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Overview

To update a service without an outage, kubectl supports what is called ‘rolling update’, which updates one pod at a time, rather than taking down the entire service at the same time. See the rolling update design document and the example of rolling update for more information.

Note that kubectl rolling-update only supports Replication Controllers. However, if you deploy applications with Replication Controllers, consider switching them to Deployments. A Deployment is a higher-level controller that automates rolling updates of applications declaratively, and therefore is recommended. If you still want to keep your Replication Controllers and use kubectl rolling-update, keep reading:

A rolling update applies changes to the configuration of pods being managed by a replication controller. The changes can be passed as a new replication controller configuration file; or, if only updating the image, a new container image can be specified directly.

A rolling update works by:

  1. Creating a new replication controller with the updated configuration.
  2. Increasing/decreasing the replica count on the new and old controllers until the correct number of replicas is reached.
  3. Deleting the original replication controller.

Rolling updates are initiated with the kubectl rolling-update command:

$ kubectl rolling-update NAME \
    ([NEW_NAME] --image=IMAGE | -f FILE)

Passing a configuration file

To initiate a rolling update using a configuration file, pass the new file to kubectl rolling-update:

$ kubectl rolling-update NAME -f FILE

The configuration file must:

Replication controller configuration files are described in Creating Replication Controllers.

Examples

// Update pods of frontend-v1 using new replication controller data in frontend-v2.json.
$ kubectl rolling-update frontend-v1 -f frontend-v2.json

// Update pods of frontend-v1 using JSON data passed into stdin.
$ cat frontend-v2.json | kubectl rolling-update frontend-v1 -f -

Updating the container image

To update only the container image, pass a new image name and tag with the --image flag and (optionally) a new controller name:

$ kubectl rolling-update NAME [NEW_NAME] --image=IMAGE:TAG

The --image flag is only supported for single-container pods. Specifying --image with multi-container pods returns an error.

If no NEW_NAME is specified, a new replication controller is created with a temporary name. Once the rollout is complete, the old controller is deleted, and the new controller is updated to use the original name.

The update will fail if IMAGE:TAG is identical to the current value. For this reason, we recommend the use of versioned tags as opposed to values such as :latest. Doing a rolling update from image:latest to a new image:latest will fail, even if the image at that tag has changed. Moreover, the use of :latest is not recommended, see Best Practices for Configuration for more information.

Examples

// Update the pods of frontend-v1 to frontend-v2
$ kubectl rolling-update frontend-v1 frontend-v2 --image=image:v2

// Update the pods of frontend, keeping the replication controller name
$ kubectl rolling-update frontend --image=image:v2

Required and optional fields

Required fields are:

as well as either:

Optional fields are:

Additional information about the kubectl rolling-update command is available from the kubectl reference.

Walkthrough

Let’s say you were running version 1.7.9 of nginx:

apiVersion: v1
kind: ReplicationController
metadata:
  name: my-nginx
spec:
  replicas: 5
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

To update to version 1.9.1, you can use kubectl rolling-update --image to specify the new image:

$ kubectl rolling-update my-nginx --image=nginx:1.9.1
Created my-nginx-ccba8fbd8cc8160970f63f9a2696fc46

In another window, you can see that kubectl added a deployment label to the pods, whose value is a hash of the configuration, to distinguish the new pods from the old:

$ kubectl get pods -l app=nginx -L deployment
NAME                                              READY     STATUS    RESTARTS   AGE       DEPLOYMENT
my-nginx-ccba8fbd8cc8160970f63f9a2696fc46-k156z   1/1       Running   0          1m        ccba8fbd8cc8160970f63f9a2696fc46
my-nginx-ccba8fbd8cc8160970f63f9a2696fc46-v95yh   1/1       Running   0          35s       ccba8fbd8cc8160970f63f9a2696fc46
my-nginx-divi2                                    1/1       Running   0          2h        2d1d7a8f682934a254002b56404b813e
my-nginx-o0ef1                                    1/1       Running   0          2h        2d1d7a8f682934a254002b56404b813e
my-nginx-q6all                                    1/1       Running   0          8m        2d1d7a8f682934a254002b56404b813e

kubectl rolling-update reports progress as it progresses:

Scaling up my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 from 0 to 3, scaling down my-nginx from 3 to 0 (keep 3 pods available, don't exceed 4 pods)
Scaling my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 up to 1
Scaling my-nginx down to 2
Scaling my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 up to 2
Scaling my-nginx down to 1
Scaling my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 up to 3
Scaling my-nginx down to 0
Update succeeded. Deleting old controller: my-nginx
Renaming my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 to my-nginx
replicationcontroller "my-nginx" rolling updated

If you encounter a problem, you can stop the rolling update midway and revert to the previous version using --rollback:

$ kubectl rolling-update my-nginx --rollback
Setting "my-nginx" replicas to 1
Continuing update with existing controller my-nginx.
Scaling up nginx from 1 to 1, scaling down my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 from 1 to 0 (keep 1 pods available, don't exceed 2 pods)
Scaling my-nginx-ccba8fbd8cc8160970f63f9a2696fc46 down to 0
Update succeeded. Deleting my-nginx-ccba8fbd8cc8160970f63f9a2696fc46
replicationcontroller "my-nginx" rolling updated

This is one example where the immutability of containers is a huge asset.

If you need to update more than just the image (e.g., command arguments, environment variables), you can create a new replication controller, with a new name and distinguishing label value, such as:

apiVersion: v1
kind: ReplicationController
metadata:
  name: my-nginx-v4
spec:
  replicas: 5
  selector:
    app: nginx
    deployment: v4
  template:
    metadata:
      labels:
        app: nginx
        deployment: v4
    spec:
      containers:
      - name: nginx
        image: nginx:1.9.2
        args: ["nginx", "-T"]
        ports:
        - containerPort: 80

and roll it out:

$ kubectl rolling-update my-nginx -f ./nginx-rc.yaml
Created my-nginx-v4
Scaling up my-nginx-v4 from 0 to 5, scaling down my-nginx from 4 to 0 (keep 4 pods available, don't exceed 5 pods)
Scaling my-nginx-v4 up to 1
Scaling my-nginx down to 3
Scaling my-nginx-v4 up to 2
Scaling my-nginx down to 2
Scaling my-nginx-v4 up to 3
Scaling my-nginx down to 1
Scaling my-nginx-v4 up to 4
Scaling my-nginx down to 0
Scaling my-nginx-v4 up to 5
Update succeeded. Deleting old controller: my-nginx
replicationcontroller "my-nginx-v4" rolling updated

You can also run the update demo to see a visual representation of the rolling update process.

Troubleshooting

If the timeout duration is reached during a rolling update, the operation will fail with some pods belonging to the new replication controller, and some to the original controller.

To continue the update from where it failed, retry using the same command.

To roll back to the original state before the attempted update, append the --rollback=true flag to the original command. This will revert all changes.

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