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Maven / GitHub Actions Interview Questions

How do you cache dependencies in GitHub Actions using actions/cache?

actions/cache saves and restores a directory between workflow runs so that package managers like npm, Maven, or pip do not re-download the same dependencies on every run. A cache hit can reduce a 3-minute install step to a few seconds.

The action requires two inputs: path (the directory to cache) and key (a string that identifies the cache). If the key matches an existing cache, the directory is restored before your install step. If not, the action records a cache miss and saves the directory at the end of the job for future runs.

- name: Cache npm dependencies
  uses: actions/cache@v4
  with:
    path: ~/.npm
    key: npm-${{ runner.os }}-${{ hashFiles('**/package-lock.json') }}
    restore-keys: |
      npm-${{ runner.os }}-

- name: Install dependencies
  run: npm ci

The hashFiles('**/package-lock.json') expression produces a hash of your lock file. When the lock file changes (new dependency added), the hash changes, the old cache is missed, and a fresh install populates a new cache. restore-keys: provides fallback prefixes — if the exact key is not found, GitHub tries caches whose key starts with npm-ubuntu-latest-, giving a partial hit that is still faster than a cold install.

Popular language setups — actions/setup-node, actions/setup-java, actions/setup-python — have a built-in cache: input that wraps actions/cache automatically:

- uses: actions/setup-node@v4
  with:
    node-version: '20'
    cache: 'npm'            # handles path + key automatically
In actions/cache, what expression is commonly used in the key: to invalidate the cache when dependencies change?
What happens in actions/cache when no cache matches the exact key but a restore-keys prefix matches?

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More Related questions...

What is GitHub Actions and what problems does it solve? What are the key components of GitHub Actions — workflows, jobs, steps, actions, and runners? How is a GitHub Actions workflow file structured, and where must it be placed? What are workflow triggers (on:) and which event types does GitHub Actions support? What is the difference between push, pull_request, and workflow_dispatch triggers? What are jobs in GitHub Actions, and how do they run in parallel by default? What are steps, and what is the difference between run: and uses: in a step? What are runners, and what is the difference between GitHub-hosted and self-hosted runners? What is the GitHub Actions Marketplace and how do you find and use actions from it? How do you use the actions/checkout action and what does it do? How do you pass environment variables and secrets to a GitHub Actions workflow? What is the difference between the env:, secrets:, and vars: contexts in GitHub Actions? How do you cache dependencies in GitHub Actions using actions/cache? How do you use matrix builds in GitHub Actions to test across multiple environments? How do you control job execution order in GitHub Actions using needs:? How do you share data between steps within a job using step outputs? How do you share build artifacts between jobs using actions/upload-artifact and actions/download-artifact? What are reusable workflows in GitHub Actions and how do you call them? What are composite actions and when should you choose them over reusable workflows? How do you set up a Docker container service for integration tests using services: in GitHub Actions? How do you use conditional steps with if: in GitHub Actions? What are the key GitHub Actions expression contexts and what information does each provide? How do you use concurrency groups to cancel outdated workflow runs in GitHub Actions? What is the GITHUB_TOKEN and what permissions does it have? How do you trigger one GitHub Actions workflow from another using workflow_run? How do you write a custom JavaScript action for GitHub Actions? How do you write a custom Docker container action for GitHub Actions? How do you implement a complete CI/CD pipeline for a container image in GitHub Actions — build, push to a registry, and deploy? How do you implement path filtering so a workflow only runs when specific files change? How do you debug failing GitHub Actions workflows — enabling debug logging and using tmate? How do you implement branch protection rules with required GitHub Actions status checks? How do you handle large monorepos with multiple services in GitHub Actions? What are OpenID Connect (OIDC) tokens in GitHub Actions and how do they replace long-lived cloud credentials? How do you prevent secret exposure and follow security hardening best practices in GitHub Actions? What are the key differences between GitHub Actions, Jenkins, and GitLab CI?
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