Custom Resource Definitions (CRDs) in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of a Custom Resource Definition (CRD) in Kubernetes?

Choose the best description of what a CRD does in a Kubernetes cluster.

A It allows users to define new resource types that behave like built-in Kubernetes objects.

B It automatically scales pods based on CPU usage.

C It manages network policies between pods.

D It schedules pods to specific nodes based on resource availability.

💡 Hint

Think about how Kubernetes can be extended to manage new types of objects.

A CRD lets you create your own resource types so Kubernetes can understand and manage them like built-in objects such as pods or services.

💻 Command Output

intermediate

What is the output of this command after creating a CRD?

You run kubectl get crds after applying a CRD manifest named widgets.example.com. What will you see?

Kubernetes

kubectl get crds

A

NAME                  CREATED AT
widgets.example.com    2024-06-01T12:00:00Z

B No resources found in default namespace.

C Error from server (NotFound): crds "widgets.example.com" not found

D

NAME                  READY   STATUS    RESTARTS   AGE
widgets.example.com    True    Running   0          5m

💡 Hint

Remember that CRDs are cluster-wide resources, not namespace-scoped.

The kubectl get crds command lists all CRDs in the cluster with their names and creation timestamps.

❓ Configuration

advanced

Identify the error in this CRD manifest snippet

Review this CRD YAML snippet and select the option that correctly identifies the error.

Kubernetes

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: gadgets.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                size:
                  type: integer
  scope: Namespaced
  names:
    plural: gadgets
    singular: gadget
    kind: Gadget
    listKind: GadgetList

A The 'versions' list is missing the 'subresources' field, which is mandatory.

B The 'schema' field should be 'schema.openAPIV3Schema' under 'versions', but here it is correct.

C The 'metadata.name' must be the plural form plus the group, which is correct here.

D The 'scope' field value 'Namespaced' is invalid; it should be lowercase 'namespaced'.

💡 Hint

Check the indentation and field names carefully under 'versions'.

The 'schema' field is correctly named 'schema.openAPIV3Schema' under 'versions'. The other options contain incorrect claims: 'scope' is case-sensitive and must be 'Namespaced' with uppercase N; 'metadata.name' is correct; 'subresources' is optional.

❓ Troubleshoot

advanced

Why does this CRD fail to create?

You apply a CRD manifest but get the error: error: unable to recognize "crd.yaml": no matches for kind "CustomResourceDefinition" in version "apiextensions.k8s.io/v1beta1". What is the cause?

A The 'kind' field should be 'CustomResource' instead of 'CustomResourceDefinition'.

B The CRD manifest is missing the 'metadata' section.

C The API version 'apiextensions.k8s.io/v1beta1' is deprecated and no longer supported in recent Kubernetes versions.

D The Kubernetes cluster does not have the CRD controller installed.

💡 Hint

Check the API version compatibility with your Kubernetes cluster version.

The 'apiextensions.k8s.io/v1beta1' API version for CRDs was removed in Kubernetes 1.22+. You must use 'apiextensions.k8s.io/v1' instead.

🔀 Workflow

expert

Order the steps to create and use a new CRD in Kubernetes

Put these steps in the correct order to successfully create and use a new Custom Resource Definition.

A 2,1,3,4

B 3,1,2,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

You must define and register the CRD before creating custom resources.

First, write the CRD manifest, then apply it to register the new resource type. After that, create instances of the custom resource and finally verify them.

Practice

1. What is the main purpose of a Custom Resource Definition (CRD) in Kubernetes?

easy

A. To add new resource types to Kubernetes that are not built-in

B. To update the Kubernetes version automatically

C. To manage user permissions in Kubernetes

D. To monitor cluster health and performance

Solution

1. Step 1: Understand what CRDs do

   CRDs allow users to create their own resource types beyond the default Kubernetes resources.

2. Step 2: Compare options

   Automatic version updates, user permissions (RBAC), and cluster monitoring are separate Kubernetes features unrelated to CRDs.

3. Final Answer:

   To add new resource types to Kubernetes that are not built-in -> Option A

4. Quick Check:

   CRDs = add custom resource types [OK]

Hint: CRDs = add your own resource types [OK]

Common Mistakes:

• Confusing CRDs with RBAC for permissions
• Thinking CRDs update Kubernetes versions
• Assuming CRDs monitor cluster health

2. Which of the following is the correct YAML key to define the API version for a CRD?

easy

A. version

B. api_version

C. apiVersion

D. apiVer

Solution

1. Step 1: Recall Kubernetes YAML syntax

   Kubernetes uses camelCase keys like apiVersion to specify API versions.

2. Step 2: Check other options

   version, api_version, and apiVer use incorrect formats not recognized by Kubernetes.

3. Final Answer:

   apiVersion -> Option C

4. Quick Check:

   Correct YAML key = apiVersion [OK]

Hint: Use camelCase keys like apiVersion in Kubernetes YAML [OK]

Common Mistakes:

• Using underscores instead of camelCase
• Using incomplete or shortened keys
• Confusing version with apiVersion

3. Given this CRD snippet, what is the scope of the custom resource?

spec:
  scope: Namespaced
  group: example.com
  names:
    kind: Widget
    plural: widgets

medium

A. Cluster-wide resource available in all namespaces

B. Resource limited to a specific namespace

C. Resource only available in the default namespace

D. Resource scoped to nodes

Solution

1. Step 1: Read the scope field

   The scope is set to Namespaced, meaning the resource exists within namespaces.

2. Step 2: Understand scope meanings

   Namespaced means the resource is limited to a specific namespace, not cluster-wide or node-scoped.

3. Final Answer:

   Resource limited to a specific namespace -> Option B

4. Quick Check:

   scope Namespaced = namespace-limited resource [OK]

Hint: scope: Namespaced means resource lives inside namespaces [OK]

Common Mistakes:

• Confusing Namespaced with Cluster scope
• Assuming default namespace only
• Thinking scope relates to nodes

4. You applied a CRD YAML but get an error: "unknown field 'kindd'". What is the most likely cause?

medium

A. Typo in the YAML key 'kindd' instead of 'kind'

B. Missing apiVersion field

C. CRD name is not unique

D. Cluster role permissions missing

Solution

1. Step 1: Analyze the error message

   The error says "unknown field 'kindd'", indicating a typo in the YAML key.

2. Step 2: Identify the correct key

   The correct key is kind, so kindd is a misspelling causing the error.

3. Final Answer:

   Typo in the YAML key 'kindd' instead of 'kind' -> Option A

4. Quick Check:

   YAML key typos cause unknown field errors [OK]

Hint: Check YAML keys carefully for typos [OK]

Common Mistakes:

• Ignoring spelling errors in YAML keys
• Assuming missing fields cause unknown field errors
• Blaming permissions for syntax errors

5. You want to create a CRD for a resource named Gadget that is cluster-scoped and has a plural name gadgets. Which YAML snippet correctly defines this?

hard

A. spec:
group: example.com
scope: Cluster
names:
kind: Gadget
plural: gadget

B. spec:
group: example.com
scope: Namespaced
names:
kind: Gadget
plural: gadgets

C. spec:
group: example.com
scope: Cluster
names:
kind: gadget
plural: gadget

D. spec:
group: example.com
scope: Cluster
names:
kind: Gadget
plural: gadgets

Solution

1. Step 1: Check scope value

   The resource must be cluster-scoped, so scope: Cluster is correct.

2. Step 2: Verify kind and plural names

   kind should be capitalized as Gadget and plural should be gadgets (plural lowercase).

3. Step 3: Compare options

   The snippet with scope: Cluster, kind: Gadget, plural: gadgets matches all requirements. Snippets with scope: Namespaced, lowercase kind: gadget, or singular plural: gadget are incorrect.

4. Final Answer:

   spec: group: example.com scope: Cluster names: kind: Gadget plural: gadgets -> Option D

5. Quick Check:

   Cluster scope + correct kind/plural = spec: group: example.com scope: Cluster names: kind: Gadget plural: gadgets [OK]

Hint: Cluster scope means scope: Cluster; kind capitalized, plural lowercase [OK]

Common Mistakes:

• Using Namespaced instead of Cluster scope
• Incorrect casing for kind or plural
• Using singular for plural name