Workflow Syntax: Matrix and Reusable Workflows

Overview

Write maintainable workflow YAML using strategy.matrix, job and step if: conditionals, job outputs, and a first reusable workflow call pattern.

Beyond a single job, production workflows need fan-out (test on multiple OS or language versions), selective execution (if: on jobs and steps), and reuse so fifty repositories do not diverge. A matrix expands one job definition into many. Outputs pass data between jobs. Reusable workflows (workflow_call) let a platform team own a canonical CI definition that callers invoke with inputs.

This is a core tutorial in Module 4 · Workflow Syntax of the REBASH Academy GitHub Actions for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• GitHub-Hosted and Self-Hosted Runners

Learning Objectives

By the end of this tutorial, you will be able to:

• Expand jobs with strategy.matrix and fail-fast
• Gate jobs and steps with if: expressions
• Publish and consume job outputs
• Sketch a workflow_call reusable workflow and a caller
• Name when to prefer composite actions vs reusable workflows

Architecture

This topic’s control points and relationships are shown below.

Theory

What it is

Workflow syntax is the YAML contract described in GitHub’s reference: top-level name, on, permissions, concurrency, env, and jobs. Each job may set needs, if, runs-on, strategy, outputs, permissions, and steps.

A matrix under strategy: lists axes such as python-version: ['3.11', '3.12'] or os: [ubuntu-latest, windows-latest]. GitHub creates one job per combination (Cartesian product), injecting matrix.* into expressions. Options such as fail-fast, max-parallel, and include / exclude refine the set.

Conditionals use if: at job or step level with expressions over contexts (github, needs, matrix, …). Prefer explicit conditions over duplicated workflow files per branch.

Inputs and outputs: reusable workflows and workflow_dispatch declare inputs:. Jobs declare outputs: mapped from step outputs (echo "name=value" >> "$GITHUB_OUTPUT"). Downstream jobs read ${{ needs.build.outputs.version }}.

Reusable workflows live in a workflow file with on: workflow_call. Callers use jobs.<id>.uses: org/repo/.github/workflows/ci.yml@v1 (or a SHA) and pass with: / secrets:. They are heavier than composite actions (which are step bundles) but ideal for whole pipelines (lint → test → scan).

Construct	Role
strategy.matrix	Fan-out across versions / OSes
if:	Include or skip job/step
needs + outputs	Ordered jobs with data
workflow_call	Callable pipeline definition
Composite action	Reusable steps, not whole jobs graph

Why it matters

Matrix builds catch “works on 3.12 only” defects before production. Poor if: design creates ghost jobs or skipped deploys that nobody notices until release day. Reusable workflows are how platform engineering productises CI: version the workflow, require callers to bump the ref, and fix a CVE in setup actions once. Without reuse, every repository drifts — different Node versions, missing security scans, inconsistent permissions.

How it works

1. GitHub expands the matrix into concrete jobs before scheduling.
2. Each matrix job runs independently (subject to max-parallel) with its own matrix context.
3. Jobs with needs: wait for dependencies; they can read outputs and use if: needs.x.result == 'success'.
4. A reusable workflow run nests under the caller; inputs become available as ${{ inputs.name }} inside the called workflow.
5. Permissions should be least privilege at caller and callee — later security modules tighten permissions: and id-token.

Author both files in one repository for learning; in production, platform teams often publish reusable workflows from a central org/workflows repository and pin tags or SHAs.

Key concepts and comparisons

Pattern	Use when
Matrix	Same steps, many versions/OS
Separate jobs	Different graphs or artefacts
Reusable workflow	Shared multi-job pipeline
Composite action	Shared install/build steps

Matrix tip	Why
Keep axes small	Combinatorial explosion burns minutes
Use include for special cases	Avoid huge Cartesian products
Name jobs with ${{ matrix.* }}	Readable Actions UI

Common pitfalls

• Forgetting that matrix failure fails the workflow when fail-fast: true (default).
• Referencing matrix outside a matrix job.
• Passing secrets to reusable workflows without secrets: inherit or explicit mapping.
• Treating reusable workflow @main as a stable contract — pin versions for production.
• Overusing continue-on-error so red builds look green in required checks.

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-github-actions/module-04/.github/workflows && cd ~/rebash-github-actions/module-04/.github/workflows

Focus: matrix workflow and reusable workflow caller/callee

Step 1 – Create reusable + matrix workflows

mkdir -p .github/workflows
cat > .github/workflows/reusable-test.yml << 'EOF'
name: Reusable test
on:
  workflow_call:
    inputs:
      python-version:
        required: true
        type: string
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: "3.12"
      - run: python --version
EOF
cat > .github/workflows/matrix.yml << 'EOF'
name: Matrix and reusable
on: [push, workflow_dispatch]
permissions:
  contents: read
jobs:
  matrix_probe:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        python: ["3.11", "3.12"]
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: "3.12"
      - run: python --version
  call_reusable:
    uses: ./.github/workflows/reusable-test.yml
    with:
      python-version: "3.12"
EOF


```yaml
# When wiring matrix python version dynamically use:
# python-version: ${{ matrix.python }}
# inputs: ${{ inputs.python-version }}
# Wrap workflow files containing those expressions in {% raw %} on docs pages.

### Step 2 – Check matrix and workflow_call

```bash
grep -E 'matrix:|workflow_call:|uses: \./' -n .github/workflows/*.yml

Final step – Cleanup note

# Keep ~/rebash-github-actions/ for later tutorials

Validation

• Lab commands run under ~/rebash-github-actions/module-04/.github/workflows/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Workflow Syntax: Matrix and Reusable Workflows always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for github-actions as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Forgetting that matrix failure fails the workflow when fail-fast: true (default).

Validate assumptions against the Theory section and official docs before changing production.

Referencing matrix outside a matrix job.

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Workflow Syntax: Matrix and Reusable Workflows changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Workflow Syntax: Matrix and Reusable Workflows is essential for Cloud and DevOps engineers working with github-actions. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. When is a build matrix the right tool?
2. How does workflow_call differ from workflow_run?
3. What does fail-fast false change in a matrix?
4. How do you version reusable workflows safely across repos?
5. What inputs/secrets patterns keep reusable workflows least-privileged?

Sample answer — question 2

Check matrix expansion in the UI, cancelled siblings from fail-fast, and whether the reusable workflow path/ref resolves.

Sample answer — question 4

Pin reusable workflows to tags/SHAs and pass only required secrets.

Related Tutorials

• Course overview
• Secrets, Variables, and OIDC

References

• Workflow syntax
• Running a matrix of jobs
• Reusing workflows