How does bundle clean fit into CircleCI cache? - circleci

Question

The CircleCI docs include some information on why bundle clean should be run when cache is involved, but I'm having a hard time understanding how. I'm confused about the code block in the Bundler (Ruby) section here:
https://circleci.com/docs/2.0/caching/#bundler-ruby
The code block in question is:
- run: bundle install & bundle clean
- restore_cache:
keys:
# when lock file changes, use increasingly general patterns to restore cache
- v1-gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
- v1-gem-cache-{{ arch }}-{{ .Branch }}-
- v1-gem-cache-{{ arch }}-
- save_cache:
paths:
- ~/.bundle
key: v1-gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
I'm reading this process as:
Run bundle install before the cache is put in place (a full gem installation time before restoring cache, which negates the time-saving benefits of a cache) and then run bundle clean
Restore the cache over the installed gems
Don't do anything (between restore_cache and save_cache steps)
Save the new cache
Am I understanding that process correctly?
This looks to me like restore_cache and save_cache steps are not going to be effective, because the full bundle install time would have already been spent.
If I'm understanding things, would this be a more effective process?
- restore_cache:
keys:
# when lock file changes, use increasingly general patterns to restore cache
- v1-gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
- v1-gem-cache-{{ arch }}-{{ .Branch }}-
- v1-gem-cache-{{ arch }}-
- run: bundle install & bundle clean
- save_cache:
paths:
- ~/.bundle
key: v1-gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
If I'm not understanding this correctly, can anyone help clarify how the suggested code block works?
Update:
It also looks like
- run: bundle install & bundle clean
needs to be modified to
- run: bundle install && bundle clean

Answer 1

I'm pretty sure you are correct and that this is a typo in the CircleCI Docs. I've opened a PR to fix: https://github.com/circleci/circleci-docs/pull/2663