# 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.

```dockerfile
# 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:**
    ```bash
    docker build -t my-app .
    ```
*   **Run a container:**
    ```bash
    docker run -p 3000:3000 my-app
    ```
    *(Maps port 3000 on host to port 3000 in container)*
*   **List running containers:**
    ```bash
    docker ps
    ```
*   **Stop a container:**
    ```bash
    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.

```yaml
version: "3.8"
services:
  web:
    build: .
    ports:
      - "5000:5000"
  redis:
    image: "redis:alpine"
```

To start everything:
```bash
docker-compose up
```

[[programming/microservices-architecture]]