importer-labs/gitlab/4-custom-transformers.md

Using Custom Transformers in a dry-run

In this lab we want to do a dry-run of the terraform-example project. We can now perform a dry-run of this project, however, we have discovered that the output will need to be customized to transform the Terraform artifact report. After some research, we have determined that the actions/upload-artifact action will be an adequate substitute for it. Additionally, we will need to change the environment variable PLAN_JSON to use a different value: custom_plan.json. This customization will be present in many pipelines and an automated way to apply this would be ideal. In this lab, we will use the --custom-transformers flag to change the behavior of Valet using its DSL built on top of the Ruby language.

• Prerequisites
• Write Custom Transformers
• Run with Custom Transformers
• Next Lab

Prerequisites

1. Followed steps to set up your codespace environment.
2. Completed the configure lab
3. Completed the dry-run lab

Write Custom Transformers

• Let’s run the dry-run command to see what information we can get from the generated action yaml.

  gh valet dry-run gitlab --output-dir tmp --namespace valet --project terraform-example
  

• Open the resulting GitHub Actions workflow by navigating to tmp/valet/custom-transformer.yml from the explorer

name: valet/custom-transformer
on:
  push:
  workflow_dispatch:
concurrency:
  group: "${{ github.ref }}"
  cancel-in-progress: true
jobs:
  plan:
    runs-on: ubuntu-latest
    timeout-minutes: 60
    env:
      PLAN: plan.cache
      PLAN_JSON: plan.json
    steps:
    - uses: actions/checkout@v2
      with:
        fetch-depth: 20
        lfs: true
    - run: terraform plan -out=$PLAN
    - run: terraform show --json $PLAN | convert_report > $PLAN_JSON
#     # 'artifacts.terraform' was not transformed because there is no suitable equivalent in GitHub Actions

• We can see from the last line that artifacts.terraform was not transformed. In order for us to write a custom transformer for this we need to know the identifier. In general, the identifier will be the value between the backticks, which in this case is artifacts.terraform. This is how our custom transformer will target the correct step.
• The custom transformers file can have any name, but it is recommended that you use an .rb extension so the codespaces editor knows it is a ruby file and can provide syntax highlighting.
• we have chosen the actions/upload-artifacts as our replacement so we should look at the action's documentation to determine the correct final yaml

  - uses: actions/upload-artifact@v3
    with:
      path: VALUE_FROM_GITLAB
  

• Now that we know the final yaml needed for the transformer, we can start to write the ruby file. In the custom transformers file we will call the transform method. This is a special method that Valet exposes, that takes the identifier we determined earlier and returns a ruby Hash of the final YAML for the pipeline. The ruby Hash can be thought of as the JSON representation of the YAML we want. Valet will call that method when it encounters the identifier and pass in an item. The item is the values defined for that step in GitLab. In this case the item is the path of the terraform report.

Note: If you were unsure what item represents, you could use some basic ruby to print item to the terminal. You can achieve this by adding the following line in the transform method: puts "This is the item: #{item}"

transform "artifacts.terraform" do |item|
  {
    uses: "actions/upload-artifact@v2",
    with: {
      path: item
    }
  }
end

• The custom transformers file also lets you replace values of variables by using the env method. Let’s replace the value for PLAN_JSON by using the below line. The first value of the env method is the target variable name and the second is the new value to be used.

  env "PLAN_JSON", "custom_plan.json"
  

• create a new file in the root of the workspace called transformers.rb with the following contents

  env "PLAN_JSON", "custom_plan.json"
  
  transform "artifacts.terraform" do |item|
    {
      uses: "actions/upload-artifact@v2",
      with: {
        path: item
      }
    }
  end
  

Run Again with Customer Transformers

To run the dry-run with our custom transformer we will add the --custom-transformers flag followed by the path of the custom transformer ruby file

gh valet dry-run gitlab --output-dir tmp --namespace valet --project terraform-example --custom-transformers transformers.rb

The custom transformer worked and now we have the upload-artifact in the place of the unsupported result.

- #     # 'artifacts.terraform' was not transformed because there is no suitable equivalent in GitHub Actions
+ uses: actions/upload-artifact@v2
+ with:
+   path: "$PLAN_JSON"

Also we can see the PLAN_JSON env has been updated to custom_plan.json

 env:
   PLAN: plan.cache
-  PLAN_JSON: plan.json
+  PLAN_JSON: custom_plan.json

Now that we have this custom transformers file we can add additional transformand env methods as needed and reuse it while running other dry-run and migrate commands.

Note: The custom transformers will only affect the pipeline being transformed if they contain the matching identifiers. If you believe a custom transformer should have altered the output, double check that the identifier is correct.

Next Lab

Forecast GitLab Usage