I’ve spent quite a few hours evenings and weekends recently building out a comprehensive suite of GitHub Actions for Sqitch. They cover a dozen versions of Perl, nearly 70 database versions amongst nine database engines, plus a coverage test and a release workflow. A pull request can expect over 100 actions to run. Each build requires over 100 direct dependencies, plus all their dependencies. Installing them for every build would make any given run untenable.

Happily, GitHub Actions include a caching feature, and thanks to a recent improvement to shogo82148/actions-setup-perl, it’s quite easy to use in a version-independent way. Here’s an example:

name: Test
on: [push, pull_request]
jobs:
  OS:
    strategy:
      matrix:
        os: [ ubuntu, macos, windows ]
        perl: [ 'latest', '5.34', '5.32', '5.30', '5.28' ]
    name: Perl ${{ matrix.perl }} on ${{ matrix.os }}
    runs-on: ${{ matrix.os }}-latest
    steps:
      - name: Checkout Source
        uses: actions/checkout@v3
      - name: Setup Perl
        id: perl
        uses: shogo82148/actions-setup-perl@v1
        with: { perl-version: "${{ matrix.perl }}" }
      - name: Cache CPAN Modules
        uses: actions/cache@v3
        with:
          path: local
          key: perl-${{ steps.perl.outputs.perl-hash }}
      - name: Install Dependencies
        run: cpm install --verbose --show-build-log-on-failure --no-test --cpanfile cpanfile
      - name: Run Tests
        env: { PERL5LIB: "${{ github.workspace }}/local/lib/perl5" }
        run: prove -lrj4

This workflow tests every permutation of OS and Perl version specified in jobs.OS.strategy.matrix, resulting in 15 jobs. The runs-on value determines the OS, while the steps section defines steps for each permutation. Let’s take each step in turn:

• “Checkout Source” checks the project out of GitHub. Pretty much required for any project.
• “Setup Perl” sets up the version of Perl using the value from the matrix. Note the id key set to perl, used in the next step.
• “Cache CPAN Modules” uses the cache action to cache the directory named local with the key perl-${{ steps.perl.outputs.perl-hash }}. The key lets us keep different versions of the local directory based on a unique key. Here we’ve used the perl-hash output from the perl step defined above. The actions-setup-perl action outputs this value, which contains a hash of the output of perl -V, so we’re tying the cache to a very specific version and build of Perl. This is important since compiled modules are not compatible across major versions of Perl.
• “Install Dependencies” uses cpm to quickly install Perl dependencies. By default, it puts them into the local subdirectory of the current directory — just where we configured the cache. On the first run for a given OS and Perl version, it will install all the dependencies. But on subsequent runs it will find the dependencies already present, thank to the cache, and quickly exit, reporting “All requirements are satisfied.” In this Sqitch job, it takes less than a second.
• “Run Tests” runs the tests that require the dependencies. It requires the PERL5LIB environment variable to point to the location of our cached dependencies.

That’s the whole deal. The first run will be the slowest, depending on the number of dependencies, but subsequent runs will be much faster, up to the seven-day caching period. For a complex project like Sqitch, which uses the same OS and Perl version for most of its actions, this results in a tremendous build time savings. CI configurations we’ve used in the past often took an hour or more to run. Today, most builds take only a few minutes to test, with longer times determined not by dependency installation but by container and database latency.