Background

Templates are fragile. While they can be bulletproofed, it is hard to tell what the impact of a template update is across all of the existing workspaces. This could show an aggregated response of what the impact is.

Research & vision

We should look at how other platforms like Spacelift or Terraform Enterprise do this. For example, are there tools that measure what impact an update to a module has on all of the underlying infrastructure? There may be better ways to do this.

We should also consider how this could be automated in Ci/Cd platforms and ran alongside other custom template tests a user might make (e.g. "does the project still build with this template update?". This probably means supporting JSON output and human-readable output

Also, perhaps this could be integrated with Infracost down the road to measure how a template update saves/resolves cloud cost across a fleet of workspaces.

How it would work

Here's my initial idea

$ coder templates pull my-template
$ vim main.tf 
# make changes (e.g. hardcode disk size to 45gb)
$ coder templates plan my-template
Running `terraform plan` against all 200 workspaces using this template 

For 150 started workspaces:
- Plan to modify `aws_instance: workspace`
- Plan to modify `ebs_volume: home`

For 48 stopped workspaces:
- Plan to modify `ebs_volume: home`

⚠️ Warning: Plan to destroy `ebs-volume: home` on 2 workspaces:
- bpmct/my_workspace
- spikecurtis/my-workspace

This would fix #7104 as the command currently does nothing