Update API Objects in Place Using kubectl patch

This task shows how to use kubectl patch to update an API object in place. The exercises in this task demonstrate a strategic merge patch and a JSON merge patch.

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:

• Katacoda
• Play with Kubernetes

Use a strategic merge patch to update a Deployment

Here's the configuration file for a Deployment that has two replicas. Each replica is a Pod that has one container:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: patch-demo
spec:
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: patch-demo-ctr
        image: nginx
      tolerations:
      - effect: NoSchedule
        key: dedicated
        value: test-team

Create the Deployment:

kubectl apply -f https://k8s.io/examples/application/deployment-patch.yaml

View the Pods associated with your Deployment:

kubectl get pods

The output shows that the Deployment has two Pods. The 1/1 indicates that each Pod has one container:

NAME                        READY     STATUS    RESTARTS   AGE
patch-demo-28633765-670qr   1/1       Running   0          23s
patch-demo-28633765-j5qs3   1/1       Running   0          23s

Make a note of the names of the running Pods. Later, you will see that these Pods get terminated and replaced by new ones.

At this point, each Pod has one Container that runs the nginx image. Now suppose you want each Pod to have two containers: one that runs nginx and one that runs redis.

Create a file named patch-file.yaml that has this content:

spec:
  template:
    spec:
      containers:
      - name: patch-demo-ctr-2
        image: redis

Patch your Deployment:

• Bash
• PowerShell

kubectl patch deployment patch-demo --patch "$(cat patch-file.yaml)"

kubectl patch deployment patch-demo --patch $(Get-Content patch-file.yaml -Raw)

View the patched Deployment:

kubectl get deployment patch-demo --output yaml

The output shows that the PodSpec in the Deployment has two Containers:

containers:
- image: redis
  imagePullPolicy: Always
  name: patch-demo-ctr-2
  ...
- image: nginx
  imagePullPolicy: Always
  name: patch-demo-ctr
  ...

View the Pods associated with your patched Deployment:

kubectl get pods

The output shows that the running Pods have different names from the Pods that were running previously. The Deployment terminated the old Pods and created two new Pods that comply with the updated Deployment spec. The 2/2 indicates that each Pod has two Containers:

NAME                          READY     STATUS    RESTARTS   AGE
patch-demo-1081991389-2wrn5   2/2       Running   0          1m
patch-demo-1081991389-jmg7b   2/2       Running   0          1m

Take a closer look at one of the patch-demo Pods:

kubectl get pod <your-pod-name> --output yaml

The output shows that the Pod has two Containers: one running nginx and one running redis:

containers:
- image: redis
  ...
- image: nginx
  ...

Notes on the strategic merge patch

The patch you did in the preceding exercise is called a strategic merge patch. Notice that the patch did not replace the containers list. Instead it added a new Container to the list. In other words, the list in the patch was merged with the existing list. This is not always what happens when you use a strategic merge patch on a list. In some cases, the list is replaced, not merged.

With a strategic merge patch, a list is either replaced or merged depending on its patch strategy. The patch strategy is specified by the value of the patchStrategy key in a field tag in the Kubernetes source code. For example, the Containers field of PodSpec struct has a patchStrategy of merge:

type PodSpec struct {
  ...
  Containers []Container `json:"containers" patchStrategy:"merge" patchMergeKey:"name" ...`

You can also see the patch strategy in the OpenApi spec:

"io.k8s.api.core.v1.PodSpec": {
    ...
     "containers": {
      "description": "List of containers belonging to the pod. ...
      },
      "x-kubernetes-patch-merge-key": "name",
      "x-kubernetes-patch-strategy": "merge"
     },

And you can see the patch strategy in the Kubernetes API documentation.

Create a file named patch-file-tolerations.yaml that has this content:

spec:
  template:
    spec:
      tolerations:
      - effect: NoSchedule
        key: disktype
        value: ssd

Patch your Deployment:

kubectl patch deployment patch-demo --patch "$(cat patch-file-tolerations.yaml)"

View the patched Deployment:

kubectl get deployment patch-demo --output yaml

The output shows that the PodSpec in the Deployment has only one Toleration:

tolerations:
      - effect: NoSchedule
        key: disktype
        value: ssd

Notice that the tolerations list in the PodSpec was replaced, not merged. This is because the Tolerations field of PodSpec does not have a patchStrategy key in its field tag. So the strategic merge patch uses the default patch strategy, which is replace.

type PodSpec struct {
  ...
  Tolerations []Toleration `json:"tolerations,omitempty" protobuf:"bytes,22,opt,name=tolerations"`

Use a JSON merge patch to update a Deployment

A strategic merge patch is different from a JSON merge patch. With a JSON merge patch, if you want to update a list, you have to specify the entire new list. And the new list completely replaces the existing list.

The kubectl patch command has a type parameter that you can set to one of these values:

For a comparison of JSON patch and JSON merge patch, see JSON Patch and JSON Merge Patch.

The default value for the type parameter is strategic. So in the preceding exercise, you did a strategic merge patch.

Next, do a JSON merge patch on your same Deployment. Create a file named patch-file-2.yaml that has this content:

spec:
  template:
    spec:
      containers:
      - name: patch-demo-ctr-3
        image: gcr.io/google-samples/node-hello:1.0

In your patch command, set type to merge:

kubectl patch deployment patch-demo --type merge --patch "$(cat patch-file-2.yaml)"

View the patched Deployment:

kubectl get deployment patch-demo --output yaml

The containers list that you specified in the patch has only one Container. The output shows that your list of one Container replaced the existing containers list.

spec:
  containers:
  - image: gcr.io/google-samples/node-hello:1.0
    ...
    name: patch-demo-ctr-3

List the running Pods:

kubectl get pods

In the output, you can see that the existing Pods were terminated, and new Pods were created. The 1/1 indicates that each new Pod is running only one Container.

NAME                          READY     STATUS    RESTARTS   AGE
patch-demo-1307768864-69308   1/1       Running   0          1m
patch-demo-1307768864-c86dc   1/1       Running   0          1m

Use strategic merge patch to update a Deployment using the retainKeys strategy

Here's the configuration file for a Deployment that uses the RollingUpdate strategy:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: retainkeys-demo
spec:
  selector:
    matchLabels:
      app: nginx
  strategy:
    rollingUpdate:
      maxSurge: 30%
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: retainkeys-demo-ctr
        image: nginx

Create the deployment:

kubectl apply -f https://k8s.io/examples/application/deployment-retainkeys.yaml

At this point, the deployment is created and is using the RollingUpdate strategy.

Create a file named patch-file-no-retainkeys.yaml that has this content:

spec:
  strategy:
    type: Recreate

Patch your Deployment:

• Bash
• PowerShell

kubectl patch deployment retainkeys-demo --type merge --patch "$(cat patch-file-no-retainkeys.yaml)"

kubectl patch deployment retainkeys-demo --type merge --patch $(Get-Content patch-file-no-retainkeys.yaml -Raw)

In the output, you can see that it is not possible to set type as Recreate when a value is defined for spec.strategy.rollingUpdate:

The Deployment "retainkeys-demo" is invalid: spec.strategy.rollingUpdate: Forbidden: may not be specified when strategy `type` is 'Recreate'

The way to remove the value for spec.strategy.rollingUpdate when updating the value for type is to use the retainKeys strategy for the strategic merge.

Create another file named patch-file-retainkeys.yaml that has this content:

spec:
  strategy:
    $retainKeys:
    - type
    type: Recreate

With this patch, we indicate that we want to retain only the type key of the strategy object. Thus, the rollingUpdate will be removed during the patch operation.

Patch your Deployment again with this new patch:

• Bash
• PowerShell

kubectl patch deployment retainkeys-demo --type merge --patch "$(cat patch-file-retainkeys.yaml)"

kubectl patch deployment retainkeys-demo --type merge --patch $(Get-Content patch-file-retainkeys.yaml -Raw)

Examine the content of the Deployment:

kubectl get deployment retainkeys-demo --output yaml

The output shows that the strategy object in the Deployment does not contain the rollingUpdate key anymore:

spec:
  strategy:
    type: Recreate
  template:

Notes on the strategic merge patch using the retainKeys strategy

The patch you did in the preceding exercise is called a strategic merge patch with retainKeys strategy. This method introduces a new directive $retainKeys that has the following strategies:

• It contains a list of strings.
• All fields needing to be preserved must be present in the $retainKeys list.
• The fields that are present will be merged with live object.
• All of the missing fields will be cleared when patching.
• All fields in the $retainKeys list must be a superset or the same as the fields present in the patch.

The retainKeys strategy does not work for all objects. It only works when the value of the patchStrategy key in a field tag in the Kubernetes source code contains retainKeys. For example, the Strategy field of the DeploymentSpec struct has a patchStrategy of retainKeys:

type DeploymentSpec struct {
  ...
  // +patchStrategy=retainKeys
  Strategy DeploymentStrategy `json:"strategy,omitempty" patchStrategy:"retainKeys" ...`

You can also see the retainKeys strategy in the OpenApi spec:

"io.k8s.api.apps.v1.DeploymentSpec": {
   ...
  "strategy": {
    "$ref": "#/definitions/io.k8s.api.apps.v1.DeploymentStrategy",
    "description": "The deployment strategy to use to replace existing pods with new ones.",
    "x-kubernetes-patch-strategy": "retainKeys"
  },

And you can see the retainKeys strategy in the Kubernetes API documentation.

Alternate forms of the kubectl patch command

The kubectl patch command takes YAML or JSON. It can take the patch as a file or directly on the command line.

Create a file named patch-file.json that has this content:

{
   "spec": {
      "template": {
         "spec": {
            "containers": [
               {
                  "name": "patch-demo-ctr-2",
                  "image": "redis"
               }
            ]
         }
      }
   }
}

The following commands are equivalent:

kubectl patch deployment patch-demo --patch "$(cat patch-file.yaml)"
kubectl patch deployment patch-demo --patch 'spec:\n template:\n  spec:\n   containers:\n   - name: patch-demo-ctr-2\n     image: redis'

kubectl patch deployment patch-demo --patch "$(cat patch-file.json)"
kubectl patch deployment patch-demo --patch '{"spec": {"template": {"spec": {"containers": [{"name": "patch-demo-ctr-2","image": "redis"}]}}}}'

Summary

In this exercise, you used kubectl patch to change the live configuration of a Deployment object. You did not change the configuration file that you originally used to create the Deployment object. Other commands for updating API objects include kubectl annotate, kubectl edit, kubectl replace, kubectl scale, and kubectl apply.

What's next

• Kubernetes Object Management
• Managing Kubernetes Objects Using Imperative Commands
• Imperative Management of Kubernetes Objects Using Configuration Files
• Declarative Management of Kubernetes Objects Using Configuration Files