Using Mill in CI

[re-thc]

What's the best way to use Mill with CI? Are there specific environment variables, settings or specifics to be aware of?

There's not much documentation around it, e.g. for Github Actions or Gitlab.

Locally we just keep the out directory, but is this the same for CI?

What is something that is downloaded once that we should cache long term vs things that change?

Where do dependencies go and how should that be cached?

Thanks!

[lihaoyi]

• Preserving the out/ dir between CI workers does help speed things up with incremental rebuilds
• Setting the FORCE_COLOR=1 environment variable helps make the logs more readable
• Use the ./mill bootstrap script so you don't need to install mill/java/etc. globally on your machine

That's most of it I think?

[lihaoyi]

Adding these to the docs in #6541

[lefou]

Typical downloaded resources are dependencies and JVMs which are both managed by coursier internally. That means, their download locations (typically ~/.cache/coursier) are good candidates to being preserved for the next CI run, to safe time and bandwidth.