Understanding Docker for Beginners: The Container Technology

Docker has revolutionized application development and deployment by enabling a faster and more efficient way to create, deploy, and scale applications. For those new to Docker, it can seem complex at first, but the fundamental concepts are straightforward. This comprehensive guide breaks down everything you need to know as a Docker beginner.

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What Exactly is Docker?

Docker is an open platform for developing, shipping, and running applications using containers. Containers allow you to package an application with all its dependencies into a standardized unit that can run seamlessly and isolated on any infrastructure.

Unlike virtual machines which launch a full operating system, containers share the host system‘s kernel and run as isolated processes. This makes them extremely lightweight, fast to spin up, and perfect for microservices architectures.

The main benefits Docker offers include:

  • Portability – You can build locally and deploy containers to any cloud or servers
  • Speed – Containers start almost instantly
  • Scalability – You can scale up instantly by launching more containers
  • Isolation – Containers are isolated from each other and the underlying infrastructure
  • Security – Containers apply a defense-in-depth approach with layered security

Docker is commonly used by developers to streamline building, testing, and deployment pipelines. Ops teams also utilize Docker to achieve consistent environments and modernize application infrastructure.

Key Components of Docker

There are a few key components that make up the Docker platform. Understanding these building blocks is essential to working with Docker:

Images

A Docker image is a read-only template that defines a container‘s filesystem and configuration. For example, you may have a Docker image for a web app server that bundles together all dependencies like binaries, libraries, code, and configs.

Images get layered on top of each other to reuse components. A base OS image can be shared by multiple application images, saving storage and memory.

Containers

A container is a running instance of a Docker image. When launching a container, you‘re starting up a process from the image in an isolated user space. Containers are assigned private IPs with networking interfaces and mount points for storage.

You can launch, start, stop, move, or delete containers without impacting the actual image. This means you can work with containers as immutable objects.

Dockerfile

A Dockerfile defines the steps to assemble an image. It contains instructions like adding files or directories, running commands, defining environment variables, and configuring ports.

Dockerfiles enable you to document and automate building reproducible images. This allows anyone to leverage the same Dockerfile to generate the exact same image.

Docker Hub

Docker Hub is the default registry for Docker images. It contains public images to use or extend in your own images. Images get pulled from a registry when running docker pull.

For private corporate images, Docker supports adding secure registries that require authentication to access images.

Docker Engine

The Docker Engine handles all container activities – building images, running containers, managing network and storage. It exposes a REST API which can be called from CLI commands or other application interfaces.

Engines can run locally on development machines or on server hosts with the docker daemon processing requests. Requests utilize your host‘s operating system so containers feel native.

Docker Compose

Compose is a tool that defines multi-container Docker apps in YAML so you can spin your entire application stack with one command.

The compose file models production deployment by configuring container networks and volumes so the containers interact just like they would in production. This takes away manual setup.

Installing Docker

Installing Docker is straightforward since packages are available for all major Linux distributions, macOS, and Windows 10. Here is a quick guide:

Linux

Setup automated script:

curl -fsSL https://get.docker.com -o get-docker.sh
sh get-docker.sh

Or install Docker Engine manually:

sudo apt install docker.io -y

macOS & Windows

Download and install Docker Desktop:

Download Docker Desktop

Double-click the install package and follow prompts. This will setup Docker Engine along with CLI tools.

Verify the install:

docker --version
# Docker version 20.10.12, build 20.10.12-0ubuntu1~20.04.2

If installed, Docker is now ready to use locally. Time to run a test container to validate it works!

Basic Docker Commands

Now that Docker is setup, I‘ll demo common CLI commands for interacting with Docker images and containers:

# Pull an image locally from Docker Hub
docker pull nginx

# Run detached container in background from image 
docker run -d --name mynginx nginx 

# List running containers
docker ps

# Start/Stop existing container
docker start mynginx
docker stop mynginx

# Execute command inside running container
docker exec mynginx ls /etc

# Remove container when done 
docker rm mynginx

That covers basic management – now you can pull images and spin up containers. Next I‘ll walk through containerizing a real world application.

Dockerizing a Sample Application

To see how Docker simplifies building and running apps, I‘ll Dockerize a basic Node.js app.

Here is the directory structure:

/app
   app.js
   package.json

/nginx
   default.conf
   nginx.conf

app.js contains a simple Express web server:

const express = require(‘express‘);

const app = express();

app.get(‘/‘, (req, res) => {
  res.send(‘Hi there!‘);
});

const port = 3000;

app.listen(port, () => {
  console.log(`App listening on port ${port}`);
})

And package.json defines app dependencies:

{
  "dependencies": {
    "express": "^4.17.1"
  }
}

I‘ll dockerize this by first creating a Dockerfile – note Dockerfile with no file extension:

FROM node:16-alpine
WORKDIR /app
COPY . .
RUN npm install
EXPOSE 3000 
CMD ["node", "/app/app.js"]

This Dockerfile:

  • Starts from a Node 16 base image
  • Sets working directory to /app
  • Copies in files from host
  • Installs app dependencies
  • Exposes port 3000
  • Sets start command to launch app

Next I‘ll build a Docker image using this Dockerfile:

docker build -t my-app .

Finally, I‘ll run a container from this image:

docker run -p 3000:3000 my-app

I now have a container running my Node.js app and exposing it on port 3000!

The app can be developed locally then shipped as an immutable container to production. Other team members could also start contributing just by cloning the code.

This is the simplicity and standardization Docker enables. Defining infrastructure as code allows you to version, replicate, and share environments.

Best Practices for Docker Development

When working with Docker in development environments or deploying to production,以下最佳实践:

  • Leverage multi-stage builds to keep image sizes small
  • Use Docker ignore file to exclude non-essential files
  • Implement health checks and monitoring to track container status
  • Standardize tagging scheme for images
  • Limit what runs as root process inside container
  • Scan images for vulnerabilities before deploying to production

Some other best practices include:

  • Persist data outside containers using mounted volumes
  • Make containers stateless and decoupled when possible
  • Set CPU and memory limits on containers
  • Use Docker networks to secure communications between containers

Following Docker best practices ensures you build secure and production-grade container environments.

Tips for Running Docker in Production

When ready to launch your Dockerized applications into production, keep these tips in mind:

  • Use Docker Compose to define production-scale multi-container apps
  • Implement CI/CD pipelines to build images automatically
  • Push images to a secure Docker registry instead of Docker Hub
  • Use Docker secrets to pass sensitive data to containers
  • Monitor metrics and logs with container health checks
  • Distribute containers across nodes using orchestrators like Kubernetes
  • Establish rollback workflows to revert containers if needed

Having the right processes around building, deploying, and monitoring is key to effectively operating Docker in production. Automating continuous delivery with Docker brings consistency across environments.

Key Takeaways and Next Steps

Docker makes containers the standardized unit for application development, shipment, scaling, and management. Utilizing containers unlocks speed, security, and availability.

We covered Docker basics from key components to common commands. We also saw hands-on how Docker packages and runs applications using images and containers.

To dig deeper into Docker:

  • Work through Docker‘s interactive tutorials
  • Create sample apps to practice Dockerizing different technologies
  • Explore production orchestrators like Kubernetes and OpenShift
  • Read the official best practices for container architecture

As companies continue modernizing infrastructure, Docker skills have become highly valued by ops teams and developers. Hopefully this overview gave you the Docker foundation to get started. Let me know if you have any other Docker-related topics you‘d like to see covered!

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