How can We use GitHub Actions in Gitlab?

One of my customers is building a monorepo, so we prepared a prototype on GitHub to test it out. It uses Monorepo Split GitHub Action and works well.

You know I'm a big fan of GitHub, so when client asked me: "how can we do it in Gitlab?"
I was like: "that's not possible... you need to switch to GitHub".

...or was I?

Critical thinking can be easily disabled by confirmation bias. When I stop thinking critically and actually realize that, I try to get back to reality. One way to do that is to describe the situation as it is in few simple words.

What we have now?

What we need?

Can We Get There? How Can We Get There?

  1. From GitHub mono repository to Gitlab mono repository. Basically, we need to switch the origin reference in git:
# remove github
git remote remove origin

# add GitLab
git remote add origin
git push origin
  1. From GitHub Split Action to Gitlab?

My first idea was: "I have no idea". Let's be honest here. It's not a very good solution, not a solution at all, not working, but we always have to start somewhere.

What is the next idea? Let's describe world in simple words.

What is a GitHub Action?

"GitHub Action is some script that you call, and it does some work for you. Mostly in CI regularly."

Good. GitHub Actions is a CI, and Gitlab CI is a CI. We have a mental bridge that we can use.

"Yea, but GitHub Action is tailor-made for GitHub CI".

Ok, what exactly is GitHub CI doing when you run a GitHub Action in it?

"Well, it gets input arguments that we set and runs a script with them."

What do you mean by a script?

"There is Docker image that we download and run. The Docker image can call bash, PHP, or whatever other inner script and just pass the arguments in it."

Ok, so GitHub Action is basically a Docker image that accepts arguments and runs them?

docker run some-image $ARGUMENTS


Note: there is 2nd way to build GitHub Action via Javascript. I didn't follow that path, as I know Docker better than Javascript.

1. From GitHub Action Syntax to Pseudo Syntax

Let's look at CI configuration. In GitHub, it's YAML files located in the .github/workflows directory, in Gitlab, it's .gitlab-ci.yml.

This is how looks real GitHub Action in normal syntax:

# .github/workflows/monorepo_split.yaml
            # ...
                uses: "symplify/monorepo-split-github-action@2.0"
                    package-directory: 'packages/easy-coding-standard'
                    split-repository-organization: 'symplify'
                    split-repository-name: 'easy-coding-standard'

How would the same action look like in pseudo syntax without GitHub syntax sugar?

# use this docker image
image: "symplify/monorepo-split-github-action"

# setup input variables
    FROM_PACKAGE: "packages/easy-coding-standard"

# run docker image with input variables
    - docker run symplify/monorepo-split-github-action $FROM_PACKAGE $TO_REPOSITORY

This is great! Now we know the exact steps that have to be reproduced in the next CI.

2. From Pseudo Syntax to GitLab CI Syntax

Now we take the pseudo syntax and try to fit in Gitlab conventions:

# .gitlab-ci.yml
    # ...
    # set envs
        FROM_PACKAGE: "packages/easy-coding-standard"
        TO_REPOSITORY: ""

        - docker run symplify/monorepo-split $FROM_PACKAGE $TO_REPOSITORY

That's it! Now we're using GitHub Action in a Gitlab CI.

How to make GitHub Actions that GitLab Developers can use?

Not every GitHub Action will work out of the box. There are steps we have to think about:

  1. build GitHub Action as a Docker image and not Javascript one
  2. publish the Docker image to Docker Hub (basically Packagist for Docker images), so anyone can use it
  3. allow passing Gitlab Access Token for repository access
  4. bonus: if you have many arguments, think of passing them as ENV variables to keep .gitlab-ci.yaml small

That's all, folks. I could not believe how simple this is.

Can't wait to give it a try? Let me know what GitHub Actions you're using in Gitlab!

Happy coding!

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