How to update a set of pods running in kubernetes?
What is the preferred way of updating a set of pods (e.g. after making code changes & pushing underlying docker image to docker hub) controlled by a replication controller in kubernetes cluster?
I can see 2 ways:
- Deleting & re-creating replication controller manually
- Using
kubectl rolling-update
With the rolling-update I have to change the replication controller name. Since I'm storing replication controller definition in YAML file and not generating it manually, having to change the file to push out a code update seems to bring about bad habits like alternating between 2 names for the replication controller (e.g. controllerA and controllerB) to avoid name conflict.
What is the better way?
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3 Answers
The best option I've found so far is Skaffold, which automatically builds the image, pushes it the image registry and updates the corresponding pods/controllers. It can even watch for code changes and rebuild the image as soon as changes are saved with skaffold dev command. This only requires adding a simple skaffold.yaml that specifies the image on the registry and path to the Kubernetes manifests. This workflow is described in details in the Getting Started guide.
The following explanations are from Kubernetes In Action's book
Deleting & re-creating replication controller manually
Doing a rolling update manually is laborious and error-prone. Depending on the number of replicas, you’d need to run a dozen or more commands in the proper order to perform the update process.Luckily, Kubernetes allows you to perform the rolling update with a single command.
Using kubectl rolling-update
Instead of performing rolling updates using ReplicationControllers manually, you can have kubectl perform them. Using kubectl to perform the update makes the process much easier, but, this is now an out dated way of updating apps.
Why performing an update like this isn’t as good as it could be is because it’s imperative. How Kubernetes is about you telling it the desired state of the system and having Kubernetes achieve that state on its own, by figuring out the best way to do it.
Using Deployments for updating apps declaratively --THE BEST ALTERNATIVE--
A Deployment is a higher-level resource meant for deploying applications and updating them declaratively, instead of doing it through a ReplicationController or a ReplicaSet, which are both considered lower-level concepts.
Using a Deployment instead of the lower-level constructs makes updating an app much easier, because you’re defining the desired state through the single Deployment resource and letting Kubernetes take care of the rest.
One more thing, Rolling back a rollout is possible because Deployments.
Update: kubectl rolling-update has been deprecated and the replacement command is kubectl rollout. Also note that since I wrote the original answer the Deployment resource has been added and is a better choice than ReplicaSets as the rolling update is performed server side instead of by the client.
You should use kubectl rolling-update. We recently added a feature to do a "simple rolling update" which will update the image in a replication controller without renaming it. It's the last example shown in the kubectl help rolling-update output:
// Update the pods of frontend by just changing the image, and keeping the old name
$ kubectl rolling-update frontend --image=image:v2This command also supports recovery -- if you cancel your update and restart it later, it will resume from where it left off. Even though it creates a new replication controller behind the scenes, at the end of the update the new replication controller takes the name of the old replication controller so it appears as pure update rather than switching to an entirely new replication controller.