Imperative Management of Kubernetes Objects Using Configuration Files

Kubernetes objects can be created, updated, and deleted by using the kubectl command-line tool along with an object configuration file written in YAML or JSON. This document explains how to define and manage objects using configuration files.

Before you begin

Install kubectl.

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

To check the version, enter kubectl version.

Trade-offs

The kubectl tool supports three kinds of object management:

Imperative commands
Imperative object configuration
Declarative object configuration

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

Warning: Updating objects with the replace command drops all parts of the spec not specified in the configuration file. This should not be used with objects whose specs are partially managed by the cluster, such as Services of type LoadBalancer, where the externalIPs field is managed independently from the configuration file. Independently managed fields must be copied to the configuration file to prevent replace from dropping them.

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>

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:

You create an object from a configuration file.
Another source updates the object by changing some field.
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.

Export the live object to a local object configuration file:
```
kubectl get <kind>/<name> -o yaml > <kind>_<name>.yaml
```
Manually remove the status field from the object configuration file.
For subsequent object management, use replace exclusively.
```
kubectl replace -f <kind>_<name>.yaml
```

Defining controller selectors and PodTemplate labels

Warning: Updating selectors on controllers is strongly discouraged.

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"

Black lives matter.

Documentation