The command ‘kubectl apply’ is used to manage applications through files defining Kubernetes resources. It is used to create and update resources in a cluster.

Use case 1: Apply a configuration to a resource by file name or stdin

Code:

kubectl apply -f resource_filename

Motivation: This use case allows us to apply a configuration file to a resource in a Kubernetes cluster. It is useful when we want to create or update resources based on a configuration file.

Explanation:

• kubectl apply: Command to manage applications through files defining Kubernetes resources.
• -f resource_filename: Specifies the configuration file to be applied to the resource.

Example Output:

deployment.apps/my-deployment created

Use case 2: Edit the latest last-applied-configuration annotations of resources from the default editor

Code:

kubectl apply edit-last-applied -f resource_filename

Motivation: This use case allows us to edit the last-applied-configuration annotations of a resource using the default editor. It is useful when we need to make changes to the existing configuration of a resource.

Explanation:

• kubectl apply: Command to manage applications through files defining Kubernetes resources.
• edit-last-applied: Subcommand to edit the latest last-applied-configuration annotations of resources.
• -f resource_filename: Specifies the configuration file whose annotations need to be edited.

Example Output: The default editor opens with the last-applied-configuration annotations of the specified resource file, allowing us to make changes.

Use case 3: Set the latest last-applied-configuration annotations by setting it to match the contents of a file

Code:

kubectl apply set-last-applied -f resource_filename

Motivation: This use case allows us to set the last-applied-configuration annotations of a resource to match the contents of a file. It is useful when we want to update the annotations based on a specific configuration.

Explanation:

• kubectl apply: Command to manage applications through files defining Kubernetes resources.
• set-last-applied: Subcommand to set the latest last-applied-configuration annotations of resources.
• -f resource_filename: Specifies the configuration file whose contents will be used to set the annotations.

Example Output:

annotation.kubectl.kubernetes.io/last-applied-configuration configured

Use case 4: View the latest last-applied-configuration annotations by type/name or file

Code:

kubectl apply view-last-applied -f resource_filename

Motivation: This use case allows us to view the latest last-applied-configuration annotations of a resource by type/name or file. It is useful when we want to inspect the current configuration of a resource.

Explanation:

• kubectl apply: Command to manage applications through files defining Kubernetes resources.
• view-last-applied: Subcommand to view the latest last-applied-configuration annotations of resources.
• -f resource_filename: Specifies the configuration file or type/name of the resource to view.

Example Output:

metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","kind":"Service"...}

Conclusion:

The ‘kubectl apply’ command is a powerful tool for managing Kubernetes resources. It allows us to create, update, edit, set, and view the configuration of resources in a cluster. By using different subcommands and arguments, we can perform a variety of operations on resources efficiently.