Capabilities of the Downward API
feildRef，即可以暴露的pod定义中的项目：
spec.nodeName - the node’s name
status.hostIP - the node’s IP
metadata.name - the pod’s name
metadata.namespace - the pod’s namespace
status.podIP - the pod’s IP address
spec.serviceAccountName - the pod’s service account name
metadata.uid - the pod’s UID
metadata.labels['<KEY>'] - the value of the pod’s label <KEY> (for example, metadata.labels['mylabel']); available in Kubernetes 1.9+
metadata.annotations['<KEY>'] - the value of the pod’s annotation <KEY> (for example, metadata.annotations['myannotation']); available in Kubernetes 1.9+
resourceFieldRef，即可以暴露的container定义中的项目：
- A Container’s CPU limit
- A Container’s CPU request
- A Container’s memory limit
- A Container’s memory request
