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Welcome to Part 2 of our deep dive into FluxCD Architecture. In this lesson, we’ll explore each Flux controller and see how they collaborate to implement GitOps workflows on Kubernetes. Below is an overview of the five core controllers deployed in the flux-system namespace when you install Flux:
ControllerResource TypesPrimary Function
Source ControllerGitRepository, HelmRepository, OCIRepository, BucketFetches and makes external artifacts available
Kustomize ControllerKustomizationBuilds and applies Kustomize overlays
Helm ControllerHelmReleaseManages Helm chart lifecycle (install, upgrade, rollback)
Image Reflector & AutomationImageRepository, ImagePolicy, ImageUpdateAutomationDetects new image tags and updates Git manifests
Notification ControllerAlert, ReceiverRoutes inbound webhooks and outbound notifications

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1. Source Controller

The Source Controller offers a consistent interface to pull artifacts from various external systems into your cluster. Supported sources:
  • Git repositories via GitRepository
  • Helm chart repositories via HelmRepository
  • OCI registries via OCIRepository
  • S3-compatible buckets via Bucket
Example: A simple Kubernetes Deployment stored in Git 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: game
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: game
    spec:
      containers:
        - name: game
          image: game:v1
The Source Controller will clone or pull the repo, then expose the manifest to downstream Flux controllers.
Ensure you provide credentials for private Git, OCI or S3 repositories via Kubernetes Secrets.

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2. Kustomize Controller

Use the Kustomize Controller when your repository holds raw YAML or Kustomize overlays. Define a Kustomization resource that:
  1. References a fetched source (e.g., GitRepository).
  2. Builds the Kustomize overlays.
  3. Applies the resulting manifests to your Kubernetes cluster.
apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
kind: Kustomization
metadata:
  name: game-app
spec:
  interval: 5m0s
  sourceRef:
    kind: GitRepository
    name: game-repo
  path: "./overlays/production"
  prune: true
  validation: client
Every commit to the Git repo triggers the Source Controller, which in turn prompts the Kustomize Controller to reconcile your desired state.

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3. Helm Controller

If you prefer Helm charts, Flux can fetch them via the Source Controller from Git, Helm repos, or OCI registries. Declare a HelmRelease to manage:
  • Chart source and version
  • Custom values
  • Release settings (rollback, tests)
apiVersion: helm.toolkit.fluxcd.io/v2beta1
kind: HelmRelease
metadata:
  name: game-chart
spec:
  chart:
    spec:
      chart: ./charts/game
      sourceRef:
        kind: GitRepository
        name: game-repo
      version: ">=1.2.0"
  interval: 10m
  values:
    replicaCount: 3
The Helm Controller watches these resources and automates installs, upgrades, rollbacks, tests, and uninstalls.

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4. Image Controllers

Flux splits image automation into two controllers for granular control:
ControllerRole
Image ReflectorPeriodically scans registries for new image tags, populating ImageRepository resources
Image AutomationObserves ImagePolicy updates and rewrites Git manifests via ImageUpdateAutomation
When a new tag (e.g., game:v2) matches your policy, Flux commits the updated image reference back to Git: 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: game
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: game
    spec:
      containers:
        - name: game
          image: game:v2
The Image Automation Controller requires write access to your Git repository. Ensure branch protections and commit permissions are configured securely.

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5. Notification Controller

The Notification Controller connects Flux events with external systems:
  • Inbound: Listens for Git webhooks (GitHub, GitLab) or registry events to trigger immediate reconciliation.
  • Outbound: Sends events (deployment success/failure, image updates, reconciliation errors) to channels like Slack, Teams, Discord, or email.
You configure Alert and Receiver resources to route events: 
apiVersion: notification.toolkit.fluxcd.io/v1beta1
kind: Alert
metadata:
  name: slack-alert
spec:
  eventSources:
    - kind: GitRepository
      name: game-repo
  eventSeverity: info
  receiverRefs:
    - name: team-slack
Whenever the Source Controller pulls commits or the Image Automation Controller pushes an update, the Kustomize or Helm Controller reconciles your cluster to match the latest Git state—completing the GitOps loop.

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