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What is a Pod?

A Pod is a single instance of an application and is the smallest unit of a Kubernetes application. It provides an abstraction layer over containers. Normally, a pod runs only one application container inside it (one-to-one relationship). Key characteristics:
  • Each pod gets its own IP address. When a pod is recreated, a new IP address is assigned.
  • Containers inside the pod can communicate with each other using localhost as they share the same network space.
  • They can also share the same storage space (called a volume).

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Pod Statuses

StatusDescription
CrashLoopBackOffThe container is crashing repeatedly. The pod keeps trying to restart the container but it keeps failing on start.

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Commands

# Deploy an image to a pod
kubectl run <pod-name> --image=<image>

# Examples
kubectl run nginx --image=nginx
kubectl run nginx --image=nginx --port=8000
kubectl run nginx --image=nginx --port=8080 --expose  # also creates a ClusterIP service

kubectl get pods
kubectl get pod <pod-name>
kubectl describe pod <pod-name>
kubectl delete pod <pod-name>

kubectl exec <pod-name> -- <command>
kubectl exec mypod -- /bin/bash

kubectl apply -f <filename>
kubectl create -f <filename>

kubectl edit pod <pod-name>

# Extract pod definition to a file
kubectl get pod <pod-name> -o yaml > pod.yaml

# Get more pod information (IP address, etc.)
kubectl get pods -o wide

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Pod YAML

Apply with kubectl apply -f pod.yaml.
KindapiVersion
Podv1
Servicev1
ReplicaSetapps/v1
Deploymentapps/v1
pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: myapp
  labels:
    name: myapp
spec:
  containers:
    - name: myapp
      image: nginx
      ports:
        - containerPort: 8080

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Restart Policy

Restart policy defines the behavior of the pod when the container crashes.
PolicyDescription
Always (default)The container always restarts on any termination.
OnFailureThe container restarts only on failure (non-zero exit status).
NeverThe container never restarts.
pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: myapp
spec:
  containers:
    - name: ubuntu
      image: ubuntu
  restartPolicy: Never

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Init Containers

An init container is a special type of container that runs before the main container. It runs only once and must complete successfully before the main container starts. Use cases:
  • Wait for a database to be ready
  • Prepare a configuration file
  • Load data
init-container.yaml
apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: main-container
      image: api:1.0.0
  initContainers:
    - name: init-database
      image: mysql
      command: ['sh', '-c', 'mysql -h db -u root -p $MYSQL_ROOT_PASSWORD < /scripts/init.sql']
If you specify multiple init containers, they will be run in order.

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Multi-Container Pods

Multi-container pods act as helper containers. The containers can communicate directly via localhost as they share the same network space, the same storage space, and the same lifecycle (created and destroyed together).

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Design Patterns

A sidecar container is attached to the main container to extend or enhance it. For example, a sidecar can collect logs from the main container and forward them to a centralized logging system.Use cases: Logging, Monitoring, Security, Data synchronization
sidecar.yaml
apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: app-container
      image: alpine
      command: ["/bin/sh"]
      args: ["-c", "while true; do date >> /var/log/index.html; sleep 2;done"]
      volumeMounts:
        - name: logs
          mountPath: /var/log
    - name: log-agent
      image: nginx
      ports:
        - containerPort: 80
      volumeMounts:
        - name: logs
          mountPath: /usr/share/nginx/html
  volumes:
    - name: logs
      emptyDir: {}
An adapter container acts as a translator between the main container and external systems, transforming data into a standardized format before sending it out.Use cases: Data transformation, Protocol translation, API adaptation, Standardizing data format
adapter.yaml
apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: app-container
      image: alpine
      command: ["/bin/sh"]
      args: ["-c", "while true; do date >> /var/log/debug.log; sleep 2;done"]
      volumeMounts:
        - name: logs
          mountPath: /var/log
    - name: log-adapter
      args: ["/bin/sh"]
      args: ["-c", "tail -f /var/log/debug.log|sed 's/^/DEBUG: /' > /var/log/transformed.log"]
      volumeMounts:
        - name: logs
          mountPath: /var/log
  volumes:
    - name: logs
      emptyDir: /var/log
An ambassador container acts as a proxy to external services. It hides the complexity of external services from the main container, managing all incoming and outgoing network traffic.Use case: Your application connects to different databases based on the environment (dev/staging/prod). The ambassador manages the connection so the main app always connects to localhost.
ambassador.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: db-config
data:
  development: "dev-db.example.com"
  staging: "staging-db.example.com"
  production: "prod-db.example.com"
---
apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
  labels:
    app: ambassador
    environment: development
spec:
  containers:
    - name: backend
      image: backend
      env:
        - name: DB_HOST
          value: "localhost"
      ports:
        - containerPort: 8080
    - name: ambassador
      image: ambassador-image
      env:
        - name: ENVIRONMENT
          valueFrom:
            fieldRef:
              fieldPath: metadata.labels.environment
        - name: DB_HOST
          valueFrom:
            configMapKeyRef:
              name: db-config
              key: $(ENVIRONMENT)
      ports:
        - containerPort: 80
  • ConfigMap stores database connections per environment.
  • The ambassador container reads env variables and connects to the correct database.
  • The backend container connects to the ambassador as localhost.

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