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.
- Trade-offs
- How to create objects
- How to update objects
- How to delete objects
- How to view an object
- Limitations
- Creating and editing an object from a URL without saving the configuration
- Migrating from imperative commands to imperative object configuration
- Defining controller selectors and PodTemplate labels
- What’s next
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.
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 --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"
What’s next
- Managing Kubernetes Objects Using Imperative Commands
- Managing Kubernetes Objects Using Object Configuration (Declarative)
- Kubectl Command Reference
- Kubernetes Object Schema Reference