DevOps / GitHub Actions Interview Questions

How do you use matrix builds in GitHub Actions to test across multiple environments?

A matrix strategy tells GitHub Actions to spawn multiple parallel job instances from a single job definition, varying one or more parameters across those instances. This is ideal for testing against several language versions, operating systems, or configuration combinations without duplicating YAML.

jobs:
  test:
    runs-on: ${{ matrix.os }}
    strategy:
      matrix:
        os: [ubuntu-latest, windows-latest, macos-latest]
        node: ['18', '20', '22']
      fail-fast: false   # continue other matrix jobs if one fails

    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ matrix.node }}
      - run: npm test

This definition spawns 3 × 3 = 9 parallel jobs, one for each OS/Node combination. Matrix values are referenced with ${{ matrix.<variable> }} anywhere in the job definition — including runs-on:, step inputs, and environment variables.

Key options:

fail-fast: false — by default, if any matrix job fails all remaining ones are cancelled. Set to false to let every combination finish regardless.
include: — add extra combinations or inject extra variables into specific cells. For example, add a code-coverage flag only on Node 20/Ubuntu.
exclude: — remove specific combinations from the matrix (e.g. skip macOS on an older Node version).
max-parallel: — cap the number of concurrent jobs to avoid exhausting runner capacity.

Matrices can also be generated dynamically at runtime by having a prior job output a JSON array and referencing it with fromJSON(needs.setup.outputs.matrix).

Take quiz

With a matrix of 3 operating systems and 3 Node.js versions, how many parallel jobs does GitHub Actions create? 3

✗ Try again — not quite right.

6

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9

✓ Well done — that is correct!

1 per matrix key

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What does setting fail-fast: false on a matrix strategy do? Prevents any matrix job from failing

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Allows all matrix jobs to complete even if one fails, rather than cancelling them

✓ Well done — that is correct!

Makes jobs run sequentially instead of in parallel

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Disables the matrix strategy entirely

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