kubernetes/staging/src/k8s.io/component-helpers/node/topology/helpers.go

// GetZoneKey is a helper function that builds a string identifier that is unique per failure-zone;
// it returns empty-string for no zone.
// Since there are currently two separate zone keys:
//   - "failure-domain.beta.kubernetes.io/zone"
//   - "topology.kubernetes.io/zone"
//
// GetZoneKey will first check failure-domain.beta.kubernetes.io/zone and if not exists, will then check
// topology.kubernetes.io/zone
func GetZoneKey(node *v1.Node) string { … }