Containerization-devops

🐳 Docker & Containerization – Important Theory Notes

Virtual Machines vs Containers

Virtual Machines (VM)

Virtual Machines run on a hypervisor such as VirtualBox or VMware. Each VM includes:

• Full Guest Operating System
• Virtual Hardware
• Separate Kernel
• Application Layer

Architecture: Host OS → Hypervisor → Guest OS → Applications

Characteristics:

• Heavyweight
• High memory usage
• Strong isolation
• Slower boot time
• Size in GBs

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Containers

Containers run using Docker Engine and share the host OS kernel.

Architecture: Host OS → Docker Engine → Containers → Applications

Characteristics:

• Lightweight
• Fast startup
• Efficient resource usage
• Portable
• Size in MBs

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Key Differences

Feature	Virtual Machine	Container
OS	Full Guest OS	Shared Kernel
Boot Time	Minutes	Seconds
Size	Large (GBs)	Small (MBs)
Isolation	Strong	Process-Level
Performance	Slower	Faster

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Docker Basic Commands

Check Docker Version: docker –version

Pull Image: docker pull nginx

Run Container: docker run -d -p 8080:80 nginx

List Running Containers: docker ps

List All Containers: docker ps -a

List Images: docker images

Stop Container: docker stop

Remove Container: docker rm

Remove Image: docker rmi

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Preserving Changes Inside Containers

Containers are temporary (ephemeral). Data is lost when container is removed.

Method 1 – Commit Container: docker commit new_image_name

Method 2 – Create Volume: docker volume create myvolume

Run Container with Volume: docker run -v myvolume:/data image_name

Method 3 – Bind Mount: docker run -v /host/path:/container/path image_name

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Dockerfile Important Instructions

Dockerfile is used to build custom images.

Common Instructions:

FROM → Base image RUN → Execute commands COPY → Copy files ADD → Add files WORKDIR → Set working directory EXPOSE → Expose port ENV → Environment variable CMD → Default command ENTRYPOINT → Main container command

Build Image: docker build -t image_name .

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Docker API

Docker Engine provides REST API for automation.

Check Docker API Version: curl –unix-socket /var/run/docker.sock http://localhost/version

Used For:

• Automation
• Remote management
• CI/CD integration
• Container orchestration

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Exposing Docker API

Modify daemon.json:

{ “hosts”: [“tcp://0.0.0.0:2375”, “unix:///var/run/docker.sock”] }

Restart Docker after modification.

⚠ Security Warning: Always secure Docker API with TLS authentication when exposing externally.

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Core Docker Commands for Tasks

Check Logs: docker logs

Inspect Container: docker inspect

Monitor Resource Usage: docker stats

Execute Command Inside Container: docker exec -it bash

Restart Container: docker restart

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Multistage Dockerfiles (Best Practices)

Used to reduce image size and improve security.

Example Concept:

FROM node:18 AS builder WORKDIR /app COPY . . RUN npm install

FROM alpine COPY –from=builder /app /app CMD [“node”, “app.js”]

Benefits:

• Smaller final image
• Faster deployment
• Reduced vulnerabilities
• Better security

Best Practices:

• Use minimal base image (Alpine)
• Reduce layers
• Avoid installing unnecessary packages
• Use .dockerignore
• Avoid running as root
• Use multistage builds

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Attach to Stopped Containers

Start Stopped Container: docker start

Attach to Running Container: docker attach

Run Interactive Shell: docker exec -it bash

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Data Management in Docker

Volume (Docker Managed Storage): docker volume create myvolume

Run with Volume: docker run -v myvolume:/app image_name

Bind Mount (Host Directory Mapping): docker run -v /host/path:/container/path image_name

tmpfs (Temporary In-Memory Storage): docker run –tmpfs /app image_name

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Networking in Docker

List Networks: docker network ls Here it provde to ping the other container.

Create Network: docker network create mynetwork

Connect Container to Network: docker network connect mynetwork container_name

Default Network Types:

• bridge
• host
• none
• custom bridge

Docker Compose

Theory —

These are essential concepts for containerized application deployment in modern DevOps environments.

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