The other day someone asked me if Kubernetes label selectors are case insensitive when filtering resources. I said they definitely were, but found it difficult to find examples online. Let this post be one small step toward remedying that.
setup
I’m doing this all on minikube using the hello-node container image.
I’ve used it to create two deployments:
$ kubectl create deployment deployment-green --image=gcr.io/hello-minikube-zero-install/hello-node
deployment.extensions "deployment-green" created
$ kubectl create deployment deployment-blue --image=gcr.io/hello-minikube-zero-install/hello-node
deployment.extensions "deployment-blue" created
I then added the following labels to these deployments via kubectl label:
$ kubectl label deployment deployment-green COLOR=green
deployment.extensions "deployment-green" labeled
$ kubectl label deployment deployment-blue COLOR=blue
deployment.extensions "deployment-blue" labeled
$ kubectl label deployment deployment-blue color=orange
deployment.extensions "deployment-blue" labeled
$ kubectl label deployment deployment-green color=orange
deployment.extensions "deployment-green" labeled
$ kubectl label deployment deployment-green environment=PRODUCTION
deployment.extensions "deployment-green" labeled
$ kubectl label deployment deployment-blue environment=production
deployment.extensions "deployment-blue" labeled
So these two deployments each had the following three labels:
$ kubectl get deployments --show-labels
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE LABELS
deployment-blue 1 1 1 1 15m COLOR=blue,app=deployment-blue,color=orange,environment=production
deployment-green 1 1 1 1 15m COLOR=green,app=deployment-green,color=orange,environment=PRODUCTION
note on label prefixes ⚠️
Label prefixes only support lowercase alphanumeric characters so you don’t have to worry about this for them!
$ kubectl label deployment deployment-green INVALID.EXAMPLE.COM/hello=there
The Deployment "deployment-green" is invalid: metadata.labels: Invalid value: "INVALID.EXAMPLE.COM/hello": prefix part a DNS-1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')
label selector examples
First, let’s demonstrate that the key name itself is case-sensitive. Each of our deployments has an uppercase COLOR label along with a lowercase color label. The fact that these can both be applied to the same resource demonstrates the need for case-sensitivity. Let’s see some label selector queries.
Querying for lowercase color:
$ kubectl get deployment --selector "color=orange"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-blue 1 1 1 1 24m
deployment-green 1 1 1 1 24m
$ kubectl get deployment --selector "color in (blue, green)"
No resources found.
Querying for uppercase COLOR:
$ kubectl get deployment --selector "COLOR=orange"
No resources found.
$ kubectl get deployment --selector "COLOR in (blue, green)"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-blue 1 1 1 1 25m
deployment-green 1 1 1 1 25m
$ kubectl get deployment --selector "COLOR=green"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-green 1 1 1 1 26m
$ kubectl get deployment --selector "COLOR=blue"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-blue 1 1 1 1 26m
Now let’s check out some examples that demonstrate the case-sensitivity of label values.
Querying for “production” deployments:
$ kubectl get deployment --selector "environment=PRODUCTION"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-green 1 1 1 1 29m
$ kubectl get deployment --selector "environment=production"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-blue 1 1 1 1 29m
$ kubectl get deployment --selector "environment=Production"
No resources found.
$ kubectl get deployment --selector "environment in (PRODUCTION, production)"
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment-blue 1 1 1 1 29m
deployment-green 1 1 1 1 29m
As you can see above, PRODUCTION definitely does not equal production – a sign that it’s important to agree and standardize on the labels you use within your team! 🚣