block

Now that’s not what we want, the script field was overwritten! Good for us that Gitlab has a solution: !reference[].

!reference

!reference[] is an extension to YAML Gitlab provides, it allows us to reference another block and key which will be – well – referenced!

my-base-block:
  stage: deploy
  script:
    - echo "Hello, Gitlab CI!"

my-other-block:
  extends:
    - my-base-block
  script:
    - !reference [my-base-block, script]
    - echo "Bye, Gitlab CI!"

And the merged YAML:

my-base-block:
  stage: deploy
  script:
  - echo "Hello, Gitlab CI!"
my-other-block:
  stage: deploy
  script:
  - - echo "Hello, Gitlab CI!"
  - echo "Bye, Gitlab CI!"
  extends:
  - my-base-block

Now we have both script parts! I’m exited to explore Gitlab further in the future, so far my experience with it has been a real pleasure. 🧡

Thanks for coming to my TED Talk!

Extends

!reference

Further reading