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Not to be afraid of GitLab's dynamic parent-child pipelines

June 29, 2022

The time has finally come where I’ve truly needed to create a set of dynamic pipelines in GitLab, ones that can only be determined during runtime. So, what do I do? I crack open the official documentation and see that their example project uses something called Jsonnet, which is something I’ve never used before.

The example seems simple enough to mold into what I need, but fairly quickly it becomes apparent that while the saved file in GitLab’s own example, and in various examples across the Internet, is saved as .yml indicating that it’s YAML. It is actually formatted as JSON1. As soon as I needed to do more complicated things like multi-line script blocks it became unwieldy if not impossible to work with JSON without destroying readability.

So, a lightbulb went off for my colleague. If it’s supposedly YAML, but looks like JSON, and is parsed fine by GitLab then that means one should just be able to pass in a YAML file like in a regular include. How does one dynamically create a valid YAML file? With Jinja2 of course! Since it’s usually used as a library, for example within Ansible and Python, it was necessary to install an additional tool called j2cli. After reading how to use the tool I had hoped I could just pass in a list through an environment variable, have it be looped over in a normal for-loop, and be done with it, but I had to first create a rudimentary INI file that contained all the elements of my list2 and pass that to the j2 executable:

$ cat jobs.ini
$ j2 job-template.j2 jobs.ini

After that it was off to the races as I was able to replace something that far fewer people have used (i.e. Jsonnet) with something that is quite commonplace. I think if people knew that you don’t have to necessarily use Jsonnet or Dhall or ytt, as they mention in the documentation, then that would significantly lower the barriers to entry in creating more dynamic pipelines.


  1. Supposedly YAML is a superset of JSON, but that’s neither here nor there when it comes to setting useful file extensions.

  2. The names of the jobs are duplicated and separated by an equals sign to make the syntax valid, otherwise j2 complains.

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