A label is a key–value pair attached to a Kubernetes object to identify its role. Labels help people organize pods and other objects by application, release, or another useful criterion. Unlike namespaces, which separate groups of objects, labels let you group the same objects in several ways. section 7.2.1 section 7.2
Identify an application and its release
Give a stable pod the labels app=kiada and rel=stable. Give a canary pod app=kiada and rel=canary. The shared app value identifies the application; the differing rel values distinguish its releases. Put these pairs in the object's metadata.labels map when creating it. section 7.2.1 section 7.2.2
Listing 7.3 A pod with labels
apiVersion: v1
kind: Pod
metadata:
name: kiada-001
labels: #1
app: kiada #2
rel: stable #3
spec:
...
The Pod manifest shows exactly where app and rel belong under metadata.labels. section 7.2.2
An object can have multiple labels, but each key must be unique on that object. Keys and values are strings you choose, subject to syntax rules. A key may have a prefix, as in kubernetes.io/arch; reserve the kubernetes.io/ and k8s.io/ prefixes for Kubernetes components. A value may be empty but cannot contain whitespace or exceed 63 characters. Use annotations for nonidentifying information that does not fit label values. section 7.2.1 section 7.2.3
You can inspect and change labels without recreating a pod:
- Run kubectl get pods --show-labels to see every label, or kubectl get pods -L app,rel to show those keys as columns. section 7.2.2
- Run kubectl label pod kiada-canary app=kiada rel=canary to add labels. To replace an existing value, add --overwrite; to remove a key, use its name followed by a hyphen, such as suite-. section 7.2.2
Labels also let label selectors find subsets of objects for bulk operations. A Service can select pods by label: an enabled=true selector includes a pod carrying that label, while removing the label removes it from the Service. Consistent application labels make related components recognizable to people and tools; Kubernetes also uses certain labels on system objects, such as nodes. section 7.2.4 section 11.6.2
