How does --save-config work with resource definition? - kubernetes

Question

With the below yml file:
apiVersion: v1
kind: Pod
metadata:
name: my-nginx
spec:
containers:
- name: my-nginx
image: nginx:alpine
On running kubectl create -f nginx.pod.yml --save-config, then as per the documentation: If true, the configuration of current object will be saved in its annotation.
Where exactly is this annotation saved? How to view this annotation?

Answer 1

Below command would print all the annotations present in the pod my-nginx:
kubectl get pod my-nginx -o jsonpath='{.metadata.annotations}'
Under kubectl.kubernetes.io/last-applied-configuration of the above output, your configuration used is stored.
Here is an example showing the usage:
Original manifest for my deployment:
apiVersion: apps/v1
kind: Deployment
metadata:
creationTimestamp: null
labels:
app: my-deploy
name: my-deploy
spec:
replicas: 1
selector:
matchLabels:
app: my-deploy
strategy: {}
template:
metadata:
creationTimestamp: null
labels:
app: my-deploy
spec:
containers:
- image: nginx
name: nginx
resources: {}
status: {}
Created the deployment as follow:
k create -f x.yml --save-config
deployment.apps/my-deploy created
kubectl get deployments.apps my-deploy -o jsonpath='{.metadata.annotations.kubectl\.kubernetes\.io\/last-applied-configuration}' |jq .
{
"apiVersion": "apps/v1",
"kind": "Deployment",
"metadata": {
"annotations": {},
"creationTimestamp": null,
"labels": {
"app": "my-deploy"
},
"name": "my-deploy",
"namespace": "default"
},
"spec": {
"replicas": 1,
"selector": {
"matchLabels": {
"app": "my-deploy"
}
},
"strategy": {},
"template": {
"metadata": {
"creationTimestamp": null,
"labels": {
"app": "my-deploy"
}
},
"spec": {
"containers": [
{
"image": "nginx",
"name": "nginx",
"resources": {}
}
]
}
}
},
"status": {}
}
kubectl get deployments.apps my-deploy -o jsonpath='{.spec.template.spec.containers[*].image}'
nginx
Now some user came and changed the image on nginx from nginx to httpd, using imperative commands.
k set image deployment/my-deploy nginx=httpd --record
deployment.apps/my-deploy image updated
kubectl get deployments.apps my-deploy -o jsonpath='{.spec.template.spec.containers[*].image}'
httpd
However, we can check that the last applied declarative configuration is not updated.
kubectl get deployments.apps my-deploy -o jsonpath='{.metadata.annotations.kubectl\.kubernetes\.io\/last-applied-configuration}' |jq .
{
"apiVersion": "apps/v1",
"kind": "Deployment",
"metadata": {
"annotations": {},
"creationTimestamp": null,
"labels": {
"app": "my-deploy"
},
"name": "my-deploy",
"namespace": "default"
},
"spec": {
"replicas": 1,
"selector": {
"matchLabels": {
"app": "my-deploy"
}
},
"strategy": {},
"template": {
"metadata": {
"creationTimestamp": null,
"labels": {
"app": "my-deploy"
}
},
"spec": {
"containers": [
{
"image": "nginx",
"name": "nginx",
"resources": {}
}
]
}
}
},
"status": {}
}
Now, change the image name in the original manifest file from nginx to flask, then do kubectl apply(a declarative command)
kubectl apply -f orig.yml
deployment.apps/my-deploy configured
kubectl get deployments.apps my-deploy -o jsonpath='{.spec.template.spec.containers[*].image}'
flask
Now check the last applied configuration annotation, this would have flask in it. Remember, it was missing when kubectl set image command was used.
kubectl get deployments.apps my-deploy -o jsonpath='{.metadata.annotations.kubectl\.kubernetes\.io\/last-applied-configuration}' |jq .
{
"apiVersion": "apps/v1",
"kind": "Deployment",
"metadata": {
"annotations": {},
"creationTimestamp": null,
"labels": {
"app": "my-deploy"
},
"name": "my-deploy",
"namespace": "default"
},
"spec": {
"replicas": 1,
"selector": {
"matchLabels": {
"app": "my-deploy"
}
},
"strategy": {},
"template": {
"metadata": {
"creationTimestamp": null,
"labels": {
"app": "my-deploy"
}
},
"spec": {
"containers": [
{
"image": "flask",
"name": "nginx",
"resources": {}
}
]
}
}
},
"status": {}
}
Where is the "last-applied" annotation saved:
Just like everything else, Its saved in etcd , created the pod using the manifest provided in the question and ran raw etcd command to print the content. (in this dev environment, etcd was not encrypted).
ETCDCTL_API=3 etcdctl --cert /etc/kubernetes/pki/apiserver-etcd-client.crt --key /etc/kubernetes/pki/apiserver-etcd-client.key --cacert /etc/kubernetes/pki/etcd/ca.crt get /registry/pods/default/my-nginx
/registry/pods/default/my-nginx
k8s
v1Pod⚌
⚌
my-nginxdefault"*$a3s4b729-c96a-40f7-8de9-5d5f4ag21gfa2⚌⚌⚌b⚌
0kubectl.kubernetes.io/last-applied-configuration⚌{"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"name":"my-nginx","namespace":"default"},"spec":{"containers":[{"image":"nginx:alpine","name":"my-nginx"}]}}