charmcraftcache

Reinventing the wheel

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

Installation

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

pipx install charmcraftcache

Usage

ccc add
ccc pack

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

How it works

Why are charmcraft builds slow?

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).

Caching mechanism

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 use a new machine/VM, or when you contribute to a new charm.

charmcraftcache solves the slow first build.

charmcraftcache-hub maintains a list of charms. For each charm, charmcraft pack is used to build Python dependencies from source and the pip wheel cache is uploaded to a GitHub release.

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

Note: Within the GitHub release, each charm has an isolated cache. If the same charm (same GitHub repository and relative path to charmcraft.yaml) is added to the list of charms more than once (with different git refs), the wheels are combined into a single cache. If there are duplicate wheels, the wheel is selected from the ref that is earlier in the list.

Isn't this just a worse version of PyPI?

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

Footnotes

1. Unless --destructive-mode is enabled ↩