What is pipeline caching and how do you configure cache keys?

Cache in GitLab CI speeds up jobs by storing files (typically dependencies like node_modules/) between pipeline runs. The cache key determines which jobs share a cache and when the cache is invalidated.

# Basic cache with branch-scoped key:
cache:
  key: "$CI_COMMIT_REF_SLUG"     # different cache per branch
  paths:
    - .npm/
    - node_modules/

# Cache keyed to dependency file (invalidates when file changes):
cache:
  key:
    files:
      - package-lock.json         # cache key changes when lockfile changes
    prefix: "$CI_COMMIT_REF_SLUG"
  paths:
    - node_modules/

# Cache policies (pull-push is default):
job-with-readonly-cache:
  cache:
    key: $CI_COMMIT_REF_SLUG
    paths:
      - node_modules/
    policy: pull                  # only download cache, never update it

job-that-updates-cache:
  cache:
    policy: push                  # only upload, skip downloading

# Per-job cache:
build:
  cache:
    key: "build-$CI_COMMIT_REF_SLUG"
    paths: [dist/]

test:
  cache:
    key: "deps-$CI_COMMIT_REF_SLUG"
    paths: [node_modules/]

Use policy: pull for most jobs that just use the cache, and one dedicated job with policy: push or pull-push to update it. This prevents multiple parallel jobs from overwriting each other's cache updates.

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