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Imperative Management of Kubernetes Objects Using Configuration Files
Kubernetes objects can be created, updated, and deleted by using the
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
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:
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
kubectl create -f <filename|url>
How to update objects
replacecommand 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
externalIPsfield is managed independently from the configuration file. Independently managed fields must be copied to the configuration file to prevent
replacefrom dropping them.
You can use
kubectl replace -f to update a live object according to a
kubectl replace -f <filename|url>
How to delete objects
You can use
kubectl delete -f to delete an object that is described in a
kubectl delete -f <filename|url>
If configuration file has specified the
generateName field in the
section instead of the
name field, you cannot delete the object using
kubectl delete -f <filename|url>.
You will have to use other flags for deleting the object. For example:
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
-o yaml flag specifies that the full object configuration is printed.
kubectl get -h to see a list of options.
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
is executed. This can happen if a controller, such as
a HorizontalPodAutoscaler, makes updates directly to a live object. Here's
- 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
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.
selector: matchLabels: controller-selector: "apps/v1/deployment/nginx" template: metadata: labels: controller-selector: "apps/v1/deployment/nginx"