Imperative Management of Kubernetes Objects Using Configuration Files

Install kubectl.

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

See Kubernetes Object Management for a discussion of the advantages and disadvantage of each kind of object management.

How to create objects

You can use kubectl create -f to create an object from a configuration file. Refer to the kubernetes API reference for details.

• kubectl create -f <filename|url>

How to update objects

You can use kubectl replace -f to update a live object according to a configuration file.

• kubectl replace -f <filename|url>

How to delete objects

You can use kubectl delete -f to delete an object that is described in a configuration file.

• kubectl delete -f <filename|url>

kubectl delete <type> <name>
kubectl delete <type> -l <label>

How to view an object

You can use kubectl get -f to view information about an object that is described in a configuration file.

• kubectl get -f <filename|url> -o yaml

The -o yaml flag specifies that the full object configuration is printed. Use kubectl get -h to see a list of options.

Limitations

The create, replace, and delete commands work well when each object's configuration is fully defined and recorded in its configuration file. However when a live object is updated, and the updates are not merged into its configuration file, the updates will be lost the next time a replace is executed. This can happen if a controller, such as a HorizontalPodAutoscaler, makes updates directly to a live object. Here's an example:

1. You create an object from a configuration file.
2. Another source updates the object by changing some field.
3. You replace the object from the configuration file. Changes made by the other source in step 2 are lost.

If you need to support multiple writers to the same object, you can use kubectl apply to manage the object.

Creating and editing an object from a URL without saving the configuration

Suppose you have the URL of an object configuration file. You can use kubectl create --edit to make changes to the configuration before the object is created. This is particularly useful for tutorials and tasks that point to a configuration file that could be modified by the reader.

kubectl create -f <url> --edit

Migrating from imperative commands to imperative object configuration

Migrating from imperative commands to imperative object configuration involves several manual steps.

1. Export the live object to a local object configuration file:

   kubectl get <kind>/<name> -o yaml > <kind>_<name>.yaml
   

2. Manually remove the status field from the object configuration file.

3. For subsequent object management, use replace exclusively.

   kubectl replace -f <kind>_<name>.yaml
   

Defining controller selectors and PodTemplate labels

The recommended approach is to define a single, immutable PodTemplate label used only by the controller selector with no other semantic meaning.

Example label:

selector:
  matchLabels:
      controller-selector: "apps/v1/deployment/nginx"
template:
  metadata:
    labels:
      controller-selector: "apps/v1/deployment/nginx"

What's next

• Managing Kubernetes Objects Using Imperative Commands
• Managing Kubernetes Objects Using Object Configuration (Declarative)
• Kubectl Command Reference
• Kubernetes API Reference