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| Imperative Management of Kubernetes Objects Using Configuration Files |
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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.
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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 object schema reference
for details.
create -f <filename|url>
How to update objects
You can use kubectl replace -f to update a live object according to a
configuration file.
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.
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.
get -f <filename|url> -o yaml
The -o yaml flag specifies that the full object configuration is printed.
Use 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 --export > <kind>_<name>.yaml -
Manually remove the status field from the object configuration file.
-
For subsequent object management, use
replaceexclusively.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: "extensions/v1beta1/deployment/nginx"
template:
metadata:
labels:
controller-selector: "extensions/v1beta1/deployment/nginx"
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- Managing Kubernetes Objects Using Imperative Commands
- Managing Kubernetes Objects Using Object Configuration (Declarative)
- Kubectl Command Reference
- Kubernetes Object Schema Reference {% endcapture %}
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