Custom resources concept in Kubernetes - Mini Project: Build & Apply

Working with Kubernetes Custom Resources

📖 Scenario: You are managing a Kubernetes cluster and want to extend its functionality by creating a custom resource to track application versions.

🎯 Goal: Learn how to define a Custom Resource Definition (CRD), create a custom resource, and view its details using kubectl commands.

📋 What You'll Learn

Create a Custom Resource Definition YAML file named appversion-crd.yaml with specified fields

Create a custom resource YAML file named myapp-v1.yaml using the CRD

Use kubectl commands to apply and view the custom resource

💡 Why This Matters

🌍 Real World

Kubernetes Custom Resources let you add your own resource types to the cluster. This is useful for managing specialized application data or configurations beyond built-in Kubernetes objects.

💼 Career

Understanding custom resources is important for Kubernetes administrators and DevOps engineers who want to extend Kubernetes capabilities or build operators.

1

Create the Custom Resource Definition YAML

Create a file named appversion-crd.yaml with a CustomResourceDefinition named appversions.example.com in the apiextensions.k8s.io/v1 API group. The CRD should define a spec with group as example.com, versions with a single version named v1 that is served and stored, and a scope of Namespaced. The names section should specify plural as appversions, singular as appversion, kind as AppVersion, and shortNames as av. Include an openAPIV3Schema with a spec containing a version field of type string.

Kubernetes

# Create the appversion-crd.yaml file with the specified CustomResourceDefinition
# Your code here

Hint

Remember to follow the Kubernetes CRD structure carefully and indent YAML properly.

2

Create a Custom Resource YAML

Create a file named myapp-v1.yaml that defines a custom resource of kind AppVersion in the example.com/v1 API version. Set the metadata.name to myapp-v1 and the spec.version to 1.0.0.

Kubernetes

# Create the myapp-v1.yaml file with the custom resource definition
# Your code here

Hint

Use the correct API version and kind matching the CRD you created.

3

Apply the CRD and Custom Resource

Use kubectl apply -f appversion-crd.yaml to create the CustomResourceDefinition, then use kubectl apply -f myapp-v1.yaml to create the custom resource.

Kubernetes

# Run the kubectl apply commands below
# Your code here

Hint

Use the exact kubectl commands to apply both YAML files.

4

View the Custom Resource Details

Run kubectl get appversions to list the custom resources, then run kubectl get appversion myapp-v1 -o yaml to see the details of the myapp-v1 resource.

Kubernetes

# Run kubectl commands to view the custom resource
# Your code here

Hint

First list all appversions, then describe the specific one in YAML format.

Practice

1. What is the main purpose of a Custom Resource in Kubernetes?

easy

A. To delete all existing Kubernetes resources

B. To replace built-in Kubernetes objects like Pods

C. To automatically update Kubernetes itself

D. To add new object types to Kubernetes for custom app needs

Solution

1. Step 1: Understand Kubernetes object types

   Kubernetes has built-in objects like Pods and Services, but sometimes you need new types for your apps.

2. Step 2: Role of Custom Resources

   Custom Resources let you define new object types to extend Kubernetes capabilities without changing its core.

3. Final Answer:

   To add new object types to Kubernetes for custom app needs -> Option D

4. Quick Check:

   Custom Resources extend Kubernetes = A [OK]

Hint: Custom Resources add new types, not replace or delete [OK]

Common Mistakes:

• Thinking Custom Resources replace built-in objects
• Believing Custom Resources update Kubernetes itself
• Confusing Custom Resources with deleting resources

2. Which YAML kind is used to define a Custom Resource type in Kubernetes?

easy

A. CustomResourceDefinition

B. Pod

C. Deployment

D. Service

Solution

1. Step 1: Identify the YAML kind for custom types

   Custom Resource types are defined by a special Kubernetes object called CustomResourceDefinition.

2. Step 2: Differentiate from common kinds

   Pod, Deployment, and Service are built-in kinds, not for defining new types.

3. Final Answer:

   CustomResourceDefinition -> Option A

4. Quick Check:

   CustomResourceDefinition defines new types [OK]

Hint: CustomResourceDefinition is the special kind for custom types [OK]

Common Mistakes:

• Choosing Pod or Deployment as custom type definition
• Confusing Service with CustomResourceDefinition
• Using incorrect kind names in YAML

3. Given this CustomResourceDefinition snippet, what is the spec.names.kind used for?

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: widgets.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
  scope: Namespaced
  names:
    plural: widgets
    singular: widget
    kind: Widget
    shortNames:
    - wdg

medium

A. It controls the storage backend for the resource

B. It defines the kind name used when creating custom resource objects

C. It sets the namespace where the resource lives

D. It specifies the API version of the custom resource

Solution

1. Step 1: Understand spec.names.kind role

   This field sets the kind name you use in YAML when creating instances of this custom resource.

2. Step 2: Differentiate from other fields

   API version is under versions.name, scope is separate, storage backend is not set here.

3. Final Answer:

   It defines the kind name used when creating custom resource objects -> Option B

4. Quick Check:

   spec.names.kind = kind name for objects [OK]

Hint: spec.names.kind is the object kind name in YAML [OK]

Common Mistakes:

• Confusing kind with API version
• Thinking it sets namespace or storage
• Mixing plural and kind meanings

4. You applied a CustomResourceDefinition YAML but get an error: error: unable to recognize "crd.yaml": no matches for kind "CustomResourceDefinition" in version "v1beta1". What is the likely cause?

medium

A. Missing metadata.name field in the YAML

B. Trying to create a Pod instead of a CustomResourceDefinition

C. Using deprecated API version v1beta1 instead of apiextensions.k8s.io/v1

D. Incorrect indentation in the YAML file

Solution

1. Step 1: Analyze the error message

   The error says no matches for kind "CustomResourceDefinition" in version "v1beta1" which means the API version is not supported.

2. Step 2: Check Kubernetes API version support

   Since Kubernetes 1.22+, the v1beta1 version for CustomResourceDefinition is removed; use apiextensions.k8s.io/v1 instead.

3. Final Answer:

   Using deprecated API version v1beta1 instead of apiextensions.k8s.io/v1 -> Option C

4. Quick Check:

   Deprecated API version causes no match error [OK]

Hint: Use apiextensions.k8s.io/v1 for CRD, not v1beta1 [OK]

Common Mistakes:

• Ignoring API version deprecation
• Assuming missing metadata causes this error
• Blaming YAML indentation without checking version

5. You want to create a custom resource named Gadget with group devices.example.com and version v1. Which is the correct minimal spec section of the CustomResourceDefinition YAML?

hard

A. group: devices.example.com
versions:
- name: v1
served: true
storage: true
scope: Namespaced
names:
plural: gadgets
singular: gadget
kind: Gadget

B. group: devices.example.com
version: v1
scope: Cluster
names:
plural: gadgets
kind: Gadget

C. apiVersion: v1
group: devices.example.com
versions:
- name: v1
served: false
storage: true
scope: Namespaced
names:
plural: gadgets
kind: Gadget

D. group: devices.example.com
versions:
- name: v2
served: true
storage: true
scope: Namespaced
names:
plural: gadgets
singular: gadget
kind: Gadget

Solution

1. Step 1: Check group and version correctness

   The group must be devices.example.com and version v1 served and storage true.

2. Step 2: Validate scope and names

   Scope should be Namespaced (common default), and names must include plural, singular, and kind Gadget.

3. Step 3: Eliminate wrong options

   group: devices.example.com version: v1 scope: Cluster names: plural: gadgets kind: Gadget uses singular version field and Cluster scope, apiVersion: v1 group: devices.example.com versions: - name: v1 served: false storage: true scope: Namespaced names: plural: gadgets kind: Gadget has served false, group: devices.example.com versions: - name: v2 served: true storage: true scope: Namespaced names: plural: gadgets singular: gadget kind: Gadget uses version v2 instead of v1.

4. Final Answer:

   group: devices.example.com versions: - name: v1 served: true storage: true scope: Namespaced names: plural: gadgets singular: gadget kind: Gadget -> Option A

5. Quick Check:

   Correct group, version, scope, and names = A [OK]

Hint: Use served: true and storage: true for active versions [OK]

Common Mistakes:

• Using version instead of versions list
• Setting served: false disables API
• Mismatching version name or group
• Omitting singular name