🎯 1. Objective
Before working on real DevOps projects or advanced Docker concepts, it’s essential to first understand:
This session focuses purely on concepts, not hands-on commands.
💡 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.
⚙️ 3. The Problem with Virtual Machines
Let’s say:
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.
🧩 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:
Directly on a physical server, or
On top of a virtual machine
In both cases, you install a containerization platform (like Docker, Podman, or Buildah) over the host OS.
⚖️ 5. Containers vs Virtual Machines
| Feature | Virtual Machine | Container |
| OS | Full guest OS per VM | Shares host OS kernel |
| Size | Heavy (GBs) | Lightweight (MBs) |
| Startup Time | Minutes | Seconds |
| Isolation | Full hardware-level | Process-level (less secure) |
| Resource Efficiency | Moderate | Very high |
| Use Case | Legacy apps, full isolation | Microservices, cloud-native apps |
📌 In short:
Virtual machines isolate hardware.
Containers isolate processes.
🧠 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:
This makes containers:
📦 7. What’s Inside a Container
A container image includes:
Application code
Application dependencies (libraries, frameworks)
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).
⚙️ 8. How Docker Works
Docker is a containerization platform that simplifies container creation and management.
🔄 Docker Lifecycle
| Stage | Description | Command |
| 1️⃣ Write | Define image in a Dockerfile | Dockerfile |
| 2️⃣ Build | Convert Dockerfile → Image | docker build |
| 3️⃣ Run | Launch container from image | docker run |
Behind the scenes, Docker Engine executes these commands and handles:
⚠️ 9. Drawbacks of Docker
While Docker made containers popular, it has a few limitations:
🧱 Single Point of Failure (SPOF)
🧩 Layer Complexity
🔧 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.
🧭 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
🚀 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”
🧾 13. Summary Table
| Concept | Description |
| Container | Lightweight, isolated runtime for applications |
| Docker | Tool/platform to build and manage containers |
| Dockerfile | Script to define how to build an image |
| Image | Read-only template containing code + dependencies |
| Container | Running instance of an image |
| Buildah | Daemonless alternative to build container images |
| Base Image | Starting OS or runtime layer (e.g., Ubuntu, Alpine) |
✅ 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.