Day 15 - Introduction to Containers and Docker

🎯 1. Objective

Before working on real DevOps projects or advanced Docker concepts, it’s essential to first understand:

• What containers are

• How they differ from virtual machines (VMs)

• What Docker and Buildah do in the container world

This session focuses purely on concepts, not hands-on commands.

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💡 2. Background: From Physical Servers → Virtual Machines → Containers

🖥️ Physical Servers

• Earlier, organizations ran one application per physical server.

• Hardware resources (CPU, RAM, storage) were often underutilized.

• Maintaining thousands of physical servers was expensive and inefficient.

🧰 Virtualization

To improve resource utilization, the concept of virtualization was introduced using a Hypervisor.

Hypervisor:
A software layer that lets you create multiple virtual machines (VMs) on a single physical server.

Each VM has:

• Its own Operating System (OS)

• Its own applications and dependencies

• Logical isolation from other VMs

Benefits:

• Better hardware utilization

• Isolation between applications

• Easier deployment than physical servers

Drawback:

• Each VM still requires a full OS, consuming large amounts of CPU, RAM, and disk.

• Even with virtualization, many VMs remain underutilized most of the time.

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⚙️ 3. The Problem with Virtual Machines

Let’s say:

• Physical server = 100 GB RAM, 100 CPUs

• You create 4 VMs (25 GB RAM each)

Even if one VM’s application only needs 10 GB RAM, the rest (15 GB RAM) stays idle.

At scale — say, 1 million VMs — that wasted capacity means huge financial loss.

Hence, a more lightweight and resource-efficient approach was needed.

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🧩 4. Solution: Containers

Containers were introduced to solve these inefficiencies.

Definition:

A container is a lightweight, standalone package that includes everything needed to run a piece of software — code, libraries, dependencies, and minimal OS components.

🏗️ How Containers Run

Containers can be created:

1. Directly on a physical server, or

2. On top of a virtual machine

In both cases, you install a containerization platform (like Docker, Podman, or Buildah) over the host OS.

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⚖️ 5. Containers vs Virtual Machines

📌 In short:

Virtual machines isolate hardware.
Containers isolate processes.

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🧠 6. Why Containers Are Lightweight

• Containers don’t include a full OS — only minimal system libraries and dependencies.

• They share the host system’s kernel.

• Only necessary components are bundled → drastically smaller image sizes.

Example:

• VM image: ~2–3 GB

• Container image: ~100–500 MB

This makes containers:

• Faster to build

• Easier to transfer (“ship”)

• Quicker to deploy

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📦 7. What’s Inside a Container

A container image includes:

1. Application code

2. Application dependencies (libraries, frameworks)

3. System dependencies (minimal OS libraries)

If additional libraries are needed (e.g., Python, Node.js, Java), they’re added via base images (e.g., python:3.10, node:18, openjdk:17).

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⚙️ 8. How Docker Works

Docker is a containerization platform that simplifies container creation and management.

🔄 Docker Lifecycle

Behind the scenes, Docker Engine executes these commands and handles:

• Layered image building

• Container lifecycle management

• Resource sharing with host OS

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⚠️ 9. Drawbacks of Docker

While Docker made containers popular, it has a few limitations:

🧱 Single Point of Failure (SPOF)

• All containers depend on the Docker Engine daemon.

• If the Docker daemon stops, all containers stop.

🧩 Layer Complexity

• Docker builds images in layers.

• Too many layers can slow builds and consume disk space.

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🔧 10. Alternative: Buildah

To overcome Docker’s limitations, Buildah was introduced.

🏗️ What Is Buildah?

A container image–building tool that doesn’t depend on a daemon like Docker Engine.

🔍 Benefits of Buildah

• No single point of failure

• No background daemon

• Works well with Podman and Skopeo

• Compatible with Docker images (OCI compliant)

• Simpler scripting via shell commands

Unlike Docker (which uses Dockerfile), Buildah can create images directly from shell scripts.

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🧭 11. Real-World Architecture

Model 1: Containers on Physical Server

Physical Server
 └── OS
     └── Docker Engine / Container Platform
         ├── Container 1
         ├── Container 2
         └── Container 3

Model 2: Containers on Virtual Machine (Most Common)

Physical Server (Cloud / Data Center)
 └── Virtual Machine
     └── OS
         └── Docker Engine / Podman / Buildah
             ├── Container 1
             ├── Container 2
             └── Container 3

📌 Most organizations today use Model 2 because:

• They rely on cloud providers (AWS, Azure, GCP)

• They avoid maintaining physical data centers

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🚀 12. Why Docker Became So Popular

• Simple to learn and use (docker build, docker run)

• Strong community support

• Easy image sharing through Docker Hub

• Integrated with orchestration tools like Kubernetes

• Lightweight and portable — “Build once, run anywhere”

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🧾 13. Summary Table

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✅ 14. Final Summary

• Containers evolved to improve resource efficiency over VMs.

• Docker popularized containers through ease of use and portability.

• Containers are lightweight, fast, and ideal for microservices.

• Docker’s limitations (single point of failure, layer bloat) led to tools like Buildah and Podman.

• Today, containerization is the foundation of modern DevOps and cloud-native computing.