feat: ADR - TF tagging

[MichaelThamm]

Reference to discussion:

• Tagging for TF modules in charm repos #427

[lucabello]

Why the ambiguity in the tag? Can we swap this with a concrete number like the other examples?

[MichaelThamm]

❗️Some issues with our approach

A tag is based on a commit on track/0 and we are now on track/1, but we never had to make any TF changes until track/2 is released. Now we bump the major to e.g. 1.0.0 and this tag is based on a commit on track/2. However, now we want to go back and add an endpoint to the 0.x.x TF module, but I don't know which track to tag a commit from.

Solution idea: we can use branches named after major TF tag versions to avoid outdated TF in older branches.

---

If we have charms in a product that do not have breaking changes, then they have minor bumps per cycle. What do we do with the product version to stay up-to-date with docs? The concern Luca brought up is that we will not be able to version our stack docs separately from the i.e., we have a core mapping conflict between TF modules vs. RTD.

We might have a workaround if we name tracks after major.minor of the product tf module
Then you'd have separate docs every time we change the charm channels (likely every 6 months); the downside is of course that if most documentation stays the same, we need to backport docs changes, but that's probably fine

[lucabello]

Note that the validation on tracks also implies users can't deploy chosen version of the workloads besides the few combinations that we explicitly allow.

If we have:

• 1.0.0 with Loki 2.9 and Grafana 10
• 2.0.0 with Loki 3.6 and Grafana 12

If a user wants to deploy Loki 3.6 with Grafana 10, they'll have to fork our Terraform module and write their own; I would add that here.

[MichaelThamm]

This is not necessarily true. The bumping of applications/components within COS (likely every cycle) may not require a major change in the product module. E.g., this is also possible if we did not break the product API:

• 1.0.0 with Loki 2.9 and Grafana 10
• 1.1.0 with Loki 3.6 and Grafana 12

Regardless, the user can still override the channel for a specific application if they choose to. E.g.,

module "cos" {
  source = 1.1.0
  grafana = {channel = 10.1/stable}
}

[lucabello]

Superseded by https://github.com/canonical/observability-team/blob/bc22671f6c40bbad2e13be4962036d1f3f26d765/notes/discussions/2026-04-23--terraform-tagging.md