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Welcome to this hands-on tutorial on using the Kustomize controller to apply overlays via Flux CD. In this lesson, you’ll learn how to:
  1. Switch to the 3-demo branch containing a kustomization.yaml
  2. Create a Flux GitRepository source
  3. Define a Flux Kustomization resource
  4. Verify the setup with Flux commands
  5. Inspect the deployed Kubernetes resources
  6. View the updated Block Buster application
By the end, you’ll have a working overlay that skips the build phase and applies changes automatically.

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1. Switch to the 3-demo Branch

First, clone or navigate to your application source and check out the 3-demo branch where the kustomization.yaml is already defined: 
cd ~/bb-app-source
git checkout 3-demo
You should see: 
Branch '3-demo' set up to track remote branch '3-demo' from 'origin'.
Switched to a new branch '3-demo'
List the directory to confirm all files: 
ls
FileDescription
deployment.ymlDeployment specification for block-buster
service.ymlService exposing your application
namespace.ymlNamespace definition for isolation
kustomization.yamlKustomize resource list for overlay
Contents of kustomization.yaml: 
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
  - deployment.yml
  - service.yml
  - namespace.yml
Because the kustomization.yaml is present, the Kustomize controller will skip the normal build phase and apply resources directly.

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2. Create a Git Source in the Flux Cluster

Switch to your Flux configuration repository: 
cd ~/block-buster/flux-clusters/dev-cluster/
Define a GitRepository resource that points to branch 3-demo of your app repo: 
flux create source git 3-demo-source-git-bb-app \
  --url https://github.com/sidd-harth-2/bb-app-source \
  --branch 3-demo \
  --interval 10s \
  --export > 3-demo-source-git-bb-app.yaml
3-demo-source-git-bb-app.yaml: 
apiVersion: source.toolkit.fluxcd.io/v1beta2
kind: GitRepository
metadata:
  name: 3-demo-source-git-bb-app
  namespace: flux-system
spec:
  interval: 10s
  ref:
    branch: 3-demo
  url: https://github.com/sidd-harth-2/bb-app-source
Flux will poll the Git repository every 10 seconds and store an artifact for the specified branch.

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3. Create the Kustomization Resource

Next, define a Flux Kustomization that applies the overlay found under kustomize: 
flux create kustomization 3-demo-kustomize-git-bb-app \
  --source GitRepository/3-demo-source-git-bb-app \
  --path kustomize \
  --target-namespace 3-demo \
  --prune true \
  --interval 10s \
  --export > 3-demo-kustomize-git-bb-app.yaml
Contents of 3-demo-kustomize-git-bb-app.yaml: 
apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
kind: Kustomization
metadata:
  name: 3-demo-kustomize-git-bb-app
  namespace: flux-system
spec:
  interval: 10s
  path: ./kustomize
  prune: true
  sourceRef:
    kind: GitRepository
    name: 3-demo-source-git-bb-app
  targetNamespace: 3-demo
Commit and push both Flux manifests: 
git add 3-demo-source-git-bb-app.yaml 3-demo-kustomize-git-bb-app.yaml
git commit -m "Add 3-demo overlay with Kustomization"
git push

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4. Verify with Flux

Check that the Git source is registered: 
flux get sources git
Expected output: 
NAME                        REVISION              SUSPENDED READY MESSAGE
2-demo-source-git-bb-app    2-demo@sha1:7dfa8105  False     True  stored artifact for revision '2-demo@sha1:7dfa8105'
3-demo-source-git-bb-app    3-demo@sha1:990e6de9  False     True  stored artifact for revision '3-demo@sha1:990e6de9'
flux-system                 main@sha1:548f1930    False     True  stored artifact for revision 'main@sha1:548f1930'
Then confirm the overlay is applied: 
flux get kustomizations
Expected output: 
NAME                            REVISION              SUSPENDED READY MESSAGE
2-demo-kustomize-git-bb-app     2-demo@sha1:7dfa8105  False     True  Applied revision: 2-demo@sha1:7dfa8105
3-demo-kustomize-git-bb-app     3-demo@sha1:990e6de9  False     True  Applied revision: 3-demo@sha1:990e6de9
flux-system                      main@sha1:548f1930    False     True  Applied revision: main@sha1:548f1930

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5. Inspect Deployed Resources

List all namespaces to ensure 3-demo exists: 
kubectl get ns
View resources in the 3-demo namespace: 
kubectl get all -n 3-demo
Sample output: 
NAME                                         READY   STATUS    RESTARTS AGE
pod/block-buster-68f7ccdfd7-62w6f            1/1     Running   0        69s

NAME                           TYPE       CLUSTER-IP      EXTERNAL-IP   PORT(S)        AGE
service/block-buster-service   NodePort   10.101.115.232  <none>        80:30008/TCP   69s

NAME                             READY UP-TO-DATE AVAILABLE AGE
deployment.apps/block-buster      1/1   1          1         69s

NAME                                         DESIRED CURRENT READY AGE
replicaset.apps/block-buster-68f7ccdfd7     1       1       1     69s

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6. View the Updated Application

Open your browser to http://<node-ip>:30008. You’ll notice that the game now shows three lives instead of one:
The image shows a "Block Buster" game interface with colorful blocks, a ball, and a paddle. It also displays technical details like pod name, IP, and app version on the left.

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