Terraform GitHub Actions caching example

Summary

An implementation of caching Terraform providers via actions/cache within a workflow run in an attempt to improve terraform init|plan|apply execution times.

Why?

• Providers are external to Terraform itself and require download during terraform init operations.
• Common use providers can often be very large in size (e.g. the AWS Provider at time of writing weighs in around 200MB).
• By caching these provider binaries between GitHub Action runs, we hope to have required configuration providers available to terraform sooner!

Example

See: .github/workflows/example.yaml

Breakdown of the key workflow steps:

• Git source is fetched and Terraform setup via actions/checkout and hashicorp/setup-terraform respectively.

• Terraform plugin cache is configured:

  • By default Terraform downloads providers to each .terraform/ directory within a configuration. By enabling a system wide cache, terraform downloads each provider once to a central location and symlink back into each .terraform/ directory - avoiding repeated downloads. More details here.
  • The plugin cache is set with the plugin_cache_dir= property within ~/.terraformrc.
  • With a global plugin cache location enabled, we've now got a perfect candidate for workflow run caching.

• Next, actions/cache is setup to save/restore the plugin cache location we've set at ~/.terraform.d/plugin-cache. The cache key is built using a hash of dependency lock files (.terraform.lock.hcl), introduced in Terraform 0.14.

  Configuration lock file can be created/updated via the following:

   terraform providers lock -platform=darwin_amd64 -platform=linux_amd64

• Finally a terraform init is run, which sets up a trivial Terraform configuration of main.tf.