1 Containers + Kubernetes

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🧊 Containers Overview

📌 Core Idea

• Containers = lightweight, isolated environments that run apps + dependencies
• ⚠️ Windows containers need Windows host
• Faster startup (seconds 🚀), smaller size (MBs 💡) than VMs (GBs, minutes)
• When you run Linux container on Windows === Linux container on Linux virtual machine on Windows

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📌 Docker Vs Virtual Machines

Feature	Containers	Virtual Machines
Flow	Hardware → OS → Docker → Containers	Hardware → Host OS → Hypervisor → Guest OS per VM
Boot Time	Seconds	Minutes
Size	MBs	GBs
Kernel	Shared Host Kernel	Own Kernel
Isolation	Process Level	OS Level
Efficiency	High	Medium

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☸️ Kubernetes Overview

📌 What is Kubernetes

• Open-source container orchestration (originally by Google)
• Manages hundreds or thousands of containers across environments (VMs, physical, cloud, hybrid)
• Abstracts container runtime, you interact only with K8s layer

📌 Why Kubernetes (Problems it solves)

• Microservices rise → many small independent containers
• Managing containers via scripts becomes complex / impossible
• Provides 3 major guarantees:
  • High Availability → No downtime, always accessible
  • Scalability → Fast load, high performance, better response rate
  • Disaster Recovery → Restore data to latest state, no data loss

🧩 Core Kubernetes Components

📌 Pod

• Smallest unit, abstraction over container
• Gets IP address (not container, Pod itself)
• Usually runs 1 app container
• For sidecar/helper → can run multiple containers
• Pods are ephemeral i.e short-lived (can die, restart → get new IP)

📌 Service

• Provides static IP + DNS for Pod communication (IP endpoint stays even if Pod dies)
• Acts as Load Balancer (routes to least busy Pod)
• Types:
  • ClusterIP → Internal only
  • NodePort → Exposes via Node IP + Port
  • LoadBalancer → Cloud LB integration
  • ExternalName → Maps to external DNS
• Internal vs External service:
  • For Database → Internal Service (not public)
  • For Frontend App → External Service (Exposes app outside via NodePort/LB/Ingress)

📌 Ingress

• Routes traffic via HTTPS + Domain (example: app.example.com)
• Sits before Service, handles forwarding + routing rules

📌 ConfigMap

• Like AWS SSM Parameter
• Stores non-sensitive external configs (DB URLs, service endpoints)
• Update config → no rebuild of image needed
• Mounted to Pod as env vars or properties file

📌 Secret

• Stores sensitive data (passwords, certs, tokens)
• Saved as Base64 encoded (not plain text)
• Injected into Pod like ConfigMap using:
  • Environment variables Or properties file

📌 Volumes

• Attaches physical storage to Pods (Like external hard drive)
• K8s does NOT manage backup
• User/admin responsibility:
  • Need to handle replication, backup, hardware durability yourself

📦 Deployments & StatefulSets

📌 Deployment

• Blueprint for Pods
  • You create Deployments, not Pods directly
• Provides:
  • Replica management
  • Scale up/down
  • No shared state dependency
• Used for stateless apps (Web servers, APIs)
• Commands:
  • kubectl apply -f deploy.yaml
  • kubectl get deploy
  • kubectl scale deploy app --replicas=3

📌 StatefulSet

• Used for stateful apps (DBs like MongoDB, MySQL, ES)
• Ensures:
  • sync reads/writes to shared storage
  • No data inconsistency
• Harder than Deployment, so common practice is:
  • Host DB outside the K8s cluster

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