ConfigMaps

kubectl create configmap checkout-config \
  --from-literal=APP_ENV=production \
  --from-literal=LOG_LEVEL=info \
  --from-literal=DB_HOST=postgres.payments.svc.cluster.local \
  --from-literal=DB_PORT=5432
# --from-literal: create a ConfigMap from inline key=value pairs
# Good for simple, short values — not ideal for multi-line config files
# Each --from-literal adds one key-value entry to the ConfigMap data

kubectl create configmap checkout-config \
  --from-file=app.properties
# --from-file: create a ConfigMap from a file — the filename becomes the key
# The entire file content becomes the value — ideal for config files (nginx.conf, application.yml)

kubectl create configmap checkout-config \
  --from-file=config-dir/
# --from-file with a directory: every file in the directory becomes a key in the ConfigMap
# Key = filename, Value = file contents — useful for bulk config migration

apiVersion: v1
kind: ConfigMap                 # ConfigMaps are core v1 objects
metadata:
  name: checkout-config         # Name of the ConfigMap — referenced by Pods that consume it
  namespace: payments           # Must be in the same namespace as the Pods that use it
  labels:
    app: checkout-api           # Label it so you can find it with kubectl get cm -l app=checkout-api
data:                           # data: the key-value store — all values are strings
  APP_ENV: "production"         # Simple string values — will be injected as env vars or file keys
  LOG_LEVEL: "info"
  DB_HOST: "postgres.payments.svc.cluster.local"
  DB_PORT: "5432"               # Numbers must be quoted in ConfigMap data — values are always strings
  FEATURE_NEW_CHECKOUT: "true"  # Feature flag — toggle behaviour without redeploying the image
  MAX_CONNECTIONS: "100"

  app.properties: |             # Multi-line value using YAML literal block scalar (|)
    # Checkout service configuration                    
    server.port=8080
    server.timeout=30s
    database.pool.min=5
    database.pool.max=20
    cache.ttl=300
    payment.gateway.url=https://gateway.payments.internal
    payment.retry.max=3
    logging.format=json
    # The | preserves newlines exactly — the entire block is one string value
    # This file will be mounted as /config/app.properties inside the container

apiVersion: apps/v1
kind: Deployment
metadata:
  name: checkout-api
  namespace: payments
spec:
  replicas: 2
  selector:
    matchLabels:
      app: checkout-api
  template:
    metadata:
      labels:
        app: checkout-api
    spec:
      containers:
        - name: checkout-api
          image: company/checkout-api:2.3.0
          ports:
            - containerPort: 3000
          env:
            - name: APP_ENV             # The env var name the container will see
              valueFrom:
                configMapKeyRef:        # configMapKeyRef: pull a single key from a ConfigMap
                  name: checkout-config # Which ConfigMap to read from
                  key: APP_ENV          # Which key within the ConfigMap to use
            - name: LOG_LEVEL
              valueFrom:
                configMapKeyRef:
                  name: checkout-config
                  key: LOG_LEVEL
            - name: DB_HOST
              valueFrom:
                configMapKeyRef:
                  name: checkout-config
                  key: DB_HOST
          envFrom:                      # envFrom: inject ALL keys from a ConfigMap as env vars at once
            - configMapRef:
                name: checkout-config   # Every key in checkout-config becomes an env var
                                        # Key name = env var name, value = env var value
                                        # Useful when you want all values without listing each one

apiVersion: apps/v1
kind: Deployment
metadata:
  name: payment-processor
  namespace: payments
spec:
  replicas: 2
  selector:
    matchLabels:
      app: payment-processor
  template:
    metadata:
      labels:
        app: payment-processor
    spec:
      containers:
        - name: payment-processor
          image: company/payment-processor:3.1.0
          ports:
            - containerPort: 8080
          volumeMounts:
            - name: config-volume       # Must match the volume name defined in volumes below
              mountPath: /config        # The directory path inside the container to mount into
              readOnly: true            # readOnly: container can read but not write to this directory
      volumes:
        - name: config-volume           # Declare the volume — referenced by volumeMounts above
          configMap:
            name: checkout-config       # Which ConfigMap to expose as this volume
            items:                      # items: selectively expose only specific keys (optional)
              - key: app.properties     # Which key in the ConfigMap to expose
                path: app.properties    # The filename to use inside the mountPath directory
                                        # Without items, ALL keys appear as files — sometimes too noisy

kubectl edit configmap checkout-config -n payments
# edit: opens the ConfigMap in your $EDITOR (vim by default)
# Change LOG_LEVEL from "info" to "debug", save and exit
# Kubernetes applies the change immediately upon save
# Volume-mounted consumers will see the file update within ~60 seconds
# env var consumers will NOT update — they require a Pod restart

kubectl patch configmap checkout-config -n payments \
  --type merge \
  -p '{"data":{"LOG_LEVEL":"debug"}}'
# patch: update specific fields without opening the editor
# --type merge: merge the patch with the existing data rather than replace it
# -p: the patch body as JSON — only the fields you specify are changed
# Faster than edit for scripted or automated config changes

kubectl rollout restart deployment/checkout-api -n payments
# rollout restart: gracefully restarts all Pods in the Deployment one by one
# Use this to force env-var consumers to pick up the new ConfigMap values
# It performs a rolling restart — zero downtime if you have multiple replicas