Cheat notes · Module 03 · Platform foundation

Hand the fleet over to Git

A five-minute review for when you have done this module and want it back in your head. Open the full module for the steps and the lab, or the full text to print.

The idea

Terraform built the clusters; a root Application hands everything that runs on them to Git. Root points at applicationsets/, and an ApplicationSet turns the cluster list into one Application per cluster.

Remember

Root Application: root-app.yaml points Argo CD at applicationsets/ in your repo (app of apps). Terraform creates it once the file exists.
ApplicationSet: a template plus a generator. The git files generator reads clusters/*.yaml and creates one Application per cluster: dev-platform-namespaces, prod-platform-namespaces.
AppProject fleet-platform: the project for platform-owned Applications.
After the handover Terraform only runs when clusters or the hub bootstrap change. Everything on the clusters goes through Git and Argo CD.
Self-heal: a manual change survives only until the next comparison. Per-environment differences go in Kustomize overlays, reviewed.

Pitfalls

Deleting a namespace Git declares with kubectl: it is back within seconds, with a new age. Make the change in the environment's overlay, by pull request.
Putting logic in the root app. Keep it boring; it is what you need on the day you rebuild a hub.

Commands

`kubectl --context kind-platform -n argocd get applications -w`	Watch root create the fleet's Applications
`kubectl --context kind-dev get namespaces -l environment=dev`	What Argo CD created on a spoke
`kubectl kustomize addons/namespaces/overlays/dev`	Render an environment's overlay before you push
`git diff root-app.yaml`	Check `repoURL` is your repo after `set-repo-url.sh`

Remove one object in one environment

# addons/namespaces/overlays/dev/kustomization.yaml
patches:
  - patch: |-
      $patch: delete
      apiVersion: v1
      kind: Namespace
      metadata:
        name: platform-system

Test yourself

Answer in your head first, then open the answer.

What does root-app.yaml point at?

applicationsets/. The ApplicationSets generate everything else.

You delete a namespace that Git declares. What happens?

Argo CD recreates it (self-heal). Terraform is not involved after the handover.

When does Terraform run after this module?

Only when clusters or the hub bootstrap change.

Where module 03 leaves you: Git runs the fleet. Argo CD syncs dev-platform-namespaces and prod-platform-namespaces from your repo, and from now on Terraform only runs when clusters change.