Why Custom resources concept in Kubernetes? - Purpose & Use Cases

The Big Idea

What if you could teach Kubernetes to understand and manage anything you want, just like its built-in parts?

The Scenario

Imagine you want to manage a new type of application setting in Kubernetes that is not supported by default. You try to track these settings outside Kubernetes, using spreadsheets or separate scripts.

The Problem

This manual tracking is slow and error-prone. You must constantly update external files and run scripts, risking mismatches and forgotten changes. It's hard to keep everything in sync with your cluster state.

The Solution

Custom resources let you extend Kubernetes with your own resource types. You can define and manage your new settings directly inside Kubernetes, just like built-in resources, making everything consistent and automated.

Before vs After

✗ Before

Track settings in a spreadsheet and run separate scripts to apply changes.

✓ After

kubectl apply -f my-custom-resource.yaml

What It Enables

It enables you to manage any kind of configuration or application state natively within Kubernetes, making automation and scaling much easier.

Real Life Example

A team creates a custom resource to manage database backup schedules, so they can control backups declaratively and monitor them with Kubernetes tools.

Key Takeaways

Manual tracking outside Kubernetes is slow and risky.

Custom resources let you add new resource types inside Kubernetes.

This makes management consistent, automated, and scalable.

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