terraform-github-action-cache-example's Introduction

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. For example, the AWS Provider at time of writing weighs in around 200MB uncompressed.
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.

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