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

Process Flow - Custom resources concept

Define CustomResourceDefinition (CRD)

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Apply CRD to Kubernetes API

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Kubernetes API Server Registers CRD

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Create Custom Resource (CR) Instance

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Kubernetes Stores CR Instance

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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.