Custom Resource Definitions (CRDs) in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Custom Resource Definition (CRD) in Kubernetes?

A CRD is a way to add your own custom objects to Kubernetes. It lets you define new resource types beyond the built-in ones, so you can manage your own data with Kubernetes tools.

beginner

How do you create a new custom resource type in Kubernetes?

You create a CRD YAML file that defines the new resource's name, version, and schema, then apply it with kubectl. This tells Kubernetes about your new resource type.

beginner

What command applies a CRD YAML file to the Kubernetes cluster?

kubectl apply -f applies the CRD to the cluster, registering the new custom resource type.

intermediate

Why use CRDs instead of ConfigMaps or Secrets for custom data?

CRDs let you create structured, typed resources with validation and versioning. ConfigMaps and Secrets are simpler key-value stores without schema or lifecycle management.

intermediate

What is the role of a controller with CRDs?

A controller watches custom resources and acts on changes. It helps automate tasks like creating pods or updating status based on the custom resource's state.

What does a Custom Resource Definition (CRD) allow you to do in Kubernetes?

A Upgrade Kubernetes versions

B Define new resource types

C Delete nodes automatically

D Monitor cluster health

CRDs let you define new resource types that Kubernetes can manage.

Which command is used to apply a CRD YAML file to a Kubernetes cluster?

A kubectl apply -f <file>

B kubectl create crd

C kubectl get crd

D kubectl delete crd

kubectl apply -f applies the CRD YAML to the cluster.

What is a key benefit of using CRDs over ConfigMaps for custom data?

A CRDs support schema validation

B ConfigMaps are encrypted by default

C CRDs cannot be versioned

D ConfigMaps allow custom resource types

CRDs support schema validation, making data structured and reliable.

What component typically watches CRDs to automate actions?

A Scheduler

B API Server

C Controller

D Kubelet

Controllers watch CRDs and automate responses to changes.

Which of these is NOT true about CRDs?

A They extend Kubernetes with new resource types

B They support versioning and validation

C They can be managed with kubectl

D They require writing Go code to use

You do not need to write Go code to use CRDs; you define them with YAML.

Explain what a Custom Resource Definition (CRD) is and why it is useful in Kubernetes.

Think about how Kubernetes can manage things beyond built-in resources.

Describe the steps to create and use a new custom resource in Kubernetes using a CRD.

Focus on YAML definition, applying it, and using the new resource.

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