Docker Overview¶

Docker is a containerization platform that packages applications with their dependencies into isolated, portable units called containers. This enables consistent behavior across different environments—from development to production.

What is Docker?¶

Docker uses containerization to solve the "it works on my machine" problem. Instead of shipping source code or applications that depend on the host system's specific configuration, Docker bundles everything together:

• Application code
• Runtime environment
• System libraries
• Dependencies
• Configuration files

Containers vs Images¶

Images¶

An image is a blueprint or template—a read-only snapshot containing: - Base OS filesystem - Application code - Dependencies - Environment variables - Configuration

Think of it like a class definition in programming.

Containers¶

A container is a running instance of an image—a lightweight, isolated process environment.

Think of it like an object instantiated from a class.

Visual Comparison¶

Image (Blueprint)                Container (Running Instance)
├── Base OS (Alpine)             ├── Image Layer (read-only)
├── Python Runtime               ├── Container Layer (writable)
├── Dependencies                 ├── Filesystem
├── App Code                      ├── Network Interface
└── Config Files                 └── Process ID (PID)

Multiple containers can       Each container is
run from one image            independent & isolated

Why Use Docker?¶

1. Consistency¶

Same environment everywhere—development, testing, production.

Without Docker:
Dev: Python 3.9, PostgreSQL 12, Ubuntu 20.04
Test: Python 3.10, PostgreSQL 13, CentOS 8
Prod: Python 3.8, PostgreSQL 11, Ubuntu 18.04
❌ Different behaviors, bugs in prod

With Docker:
Dev: Docker image with exact versions
Test: Same Docker image
Prod: Same Docker image
✓ Predictable behavior everywhere

2. Isolation¶

Each container has its own: - Filesystem - Network namespace - Process space - Environment variables

Prevents conflicts between applications.

3. Scalability¶

Spin up multiple identical containers instantly for load balancing:

docker run myapp:1.0  # Instance 1
docker run myapp:1.0  # Instance 2
docker run myapp:1.0  # Instance 3

4. Lightweight & Fast¶

• Containers share the host OS kernel (not full OS like VMs)
• Startup time: milliseconds
• Memory overhead: minimal
• Ideal for microservices architecture

5. Portability¶

Once Dockerized, an application runs on: - Local machine - Cloud providers (AWS, GCP, Azure) - Kubernetes clusters - On-premises servers

Without modification.

6. Development Efficiency¶

# Instead of complex setup guide:
git clone repo
python -m venv venv
source venv/bin/activate
pip install -r requirements.txt
...

# Just run:
docker-compose up

Common Use Cases¶

Docker Architecture¶

┌─────────────────────────────────────────┐
│         Docker Client (CLI)             │
│  $ docker run, build, push, etc.        │
└────────────┬────────────────────────────┘
             │
┌────────────▼────────────────────────────┐
│         Docker Daemon                   │
│  - Manages images & containers          │
│  - Manages networks & volumes           │
│  - Executes containers                  │
└────────────┬────────────────────────────┘
             │
┌────────────▼────────────────────────────┐
│      Container Runtime (containerd)     │
│  - Low-level container execution        │
└────────────┬────────────────────────────┘
             │
┌────────────▼────────────────────────────┐
│          Host OS Kernel                 │
│  - cgroups (resource limits)            │
│  - namespaces (isolation)               │
└─────────────────────────────────────────┘

Quick Facts¶

• Year Released: 2013
• Creator: Solomon Hykes (dotCloud)
• Type: Open source containerization platform
• Written in: Go
• License: Docker Community Edition (free), Docker Enterprise (paid)
• Standard: Implements OCI (Open Container Initiative) standards

Next Steps¶

• Learn Docker CLI commands
• Follow Docker best practices
• See Docker tutorials for hands-on guides

For questions, see Contributing Guide