Domain 4: Modern Development (13%)

GitHub Actions

GitHub Actions is GitHub's built-in CI/CD and automation platform. It runs workflows in response to events.

Core Concepts

Term	Definition
Workflow	A YAML file in `.github/workflows/` that defines automation
Event	A trigger (push, pull_request, schedule, etc.)
Job	A set of steps that run on a runner
Step	An individual command or action within a job
Action	A reusable unit of automation (from Marketplace or custom)
Runner	The server that executes jobs (GitHub-hosted or self-hosted)

Workflow Structure

yaml

name: CI Pipeline

on:
  push:
    branches: [main]
  pull_request:

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run tests
        run: npm test

Common Triggers (Events)

Event	Fires when
`push`	Code is pushed to a branch
`pull_request`	PR is opened, updated, or merged
`schedule`	Cron-based scheduled time
`workflow_dispatch`	Manually triggered via UI or API
`release`	A release is published
`issues`	An issue is opened, edited, closed

GitHub-Hosted vs Self-Hosted Runners

Type	Description	Use case
GitHub-hosted	Ubuntu, Windows, macOS VMs managed by GitHub	General purpose, free tier available
Self-hosted	Your own machine/server registers as a runner	Custom hardware, private networks, compliance

Exam Tip

For GH-900, focus on what Actions IS and what it can DO — not deep workflow YAML syntax. Key points: it's event-driven, YAML-based, lives in .github/workflows/, and uses the Actions Marketplace.

GitHub Codespaces

Codespaces is a cloud-hosted development environment — a full VM with VS Code running in the browser or connected to your local VS Code.

Key Features

Pre-configured via devcontainer.json (in .devcontainer/ folder)
Runs on GitHub-managed cloud VMs
Has a full terminal, file system, and can run servers
Billed per compute-hour (free tier for personal accounts)
Codespace state persists between sessions (up to 30 days inactive)

devcontainer.json

Defines the Codespace environment:

json

{
  "name": "Node.js Dev",
  "image": "mcr.microsoft.com/devcontainers/javascript-node:18",
  "postCreateCommand": "npm install",
  "extensions": ["dbaeumer.vscode-eslint"]
}

Codespace Lifecycle

Create → Running → Stopped (idle) → Deleted

Idle timeout: default 30 minutes (configurable)
Max retention: 30 days after last use

github.dev Editor

github.dev is a lightweight, browser-based code editor — essentially VS Code in the browser, but with no compute.

How to open

Press . (period) on any GitHub repo page
Change .com to .dev in the URL

Limitations vs Codespaces

Feature	GitHub Codespaces	github.dev
Primary Use	Full development (running, building, and debugging code).	Quick edits and browsing files.
Environment	A dedicated, customizable virtual machine (VM) and Docker container hosted in the cloud.	Runs entirely in your browser's sandbox; no cloud compute is attached.
Terminal Access	Yes, full integrated terminal access is available.	No terminal access.
Code Execution	Yes, you can run and debug your application with the power of the cloud VM.	No, you cannot build or run code within the environment itself.
Cost	Billed per hour of use and per GB of storage, with a free monthly quota for personal accounts.	Entirely free for everyone on GitHub.com.
Setup Time	Takes a few moments to spin up the VM and configure the container based on a devcontainer.json file.	Opens instantly with a single key press (press . on any GitHub repository page).
Extensions	Supports most extensions available from the Visual Studio Code Marketplace.	Only supports a subset of "web extensions" that can run in the browser.

Exam Trap

This is the most tested comparison in Domain 4. Remember: github.dev = editor only, no terminal. If a question mentions running tests, starting a server, or using a terminal → the answer is Codespaces.

GitHub Copilot

GitHub Copilot is an AI pair programmer that provides code suggestions inline in your editor.

What Copilot Does

Autocompletes code as you type
Suggests entire functions based on comments or context
Supports multiple languages (Python, JavaScript, TypeScript, Go, Ruby, etc.)
Available in VS Code, JetBrains IDEs, Neovim, and github.dev

Copilot Tiers (GH-900 level awareness)

Tier	For
Copilot Individual	Personal accounts
Copilot Business	Organizations with policy controls
Copilot Enterprise	Org-wide with custom org knowledge

Exam Tip

GH-900 tests basic awareness: Copilot is AI-powered, subscription-based, and works in the IDE as a code suggestion tool. Deep Copilot architecture questions are for GH-300.

GitHub Packages

GitHub Packages is a package registry integrated with GitHub — store and share packages alongside your code.

Supported package types:

npm (JavaScript)
Maven / Gradle (Java)
NuGet (.NET)
RubyGems (Ruby)
Docker / OCI container images

Packages are scoped to a user or organization and can be public or private. GitHub Actions can publish and consume packages automatically.

CI/CD Concepts

Term	Meaning
CI (Continuous Integration)	Automatically build and test code on every push
CD (Continuous Delivery)	Automatically prepare code for release after CI passes
CD (Continuous Deployment)	Automatically deploy to production after CI passes

GitHub Actions is GitHub's native CI/CD solution. Key benefits:

No external CI server needed
Runs on every push/PR by default
Status checks can block PR merges until they pass

Status Checks

A required status check in branch protection rules means a PR cannot be merged until the check (e.g., a CI workflow) passes. This is how Actions integrates with code review workflows.