Kubernetes API

Mastering the Kubernetes API

Understanding the Kubernetes API is essential for mastering Kubernetes. It serves as the backbone of the platform, allowing you to manage resources programmatically and automate cluster operations.

Overview of the Kubernetes API

The Big Picture

Kubernetes is an API-centric platform. All resources, such as Pods, Services, and StatefulSets, are defined through the API and managed by the API server. Administrators and clients interact with the cluster by sending requests to create, read, update, and delete these resources. Most interactions are done using kubectl, but they can also be crafted in code or generated through API development tools.

Key Concepts

API Server: The central component that exposes the API and handles requests.
Resources and Objects: Resources like Pods and Services are defined in the API. When deployed to a cluster, these resources are often called objects.
Serialization: The process of converting an object into a string or stream of bytes for transmission or storage. Kubernetes supports JSON and Protobuf for serialization.

How the API Works

The Kubernetes API server is the central hub through which all interactions in the cluster are routed, functioning as the front-end interface for Kubernetes' API. Picture it as the Grand Central Station of Kubernetes — every command, status update, and inter-service communication passes through the API server via RESTful calls over HTTPS. Here's a snapshot of how it operates:

kubectl commands are directed to the API server, whether it's for creating, retrieving, updating, or deleting Kubernetes objects.
Node Kubelets keep an eye on the API server, picking up new tasks and sending back their statuses.
The control plane services don't chat amongst themselves directly; they communicate through the API server.

Understanding Serialization

Serialization is essential for transmitting and storing objects. Kubernetes typically uses JSON for communication with external clients and Protobuf for internal cluster traffic due to its efficiency.

Example: Serialization in Action

When a client like kubectl posts a request, it serializes the object as JSON. The API server then processes this request and sends back a serialized response.

The API Server

Role and Function

The API server is the front-end to the Kubernetes API, handling all RESTful HTTPS requests. It manages all interactions between internal components and external clients.

Components

Control Plane Service: Runs as a set of Pods in the kube-system Namespace.
TLS and Authentication: Ensures secure communication and validates requests.
RESTful Interface: Supports CRUD operations via standard HTTP methods (POST, GET, PUT, PATCH, DELETE).

Example: Using the API

A typical kubectl command translates into a REST request:

kubectl get pods --namespace eggs

This command converts to:

GET /api/v1/namespaces/eggs/pods

Hands-On with the API

Exploring the API

1. Start a Proxy Session:

kubectl proxy --port 9000 &

This command starts a local proxy to the Kubernetes API server, allowing you to interact with the API using curl or other HTTP clients on http://localhost:9000.

2. Using curl to Interact with the API:

curl -X GET http://localhost:9000/api/v1/namespaces/eggs/pods

This command sends a GET request to the API server to retrieve information about Pods in the eggs Namespace.

Example output:

{
  "kind": "PodList",
  "apiVersion": "v1",
  "items": []
}

Creating Resources

1. Define a Namespace: Create a JSON file (ns.json):

{
  "kind": "Namespace",
  "apiVersion": "v1",
  "metadata": {
    "name": "eggs",
    "labels": {
      "chapter": "api"
    }
  }
}

2. Post the Namespace:

curl -X POST -H "Content-Type: application/json" --data-binary @ns.json http://localhost:9000/api/v1/namespaces

This command posts the JSON data to the API server, creating a new Namespace called eggs.

Example output:

{
  "kind": "Namespace",
  "apiVersion": "v1",
  "metadata": {
    "name": "eggs",
    "selfLink": "/api/v1/namespaces/eggs",
    "uid": "abcd1234-5678-90ef-ghij-klmnopqrstuv",
    "resourceVersion": "123456",
    "creationTimestamp": "2024-06-07T12:34:56Z",
    "labels": {
      "chapter": "api"
    }
  }
}

3. Verify Creation:

kubectl get namespaces

This command lists all Namespaces in the cluster, allowing you to verify the creation of the eggs Namespace.

Example output:

NAME          STATUS   AGE
default       Active   84d
kube-system   Active   84d
eggs          Active   1m

4. Delete the Namespace:

curl -X DELETE http://localhost:9000/api/v1/namespaces/eggs

This command deletes the eggs Namespace.

Example output:

{
  "kind": "Namespace",
  "apiVersion": "v1",
  "metadata": {
    "name": "eggs",
    "deletionTimestamp": "2024-06-07T12:36:00Z"
  },
  "status": {
    "phase": "Terminating"
  }
}

Inspecting the API

Useful Commands

1. List All API Resources:

kubectl api-resources

This command lists all available API resources in the cluster.

Example output:

NAME                  SHORTNAMES   APIGROUP                       NAMESPACED   KIND
pods                  po                                        true          Pod
services              svc                                       true          Service
deployments           deploy        apps                        true          Deployment
...

2. List Supported API Versions:

kubectl api-versions

This command lists all API versions supported by the cluster.

Example output:

v1
apps/v1
batch/v1
extensions/v1beta1
...

3. Inspect Specific Resources:

kubectl explain pods

This command provides detailed information about the Pod resource, including its fields and their descriptions.

Example output:

KIND:     Pod
VERSION:  v1

DESCRIPTION:
     Pod is a collection of containers that can run on a host. This resource
     is created by clients and scheduled onto hosts.

FIELDS:
   apiVersion   <string>
   kind         <string>
   metadata     <Object>
   spec         <Object>
   status       <Object>

4. Using curl to Explore:

curl http://localhost:9000/apis

This command lists all API groups and their versions available in the cluster.

Example output:

href="#__codelineno-19-1">{ "kind": "APIGroupList", "apiVersion": "v1", "groups": [ { "name": "apps", "versions": [ { "groupVersion": "apps/v1", "version": "v1" } ], "preferredVersion": { "groupVersion": "apps/v1", "version": "v1" } }, ... ] }

Extending the API

Custom Resources

Kubernetes allows you to extend the API with CustomResourceDefinitions (CRDs). These custom resources behave like native Kubernetes resources, enabling you to manage new types of objects within your cluster.

Example CRD

crd.yml:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: recipes.breakfast.com
spec:
  group: breakfast.com
  scope: Cluster
  names:
    plural: recipes
    singular: recipe
    kind: Recipe
    shortNames:
    - rp
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                bookTitle:
                  type: string
                topic:
                  type: string
                edition:
                  type: integer

Deploying the CRD

1. Apply the CRD:

kubectl apply -f crd.yml

This command creates the custom resource definition in the cluster.

Example output:

customresourcedefinition.apiextensions.k8s.io/recipes.breakfast.com created

2. Create an Instance: Create a YAML file (eggs.yml) for the custom resource: ```yaml apiVersion:

breakfast.com/v1 kind: Recipe metadata: name: scrambled spec: bookTitle: "Breakfast Recipes" topic: Eggs edition: 1 ```

3. Apply the Instance:

kubectl apply -f eggs.yml

This command creates an instance of the custom resource.

Example output:

recipe.breakfast.com/scrambled created

4. Verify Creation:

kubectl get rp

This command lists all instances of the custom resource.

Example output:

NAME        AGE
scrambled   1m

Summary

The Kubernetes API is a powerful tool for managing your cluster. By understanding its structure and capabilities, you can leverage it to automate and streamline your operations. From creating resources to extending the API with custom definitions, mastering the API is key to unlocking Kubernetes' full potential.