Custom resources concept in Kubernetes - Time & Space Complexity

Time Complexity: Custom resources concept

O(n)

Understanding Time Complexity

We want to understand how the time to process custom resources changes as we add more of them in Kubernetes.

How does the system handle more custom resources and how does that affect performance?

Scenario Under Consideration

Analyze the time complexity of the following Kubernetes custom resource definition and controller watch loop.

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
---
# Controller watches all Widget resources
while true {
  widgets = listAllWidgets()
  for widget in widgets {
    reconcile(widget)
  }
  sleep(10)
}

This code defines a custom resource called Widget and a controller that repeatedly lists and processes all Widget objects.

Identify Repeating Operations

Look at what repeats in this process.

• Primary operation: Loop over all Widget resources to reconcile each one.
• How many times: Once every 10 seconds, the controller lists and processes all Widgets.

How Execution Grows With Input

As the number of Widgets grows, the controller must process more items each cycle.

Input Size (n)	Approx. Operations
10	Processes 10 Widgets
100	Processes 100 Widgets
1000	Processes 1000 Widgets

Pattern observation: The work grows directly with the number of Widgets. Double the Widgets, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to process all custom resources grows linearly as you add more of them.

Common Mistake

[X] Wrong: "Processing custom resources takes the same time no matter how many exist."

[OK] Correct: Each resource must be handled individually, so more resources mean more work and more time.

Interview Connect

Understanding how resource counts affect processing time helps you design scalable Kubernetes controllers and troubleshoot performance.

Self-Check

"What if the controller only processed changed Widgets instead of all Widgets each cycle? How would the time complexity change?"

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