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Git Change Detection Integration ​

The git change detection integration helps to detect and mark stacks as changed.

Introduction ​

The approach is as simple as computing the changed stacks from the changed files discovered by the git diff between the revision of the last change (ie. the released revision) and the current change.

For the sake of clarity, we'll refer to the released revision as baseref, which is an abbreviation for base reference. Usually, this term corresponds to the default branch (origin/main or origin/default).

By default, the baseref can have two values, depending on if you're in the default branch or a feature branch, and they are:

  • origin/main: if you're in a feature branch.
  • HEAD^: if you're in the default branch.

The HEAD^ syntax means the first parent of the HEAD commit and the reasoning for using it for the default branch is that once you merge, your PR you need to apply the changes in the CI or locally. If the project adopts a non-fast-forward merge strategy, every commitβ€”aside from the first oneβ€”on the default branch becomes a merge commit. Utilizing HEAD^ as the baseref enables detection of modifications in the most recently merged code Having explained that, hopefully, it becomes clear that change detection in Terramate works best if the project follows a git flow defined below (by the way, this is probably the most common git flow used by the git community):

  1. The default branch (commonly main) is considered to be the stable branch that represents the deployed state of your IaC.
  2. Changes that should be planned and applied should be added through a feature or bugfix branch.
  3. The IaC project uses non fast-forwarded merge commits. (the default in GitHub and Bitbucket).

These are standard in most companies but option 3 is controversial as it means flows depending on git rebase in the main branch would not work. If that's the case for your company, it will require a bit of manual work to apply the changes after merging but alternatively, commands such as terraform apply can be run in the PR's branch just before merging using the default branch base ref (origin/main).

Configuration ​

The baseref can be manually changed by the terramate command line at any given point in time using the --git-change-base option or through the project configuration, so different strategies for computing the changes are supported.

If you adopt the rebase merge strategy and need to apply modifications to stacks affected by the last rebase, it's crucial to first identify the base commit (the commit before the merge). You can then provide this commit hash in the --git-change-base flag to accomplish the required changes.

$ git branch
$ git rev-parse HEAD
$ git pull origin main
$ terramate run --changed --git-change-base 80e581a8ce8cc1394da48402cc68a1f47b3cc646 \
    -- terraform plan

--git-change-base supports all git revision syntaxes, so if you know the number of parent commits you can use HEAD^n or HEAD@{<query>}, etc.