Concept: Strategy Matrix in GitHub Actions

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Overview

This demo shows how to scale a single job across many OS / runtime combinations without wasting runner minutes.
You’ll fork this repo, inspect the pre-built strategy.matrix workflow, then make a few edits to watch how fail-fast, continue-on-error, max-parallel, include, and exclude change the run behavior.
No local setup is required, the code only prints “OK.” You just need a GitHub account with Actions enabled.

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Step-by-Step

1. Create your own copy

   • Click Use this template → Create a new repository.
   • Name it anything you like and leave it Public (Actions are free for public repos).

2. Open the workflow file

   • Navigate to .github/workflows/matrix.yml.
     This is the only file you’ll edit.

3. Observe the default matrix

   • Two OSes (ubuntu-latest, windows-latest).
   • Two Node versions (18, 20).
   • One extra “experimental” combo added under include.
     Goal: understand what will run before changing anything.

4. Run the workflow once

   • Go to Actions → Matrix Mastery → Run workflow → Run workflow.
   • You should see 5 jobs (4 regular + 1 experimental).

5. Watch the grid while it runs

   • Only 2 jobs run at the same time (max-parallel: 2).
   • The experimental job fails, but the workflow still passes (continue-on-error).

6. Edit ① – Exclude a combo

   • In matrix.yml, find the exclude: section and add:

     exclude:
       - os: ubuntu-latest
         node: 20

   • Commit the file.
     You just removed the ubuntu + Node 20 permutation.

7. Edit ② – Add a new axis value

   • Still in matrix.yml, change

     experimental: [false]

     to

     experimental: [false, true]

   • Commit.
     This doubles the matrix by adding a second value to the experimental axis.

8. Edit ③ – Turn on fail-fast and force a real failure

   1. Set fail-fast: true in the strategy section.
   2. At the top of your steps: list, insert a temporary failing step:

      - name: Temporary failure
        run: exit 1   # intentional break

   3. Commit and run the workflow again.
      Watch: one job fails immediately and the rest cancel, saving time.

9. Undo the temporary breakage

   • Delete the Temporary failure step (or change exit 1 back to echo OK).
   • Commit so the workflow is green again.