Beyond ConfigMaps and Secrets, Kubernetes provides ways to inject Pod-specific runtime information into containers — the Pod's name, namespace, IP, node, resource limits, and labels. This is the Downward API: a way for Pods to learn about themselves without calling the API server.

1. All Environment Variable Sources

A container's env vars can come from five sources:

Complete Example: All Sources in One Pod

spec:
  containers:
    - name: app
      image: myapp:latest
      env:
        # 1. Static value
        - name: APP_ENV
          value: production
        
        # 2. From ConfigMap
        - name: LOG_LEVEL
          valueFrom:
            configMapKeyRef:
              name: app-config
              key: LOG_LEVEL
        
        # 3. From Secret
        - name: DB_PASSWORD
          valueFrom:
            secretKeyRef:
              name: db-creds
              key: password
        
        # 4. Downward API — Pod field
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        
        # 5. Downward API — Resource field
        - name: MEMORY_LIMIT
          valueFrom:
            resourceFieldRef:
              containerName: app
              resource: limits.memory

Dependent Environment Variables

env:
  - name: DB_HOST
    value: postgres.default.svc
  - name: DB_PORT
    value: "5432"
  - name: DATABASE_URL
    value: "postgresql://$(DB_HOST):$(DB_PORT)/mydb"   # ← variable interpolation!
# K8s resolves $(VAR_NAME) from previously-defined env vars
# Order matters: referenced vars must be defined above

2. Downward API — fieldRef

Inject Pod metadata into the container without calling the API server:

Available Fields

env:
  - name: MY_POD_NAME
    valueFrom:
      fieldRef:
        fieldPath: metadata.name
  - name: MY_POD_IP
    valueFrom:
      fieldRef:
        fieldPath: status.podIP
  - name: MY_NODE
    valueFrom:
      fieldRef:
        fieldPath: spec.nodeName
  - name: MY_VERSION
    valueFrom:
      fieldRef:
        fieldPath: metadata.labels['version']

resourceFieldRef — Container Resources

env:
  - name: MEMORY_LIMIT
    valueFrom:
      resourceFieldRef:
        containerName: app          # which container (required in multi-container)
        resource: limits.memory     # in bytes by default
        divisor: "1Mi"              # convert to MiB (optional)
  - name: CPU_REQUEST
    valueFrom:
      resourceFieldRef:
        resource: requests.cpu
        divisor: "1m"               # in millicores

3. Downward API via Volume

Labels and annotations can change after Pod creation (unlike most metadata). To see live updates, use a downwardAPI volume instead of env vars:

spec:
  containers:
    - name: app
      volumeMounts:
        - name: pod-info
          mountPath: /etc/podinfo
          readOnly: true
  volumes:
    - name: pod-info
      downwardAPI:
        items:
          - path: "labels"           # → /etc/podinfo/labels
            fieldRef:
              fieldPath: metadata.labels
          - path: "annotations"      # → /etc/podinfo/annotations
            fieldRef:
              fieldPath: metadata.annotations
          - path: "cpu_limit"        # → /etc/podinfo/cpu_limit
            resourceFieldRef:
              containerName: app
              resource: limits.cpu
              divisor: "1m"

# Inside the container:
cat /etc/podinfo/labels
# app="web"
# version="v2"
# environment="production"

cat /etc/podinfo/cpu_limit
# 500

Env Var vs Volume: When to Use Each

4. Practical Production Uses

Summary