Skip to content

Latest commit

 

History

History
61 lines (50 loc) · 3.62 KB

03-k8s-imperative-declarative.md

File metadata and controls

61 lines (50 loc) · 3.62 KB

Kubernetes Object Management

Ref: Kubernetes Object Management

The kubectl command-line tool supports several different ways to create and manage Kubernetes objects.

Imperative commands

  • A user operates directly on live objects in a cluster. The user provides operations to the kubectl command as arguments or flags.
  • It provides no history of previous configurations.
  • Example: Run an instance of the nginx container by creating a Deployment object using kubectl create deployment command.
  • Commands do not provide a template for creating new objects.
  • You specify the exact commands to be executed to achieve a specific state. For example, you may use the kubectl run command to create a Pod directly.

Declarative object configuration

  • A user operates on object configuration files stored locally, however the user does not define the operations to be taken on the files. Create, update, and delete operations are automatically detected per-object by kubectl.

  • The declarative approach involves declaring the desired state of your resources in a configuration file, such as a YAML file. You use tools like kubectl apply to apply the configuration file to the cluster. Kubernetes then automatically reconciles the actual state with the desired state defined in the configuration.

  • Example: Save the following YAML file, and then run kubectl apply -f YAML_FILE_NAME

    apiVersion: apps/v1
kind: Deployment # create Deployment object
metadata:
name: nginx-deployment # name of the Deployment
labels: # manage pods with label and also manage ReplicaSet
    app: nginx
spec: # spec of desired behavior of the Deployment
replicas: 2 # number of desired pods. Also create ReplicaSet.   
selector:
    matchLabels: # ReplicaSet will manage pods that have label app: nginx
    app: nginx
template:
    metadata:
    labels: # assign a label to the pod
        app: nginx
    spec: # spec of desired behavior of the pod
    containers: # list of containers belonging to the pod
    - name: nginx # container name
        image: nginx:latest # Docker image name
        resources:
        limits:
            memory: "128Mi"
            cpu: "500m"
        ports:
        - containerPort: 80

  • kubectl apply -f <directory> can be used to create all objects, except those that already exist, defined by configuration files in a specified directory.
    This sets the kubectl.kubernetes.io/last-applied-configuration: '{...}' annotation on each object. The annotation contains the contents of the object configuration file that was used to create the object.
    Add the -R flag to recursively process directories.

Useful info when creating / updating manifest files
kubectl api-versions - Print the supported API versions.

kubectl-api-versions

kubectl api-resources - Print the supported API resources. This information is useful when working with manifest files. It allows you to determine the apiVersion associated with a specific Kind. For example, the Deployment resource has an apiVersion of apps/v1.

kubectl-api-resources

To describe the fields associated with each supported API resource, you can utilize the kubectl explain command. For instance, running kubectl explain deployment --api-version=apps/v1 will provide the relevant output.

kubectl-explain-deployment