What does cache actually do?

GitLab CI/CD
1

I may be using cache in the wrong way, or I’m not seeing it do what I expect it to. The relevant bit of my yml looks like:

.install node dependencies: &import_install_node_dependencies
  stage: install-dependencies
  image: node:9.0
  script:
    - npm install -g bower
    - npm install --production
    - bower --allow-root install --production
  cache:
    key: "$CI_ENVIRONMENT_SLUG-$CI_JOB_NAME"
    paths:
      - node_modules/
      - bower_components/
      - assets/vendor/
  artifacts:
    paths:
      - node_modules/
      - bower_components/
      - assets/vendor/
    expire_in: 3 mos

install node dependencies staging:
  <<: *import_install_node_dependencies
  only:
    - features/staging
  environment:
    name: staging

The relevant output then looks like:

Checking cache for staging-install node dependencies staging...
Successfully extracted cache
$ npm install -g bower
$ npm install --production
added 114 packages in 20.546s
$ bower --allow-root install --production
Creating cache staging-install node dependencies staging...
node_modules/: found 37062 matching files          
WARNING: bower_components/: no matching files      
assets/vendor/: found 580 matching files           
Created cache
Uploading artifacts...
node_modules/: found 37062 matching files          
WARNING: bower_components/: no matching files      
assets/vendor/: found 580 matching files           
Uploading artifacts to coordinator... ok            id=2901 responseStatus=201 Created token=psdipQu5
Job succeeded

What this looks like to me is that it’s successfully finding files to cache, creating a cache from them, and downloading them on each pipeline. Great. What it’s not doing is… not running the tedious npm install / bower install etc etc that I was hoping caching all that… stuff… would prevent. It’s found cached versions, but it’s still recreating the files each time. Have I completely misunderstood what cache is supposed to be for, or am I Doing It Wrong?

Thanks :slight_smile: