pods-and-deployments.md

Kubernetes Pods and Deployments Guide

Table of Contents

1. Pods
2. Deployments
3. ReplicaSets
4. Configuration Strategies
5. Best Practices
6. Troubleshooting

Pods

What is a Pod?

A Pod is the smallest deployable unit in Kubernetes that can be created and managed. It's a group of one or more containers with shared storage/network resources.

Pod Specifications

Basic Pod

apiVersion: v1
kind: Pod
metadata:
  name: my-app
  labels:
    app: web
    tier: frontend
spec:
  containers:
  - name: web-container
    image: nginx:1.21
    ports:
    - containerPort: 80
    resources:
      requests:
        memory: "64Mi"
        cpu: "250m"
      limits:
        memory: "128Mi"
        cpu: "500m"

Multi-Container Pod

apiVersion: v1
kind: Pod
metadata:
  name: multi-container-pod
spec:
  containers:
  - name: web
    image: nginx
    volumeMounts:
    - name: shared-data
      mountPath: /usr/share/nginx/html

  - name: content-sync
    image: debian
    volumeMounts:
    - name: shared-data
      mountPath: /content
    command: ["/bin/sh"]
    args: ["-c", "while true; do date >> /content/index.html; sleep 5; done"]

  volumes:
  - name: shared-data
    emptyDir: {}

Pod Lifecycle

1. Pending
2. Running
3. Succeeded
4. Failed
5. Unknown

Pod Probes

spec:
  containers:
  - name: web
    image: nginx
    livenessProbe:
      httpGet:
        path: /healthz
        port: 80
      initialDelaySeconds: 3
      periodSeconds: 3
    readinessProbe:
      httpGet:
        path: /ready
        port: 80
      initialDelaySeconds: 5
      periodSeconds: 5
    startupProbe:
      httpGet:
        path: /startup
        port: 80
      failureThreshold: 30
      periodSeconds: 10

Deployments

What is a Deployment?

A Deployment provides declarative updates for Pods and ReplicaSets, allowing for easy scaling and rolling updates.

Deployment Specifications

Basic Deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-deployment
  labels:
    app: web
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: nginx
        image: nginx:1.21
        ports:
        - containerPort: 80

Advanced Deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-deployment
spec:
  replicas: 5
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 1
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: web
        image: nginx:1.21
        resources:
          requests:
            cpu: "100m"
            memory: "128Mi"
          limits:
            cpu: "200m"
            memory: "256Mi"
        readinessProbe:
          httpGet:
            path: /ready
            port: 80
          initialDelaySeconds: 5
          periodSeconds: 5

Deployment Strategies

Rolling Update

spec:
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 25%
      maxUnavailable: 25%

Recreate

spec:
  strategy:
    type: Recreate

Scaling

# Manual scaling
kubectl scale deployment web-deployment --replicas=5

# Autoscaling
kubectl autoscale deployment web-deployment --min=2 --max=5 --cpu-percent=80

ReplicaSets

What is a ReplicaSet?

ReplicaSets ensure that a specified number of pod replicas are running at any given time.

ReplicaSet Specification

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: web-replicaset
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: nginx
        image: nginx:1.21

Configuration Strategies

Environment Variables

spec:
  containers:
  - name: web
    image: nginx
    env:
    - name: DATABASE_URL
      value: "mysql://db:3306/myapp"
    - name: API_KEY
      valueFrom:
        secretKeyRef:
          name: app-secrets
          key: api-key

ConfigMaps

apiVersion: v1
kind: ConfigMap
metadata:
  name: app-config
data:
  database_url: "mysql://db:3306/myapp"
  api_endpoint: "https://api.example.com"
---
spec:
  containers:
  - name: web
    envFrom:
    - configMapRef:
        name: app-config

Secrets

apiVersion: v1
kind: Secret
metadata:
  name: app-secrets
type: Opaque
data:
  username: dXNlcm5hbWU=  # base64 encoded
  password: cGFzc3dvcmQ=  # base64 encoded
---
spec:
  containers:
  - name: web
    volumeMounts:
    - name: secrets
      mountPath: "/etc/secrets"
  volumes:
  - name: secrets
    secret:
      secretName: app-secrets

Best Practices

1. Resource Management

resources:
  requests:
    memory: "64Mi"
    cpu: "250m"
  limits:
    memory: "128Mi"
    cpu: "500m"

2. Health Checks

livenessProbe:
  httpGet:
    path: /healthz
    port: 80
  initialDelaySeconds: 3
  periodSeconds: 3

3. Labels and Selectors

metadata:
  labels:
    app: web
    environment: production
    version: v1.0.0

4. Security Context

securityContext:
  runAsUser: 1000
  runAsGroup: 3000
  fsGroup: 2000

Troubleshooting

Common Commands

# Get pod details
kubectl describe pod <pod-name>

# Get pod logs
kubectl logs <pod-name>
kubectl logs -f <pod-name>  # Follow logs

# Execute commands in pod
kubectl exec -it <pod-name> -- /bin/bash

# Check deployment status
kubectl rollout status deployment/<deployment-name>

# Deployment history
kubectl rollout history deployment/<deployment-name>

# Rollback deployment
kubectl rollout undo deployment/<deployment-name>

Common Issues

1. Pod in Pending State

kubectl describe pod <pod-name>
# Check for:
# - Resource constraints
# - PVC binding issues
# - Node selector issues

2. Pod in CrashLoopBackOff

kubectl logs <pod-name>
kubectl describe pod <pod-name>
# Check for:
# - Application crashes
# - Resource limits
# - Configuration issues

3. Pod in ImagePullBackOff

kubectl describe pod <pod-name>
# Check for:
# - Image name/tag
# - Registry credentials
# - Network connectivity

4. Deployment Not Progressing

kubectl rollout status deployment/<deployment-name>
kubectl get events --field-selector involvedObject.kind=Deployment
# Check for:
# - Pod scheduling issues
# - Resource constraints
# - Configuration errors