Kubernetes kind Deployment doesn't allow patch changes in spec.selector.matchLabels, so any new deployments (managed by Helm or otherwise) that want to change the labels can't use the RollingUpdate feature within a Deployment. What's the best way to achieve a rollout of a new deployment without causing downtime?
Minimum example:
apiVersion: apps/v1
kind: Deployment
metadata:
name: foo
namespace: default
spec:
replicas: 1
selector:
matchLabels:
app: foo
template:
metadata:
labels:
app: foo
spec:
containers:
- name: foo
image: ubuntu:latest
command: ["/bin/bash", "-ec", "sleep infinity"]
Apply this, then edit the labels (both matchLabels and metadata.labels) to foo2. If you try to apply this new deployment, k8s will complain (by design) The Deployment "foo" is invalid: spec.selector: Invalid value: v1.LabelSelector{MatchLabels:map[string]string{"app":"foo2"}, MatchExpressions:[]v1.LabelSelectorRequirement(nil)}: field is immutable.
The only way I can think of right now is to use a new Deployment name so the new deployment does not try to patch the old one, and then delete the old one, with the ingress/load balancer resources handling the transition. Then we can redeploy with the old name, and delete the new name, completing the migration.
Is there a way to do it with fewer k8s CLI steps? Perhaps I can edit/delete something that keeps the old pods alive while the new pods roll out under the same name?
From my experience, while using helm when I use
helm upgrade release -f values .
I do not get downtime. Also when using helm I noticed that until the new deployment get ready by X/X it does not terminate the old deployment. I can suggest using it. This way it can be as painless as it gets.
Also from the section Updating Deployment from Kubernetes docs it is said that, A Deployment's rollout is triggered if and only if the Deployment's Pod template (that is, .spec.template) is changed.
Therefore, you can use label changes with helm.
Hopefully I was a little help.
Beware! untried method: kubectl has an edit subcommand which enabled me to update ConfigMaps, PersistentVolumeClaims and etc. Maybe you can use it to update your Deployment. Syntax:
kubectl edit [resource] [resource-name]
But before doing that please choose a proper text editor since you will be dealing with yaml formatted files. Do so by using,
export KUBE_EDITOR=/bin/{nano,vim,yourFavEditor}