Apply
TL;DR
- Apply Creates and Updates Resources in a cluster through running
kubectl applyon Resource Config. - Apply manages complexity such as ordering of operations and merging user defined and cluster defined state.
Apply
Motivation
Apply is a command that will update a Kubernetes cluster to match state defined locally in files.
kubectl apply
- Fully declarative - don’t need to specify create or update - just manage files
- Merges user owned state (e.g. Service
selector) with state owned by the cluster (e.g. ServiceclusterIp)
Definitions
- Resources: Objects in a cluster - e.g. Deployments, Services, etc.
- Resource Config: Files declaring the desired state for Resources - e.g. deployment.yaml. Resources are created and updated using Apply with these files.
kubectl apply Creates and Updates Resources through local or remote files. This may be through either raw Resource Config or kustomization.yaml.
Usage
Though Apply can be run directly against Resource Config files or directories using -f, it is recommended
to run Apply against a kustomization.yaml using -k. The kustomization.yaml allows users to define
configuration that cuts across many Resources (e.g. namespace).
Command / Examples
Check out the reference for commands and examples.Users run Apply on directories containing kustomization.yaml files using -k or on raw
ResourceConfig files using -f.
Multi-Resource Configs
A single Resource Config file may declare multiple Resources separated by\n---\n.
CRUD Operations
Creating Resources
Any Resources that do not exist and are declared in Resource Config when Apply is run will be Created.
Updating Resources
Any Resources that already exist and are declared in Resource Config when Apply is run may be Updated.
Added Fields
Any fields that have been added to the Resource Config will be set on the Resource.
Updated Fields
Any fields that contain different values for the fields specified locally in the Resource Config from what is in the Resource will be updated by merging the Resource Config into the live Resource. See merging for more details.
Deleted Fields
Fields that were in the Resource Config the last time Apply was run, will be deleted from the Resource, and return to their default values.
Unmanaged Fields
Fields that were not specified in the Resource Config but are set on the Resource will be left unmodified.
Deleting Resources
Declarative deletion of Resources does not yet exist in a usable form, but is under development.
Continuously Applying The Hard Way
In some cases, it may be useful to automatically Apply changes when ever the Resource Config is changed.
This example uses the unix watch command to periodically invoke Apply against a target.
watch -n 60 kubectl apply -k https://github.com/myorg/myrepo
Resource Creation Ordering
Certain Resource Types may be dependent on other Resource Types being created first. e.g. Namespaced Resources on the Namespaces, RoleBindings on Roles, CustomResources on the CRDs, etc.
When used with a kustomization.yaml, Apply sorts the Resources by Resource type to ensure Resources
with these dependencies are created in the correct order.
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