Reinventing the wheel

Fast first-time builds for charmcraft—on a local machine or CI

Install pipx: https://pipx.pypa.io/stable/installation/

pipx install charmcraftcache

ccc add
ccc pack

For best results, add charm-strict-dependencies: true to your charmcraft.yaml.

Instead of downloading wheels from PyPI (which pip does by default), charmcraft builds Python package wheels from source (i.e. with pip install --no-binary).

charmcraft builds each charm base in a separate LXC container¹. Within each container, pip has an internal cache for wheels built from source & for HTTP responses.

charmcraft 2.5 moved the pip internal cache to the LXC host machine, so that one pip cache is used for all LXC containers. (This increases the chance of a cache hit—a faster build.)

However, charmcraft builds are still slow the first time the wheel is built. This happens on CI runners, when you update dependencies, use a new machine/VM, or contribute to a new charm.

charmcraftcache solves the slow first build.

charmcraftcache-hub maintains a list of charms. For each charm, every Python dependency is built from source & uploaded to a GitHub release.

ccc pack downloads these pre-built wheels to charmcraft's pip cache (and then runs charmcraft pack).

Pretty much. The only difference is charmcraftcache-hub wheels are built from source on our runners, instead of built by the package maintainer.