Custom resources concept in Kubernetes - Step-by-Step Execution

Process Flow - Custom resources concept

Define CustomResourceDefinition (CRD)

↓

Apply CRD to Kubernetes API

↓

Kubernetes API Server Registers CRD

↓

Create Custom Resource (CR) Instance

↓

Kubernetes Stores CR Instance

↓

Controllers Watch CR and Act

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Custom Behavior Implemented

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End

This flow shows how a custom resource is defined, registered, created, and managed in Kubernetes.

Execution Sample

Kubernetes

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

This YAML defines a CustomResourceDefinition named 'widgets.example.com' to create a new resource type 'Widget'.

Process Table

Step	Action	Kubernetes API State	Result
1	Apply CRD YAML	CRD 'widgets.example.com' registered	API server accepts new resource type
2	Create Custom Resource (CR) instance 'my-widget'	CR instance stored under 'widgets' resource	CR is available for use
3	Controller watches CR	Controller detects 'my-widget' creation	Controller triggers custom logic
4	Controller updates status of 'my-widget'	CR status updated	User sees updated status
5	Delete CR instance 'my-widget'	CR instance removed	Resource cleaned up
6	Delete CRD 'widgets.example.com'	CRD removed, all CRs deleted	Custom resource type no longer available

💡 CRD and CR lifecycle complete; Kubernetes API and controllers manage custom resources.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
CRD 'widgets.example.com'	Not present	Registered	Registered	Registered	Registered	Registered	Deleted
CR instance 'my-widget'	Not present	Not present	Created	Observed by controller	Status updated	Deleted	Deleted

Key Moments - 3 Insights

Why do we need to apply a CRD before creating a custom resource?

The CRD defines the new resource type to the Kubernetes API server. Without it, the API server won't recognize or accept custom resource instances, as shown in step 1 and 2 of the execution_table.

What happens when a controller watches a custom resource?

The controller detects changes to the custom resource and runs custom logic, such as updating status or managing related resources. This is shown in steps 3 and 4 where the controller observes and updates the CR.

What occurs when the CRD is deleted?

Deleting the CRD removes the resource type and all its instances from the cluster, as shown in step 6 where both CRD and CR instances are deleted.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Kubernetes API state after step 2?

A CR instance 'my-widget' is deleted

B Only CRD is registered, no CR instances yet

C CRD is registered and CR instance 'my-widget' is stored

D CRD is deleted

💡 Hint

Refer to the 'Kubernetes API State' column in row for step 2.

At which step does the controller update the status of the custom resource?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'Result' column for step 4 in the execution_table.

If the CRD is not applied, what will happen when creating a custom resource?

A The API server will accept the CR instance

B The API server will reject the CR instance

C The CR instance will be stored but ignored

D The controller will create the CRD automatically

💡 Hint

Look at step 1 and 2 in execution_table to understand the role of CRD.

Concept Snapshot

Custom Resources let you add new resource types to Kubernetes.
Define a CustomResourceDefinition (CRD) YAML and apply it.
Kubernetes API registers the CRD and accepts custom resource instances.
Controllers watch these instances to implement custom behavior.
Deleting the CRD removes all related custom resources.

Full Transcript

Custom resources in Kubernetes allow users to extend the cluster with new resource types. First, you define a CustomResourceDefinition (CRD) YAML that describes the new resource. When you apply this CRD to the cluster, the Kubernetes API server registers it and can accept instances of this new resource. You then create custom resource (CR) instances of this type. Controllers watch these CR instances and perform custom actions, such as updating status or managing related resources. When you delete the CRD, all custom resources of that type are also deleted, and the resource type is removed from the API server.

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