Kroc - Kubernetes Reactive Object Creator

This is an educational hobby project designed to demonstrate the creation of Kubernetes Operators using Go (Golang) and the kubebuilder framework.

The KROC operator introduces a custom resource, Kroc, which enables reactive resource derivation:

• It allows you to watch arbitrary Kubernetes objects (e.g., Deployments, Services) across the cluster.
• It creates and manages derived objects using attributes retrieved from the watched resources.

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✨ Key Features & Behavior

• Custom Resource Definition (CRD): Defines the Kroc resource where all configuration (watched objects, templates) resides.
• Go Templating: Leverages the Go Template Language (text/template) within the resourceToCreate field, enabling dynamic and flexible generation of multiple derived Kubernetes objects.
• Automatic Synchronization:
  • If the watched object changes, all of its derived child objects will be regenerated.
  • If any derived child object is manually removed, the operator automatically recreates it to maintain the desired state.
• Controller Composition: Demonstrates the technique of chaining three distinct controllers to clearly separate the responsibilities of resource observation, custom resource management, and object generation.

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🏗️ Architecture & Concept

The core architecture consists of three specialized and interconnected controllers:

1. Kroc Controller (The Config Manager)

This is the primary controller that reconciles the custom Kroc resource. Its main role is to consume the configuration:

• Monitors all instances of the custom Kroc resource.
• Initializes and configures the Watch Controller based on the object specifications in the Kroc spec.
• Manages the overall lifecycle and status of the Kroc object.

2. Watch Controller (The Observer)

This controller handles the dynamic tracking of external resources defined in the Kroc configuration.

• Sets up watches on the arbitrary target objects specified by fields like apiVersion, kind, nameRegex, and namespaceRegex.
• When a target object changes, it extracts the required attributes (data to be used in the template) and triggers the Creator Controller to regenerate the children.

3. Creator Controller (The Derivation Handler)

This controller manages the creation and lifecycle of the final derived objects.

• Applies the resulting YAML/JSON to the cluster, handling creation logic to ensure the derived object's state matches the template.

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💡 Example Usage

The resourceToCreate field uses the Go Template Language, allowing you to generate one or more Kubernetes objects dynamically. Data from the watched object is available via the {{.parent}} object (e.g., {{.parent.metadata.uid}}).

Defining a Watch and Creating Multiple Pods

The following manifest creates a Kroc resource that watches Deployments matching the regex pattern "pawel" in namespaces matching "pawel". When a match is found, it uses templating to create two separate Pods whose existence is automatically managed by the operator.

apiVersion: apps.pwlctk.ovh/v1alpha1
kind: Kroc
metadata:
  name: kroc-sample
spec:
  watchObject:
    apiVersion: apps/v1
    kind: Deployment
    nameRegex: pawel
    namespaceRegex: pawel
  resourceToCreate: |
    # Define variables for use in the template
    {{- $pod1 := 1 -}}
    {{- $pod2 := 2 -}}
    
    # --- Pod 1: Dynamically named and labeled using parent UID ---
    apiVersion: v1
    kind: Pod
    metadata:
      name: nginx-pod-{{ $pod1 }}
      namespace: pawel
      labels:
        environment: development
        # Inject attribute from the watched (parent) object
        parentuid: "{{.parent.metadata.uid}}"
    spec:
      containers:
      - name: nginx-container
        image: nginx:latest
        ports:
        - containerPort: 80
          protocol: TCP
    ---
    # --- Pod 2: Second object definition in the same template ---
    apiVersion: v1
    kind: Pod
    metadata:
      name: nginx-pod-{{ $pod2 }}
      namespace: pawel
      labels:
        environment: development
    spec:
      containers:
      - name: nginx-container
        # Corrected image tag for example clarity
        image: nginx:latest 
        ports:
        - containerPort: 80 
          protocol: TCP