Docker and Containers
Docker is a platform for developing, shipping, and running applications in containers. A container is a lightweight, standalone, executable package of software that includes everything needed to run an application: code, runtime, system tools, system libraries, and settings.
1. Containers vs. Virtual Machines (VMs)
- Virtual Machines: Virtualize the hardware. Each VM runs a full Operating System (OS) on top of a Hypervisor. This is heavy (GBs in size) and slow to boot.
- Containers: Virtualize the Operating System. Containers share the host machine's OS kernel but isolate the application processes. This is lightweight (MBs in size) and starts instantly.
2. Key Concepts
Image
A read-only template with instructions for creating a Docker container. It's like a "snapshot" or a "class" in OOP.
- Example:
node:18-alpine,python:3.9,ubuntu:latest.
Container
A runnable instance of an image. It is the actual running process. It's like an "object" in OOP.
Dockerfile
A text document that contains all the commands a user could call on the command line to assemble an image.
# Example Dockerfile for a Node.js app
FROM node:18
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
CMD ["node", "index.js"]Registry (Docker Hub)
A repository where images are stored and shared (like GitHub for code).
3. Basic Commands
- Build an image:
docker build -t my-app . - Run a container:
docker run -p 3000:3000 my-app(Maps port 3000 on host to port 3000 in container)
- List running containers:
docker ps - Stop a container:
docker stop <container_id>
4. Docker Compose
Docker Compose is a tool for defining and running multi-container Docker applications. You use a YAML file to configure your application's services.
version: "3.8"
services:
web:
build: .
ports:
- "5000:5000"
redis:
image: "redis:alpine"To start everything:
docker-compose up