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+# Using custom transformers to customize Valet's behavior
+
+In this lab you will build upon the `dry-run` command to override Valet's default behavior and customize the converted workflow using "custom transformers". Custom transformers can be used to:
+
+1. Convert items that are not automatically converted.
+2. Convert items that were automatically converted using different actions.
+3. Convert environment variable values differently.
+4. Convert references to runners to use a different runner name in Actions.
+
+## Prerequisites
+
+1. Followed the steps [here](./readme.md#configure-your-codespace) to set up your GitHub Codespaces environment.
+2. Completed the [configure lab](./1-configure.md#configuring-credentials).
+3. Completed the [dry-run lab](./3-dry-run.md).
+
+## Perform a dry-run
+
+You will be performing a `dry-run` command to inspect the workflow that is converted by default. Run the following command within the codespace terminal:
+
+```bash
+  gh valet dry-run travis-ci --travis-ci-repository "deploy-example" --output-dir tmp/travis/dry-run
+```
+
+The converted workflow that is generated by the above command can be seen below:
+
+<details>
+  <summary><em>Converted workflow 👇</em></summary>
+
+```yaml
+    name: valet-travis-labs/deploy-example
+    on:
+      push:
+        branches:
+        - "**/*"
+      pull_request:
+    concurrency:
+    #   # This item has no matching transformer
+    #   maximum_number_of_builds: 0
+    env:
+      DB: mysql
+    jobs:
+      test:
+        runs-on: ubuntu-latest
+        steps:
+        - name: checkout
+          uses: actions/checkout@v2
+        - run: gem install bundler
+        - run: bundle install --jobs=3 --retry=3
+        - run: rake
+        - run: echo 'ready?'
+    #     # This item has no matching transformer
+    #     - codedeploy:
+    #         provider: codedeploy
+    #         access_key_id: YOUR AWS ACCESS KEY
+    #         secret_access_key: YOUR AWS SECRET KEY
+    #         bucket: S3 Bucket
+    #         key: latest/MyApp.zip
+    #         application: MyApp
+    #         deployment_group: MyDeploymentGroup
+    #       if: "${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}"
+        - run: "./after_deploy_1.sh"
+        - run: "./after_deploy_2.sh"
+        - uses: desiderati/github-action-pushover@v1
+          with:
+            job-status: "${{ job.status }}"
+            pushover-api-token: '12345'
+            pushover-user-key: "${{ secrets.PUSHOVER_USER_KEY }}"
+          if: "${{ github.event_name != 'pull_request' }}"
+
+```
+
+</details>
+
+_Note_: You can refer to the previous [lab](./3-dry-run.md) to learn about the fundamentals of the `dry-run` command.
+
+## Custom transformers for an unknown step
+
+The converted workflow above contains an `codedeploy` step that was not automatically converted. Answer the following questions before writing a custom transformer:
+
+1. What is the "identifier" of the step to customize?
+    - __codedeploy__
+
+2. What is the desired Actions syntax to use instead?
+    - After some research, you have determined that we can login to AWS with the `aws-actions/configure-aws-credentials@v1` action, and deploy the app to AWS using a run step to replace the functionality of `codedeploy`:
+
+      ```yaml
+        - uses: aws-actions/configure-aws-credentials@v1
+          with:
+            role-to-assume: owner
+            role-session-name: GitHub-Action-Role
+            aws-region: east-2
+        - run: |
+            echo "Deploying branch ${{ env.GITHUB_REF }} to ${{ github.event.inputs.environment }}"
+            commit_hash=`git rev-parse HEAD`
+            aws deploy create-deployment --application-name MyApp --deployment-group-name MyDeploymentGroup --github-location repository=$GITHUB_REPOSITORY,commitId=$commit_hash --ignore-application-stop-failures
+      ```
+
+Now you can begin to write the custom transformer. Custom transformers use a DSL built on top of Ruby and should be defined in a file with the `.rb` file extension. You can create this file by running the following command in your codespace terminal:
+
+```bash
+code transformers.rb
+```
+
+Next, you will define a `transform` method for the `codedeploy` identifier by adding the following code to `transformers.rb`:
+
+```ruby
+transform "codedeploy" do |_item|
+  [
+    {
+      "uses": "aws-actions/configure-aws-credentials@v1",
+      "with": {
+        "role-to-assume": "owner",
+        "role-session-name": "GitHub-Action-Role",
+        "aws-region": "east-2"
+      }
+    },
+    {
+      "run": "echo \"Deploying branch ${{ env.GITHUB_REF }} to ${{ github.event.inputs.environment }}\"\ncommit_hash=`git rev-parse HEAD`\naws deploy create-deployment --application-name MyApp --deployment-group-name MyDeploymentGroup --github-location repository=$GITHUB_REPOSITORY,commitId=$commit_hash --ignore-application-stop-failures\n"
+    }
+  ]
+end
+```
+
+This method can use any valid ruby syntax and should return a `Hash`, or an array of `Hashes` that represent the YAML that should be generated for a given step. Valet will use this method to convert a step with the provided identifier and will use the `item` parameter for the original values configured in Travis CI.
+
+Now you can perform another `dry-run` command and use the `--custom-transformers` CLI option to provide this custom transformer. Run the following command within your codespace terminal:
+
+```bash
+  gh valet dry-run travis-ci --travis-ci-repository "deploy-example" --output-dir tmp/travis/dry-run --custom-transformers transformers.rb
+```
+
+The converted workflow that is generated by the above command will now use the custom logic for the `codedeploy` step.
+
+```diff
+- #     # This item has no matching transformer
+- #     - codedeploy:
+- #         provider: codedeploy
+- #         access_key_id: YOUR AWS ACCESS KEY
+- #         secret_access_key: YOUR AWS SECRET KEY
+- #         bucket: S3 Bucket
+- #         key: latest/MyApp.zip
+- #         application: MyApp
+- #         deployment_group: MyDeploymentGroup
+- #       if: "${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}"
++  - uses: aws-actions/configure-aws-credentials@v1
++    with:
++      role-to-assume: owner
++      role-session-name: GitHub-Action-Role
++      aws-region: east-2
++  - run: |
++      echo "Deploying branch ${{ env.GITHUB_REF }} to ${{ github.event.inputs.environment }}"
++      commit_hash=`git rev-parse HEAD`
++      aws deploy create-deployment --application-name MyApp --deployment-group-name MyDeploymentGroup --github-location repository=$GITHUB_REPOSITORY,commitId=$commit_hash --ignore-application-stop-failures
+```
+
+_Note_: We hard coded certain values such as the `application-name`, but we can apply these properties programmatically as well by using the item passed into the transform method. If you were unsure what the data structure of `item` was, you could use the following code in the custom transformer to print `item` to the console:
+
+```ruby
+transform "codecov_codecov_upload" do |item|
+  puts item
+end
+```
+
+## Custom transformers for environment variables
+
+You can use custom transformers to edit the values of environment variables in converted workflows. In this example, you will update the `DB` environment variable to be `sqlite` instead of `mysql`.
+
+To do this, add the following code to the `transformers.rb` file.
+
+```ruby
+env "DB", "sqlite"
+```
+
+In this example, the first parameter to the `env` method is the environment variable name and the second is the updated value.
+
+Now you can perform another `dry-run` command with the `--custom-transformers` CLI option.  When you open the converted workflow the `DB` environment variable will be set to `sqlite`:
+
+```diff
+ env:
+-  DB: "mysql"
++  DB: "sqlite"
+```
+
+## Custom transformers for runners
+
+Finally, you can use custom transformers to dictate which runners converted workflows should use. First, answer the following questions:
+
+1. What is the label of the runner in Travis to update?
+    - __linux__
+
+2. What is the label of the runner in Actions to use instead?
+    - __new-runner__
+
+With these questions answered, you can add the following code to the `transformers.rb` file:
+
+```ruby
+runner "linux", ["new-runner", "self-hosted"]
+```
+
+In this example, the first parameter to the `runner` method is the Azure DevOps label and the second is the Actions runner labels.
+
+Now you can perform another `dry-run` command with the `--custom-transformers` CLI option.  When you open the converted workflow, the `runs-on` statement will use the customized runner labels:
+
+```diff
+-    runs-on: ubuntu-latest
++    runs-on:
++      - new-runner
++      - self-hosted
+```
+
+At this point, the file contents of `transformers.rb` should match this:
+
+<details>
+  <summary><em>Custom transformers 👇</em></summary>
+
+```ruby
+transform "codedeploy" do |_item|
+  [
+      {
+      "uses": "aws-actions/configure-aws-credentials@v1",
+      "with": {
+          "role-to-assume": "owner",
+          "role-session-name": "GitHub-Action-Role",
+          "aws-region": "east-2"
+          }
+      },
+      {
+      "run": "echo \"Deploying branch ${{ env.GITHUB_REF }} to ${{ github.event.inputs.environment }}\"\ncommit_hash=`git rev-parse HEAD`\naws deploy create-deployment --application-name MyApp --deployment-group-name MyDeploymentGroup --github-location repository=$GITHUB_REPOSITORY,commitId=$commit_hash --ignore-application-stop-failures\n"
+      }
+  ]
+end
+
+env "DB", "sqlite"
+
+runner "linux", ["new-runner", "self-hosted"]
+```
+
+</details>
+
+That's it! Congratulations, you have overridden Valet's default behavior by customizing the conversion of:
+
+- Unknown steps
+- Environment variables
+- Runner
+
+## Next lab
+
+[Perform a production migration of a Travis CI pipeline](5-migrate.md)