Docker

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1. Why use Docker?

• Docker makes it realy → really easy to install and run software without worrying about setup or dependencies.

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2. What is Docker?

Before diving in, it's important to understand the Docker Ecosystem:

classDiagram
    class Docker_Ecosystem {
        Docker_Client
        Docker_Server
        Docker_Machine
        Docker_Images
        Docker_Hub
        Docker_Compose
    }

Docker is a platform or ecosystem designed around creating and running containers.

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Example:

• Hello-world or Ubuntu:
  • Image → A single file with all the dependencies and configurations required to run a program.
  • Container → An instance of an image that runs a program.

stateDiagram-v2
    Image --> Container : Instance of app image
    Image --> Container : Runs a program

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3. What is a Container?

A container is a subset of processes with a set of resources assigned to it.

classDiagram
    class Container {
        Program
        Kernel
        RAM
        Network
        CPU
        Filesystem
    }

Components of an Image:

classDiagram
    class Image {
        FS_snapshot
        Startup_Command
        Libraries
        Runtime
    }

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Docker Commands:

(i) To create and run a container:

docker run <image-name>

• Docker client interacts with the Docker server.

(ii) To list all running containers:

docker ps     # Currently running
docker ps -a  # All

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Notes:

docker run = docker create + docker start

(iii) Removing/Deleting Containers:

docker system prune

(iv) Start a specific container:

docker start -a <container-id>

(v) Get logs from a container:

docker logs <container-id>

(vi) Stop a container:

docker stop <container-id>     # SIGTERM (Terminate signal; allows cleanup)
docker kill <container-id>     # SIGKILL (Forcefully terminates immediately)

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(vii) Multi-command in Docker:

docker exec -it <container-id> <command>

• Interactive Mode: Allows running commands inside containers.
  • Because containers are isolated.

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Creating a dedicated command prompt for a container:

docker exec -it <container-id> sh/bash

• Examples:
  • ls
  • pwd
  • redis-cli

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Attach to Docker Run:

docker run -it <image-name> sh/bash

• Note: Not recommended.

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Containers are isolated:

Containers are isolated from the host machine and from each other.

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Docker Images

classDiagram
    class Docker_Images {
        Dockerfile
        Docker_Client
        Docker_Server
        Usable_Image
    }

Creating a Dockerfile:

1. Specify a base image.
2. Run some commands to install additional programs.
3. Specify a command to run when the container starts.

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Writing Dockerfiles

Example of a Dockerfile:

# Use an existing Docker image as a base
FROM alpine 
# Download & install a dependency
RUN apk add --update redis  # (For alpine Linux, install redis)
# Tell the image what to do when it starts as a container
CMD ["redis-server"]

Docker Build Process

Command: docker build

FROM
# Pull image from Dockerhub

RUN
# Create new image & run this container for this particular command

CMD
# Get container ready by running the container & removing intermediate layers (startup command)

Notes:

• Docker makes great use of cache & runs commands swiftly.

Common Docker Commands:

docker run <container-id>
docker build -t <desired-name/project-name:version>

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Manually Creating Commits

Example commands:

docker run -it alpine
apk add --update redis
CMD ["redis-server"] # Containerized
docker commit -c 'CMD ["redis-server"]' <container-id>

Creates a new SHA1 commit. Note: Ensure the container is stable enough before committing.

Running a Container:

docker run <new-id>

Port Mapping

Example command:

docker run -p 5000:8080 <image-name>

• Maps incoming requests on port 5000 to port 8080 on the container.

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Networking Between Containers

Option 1: Use Docker CLI's network features.

Option 2: Docker Compose

• Purpose: A tool for defining & running multi-container Docker applications.
• Separate CLI that gets installed alongside Docker.
• Used to start up multiple Docker containers at the same time.
• Automates some of the long-winded arguments to docker run.

Docker Compose Example:

version: '3'
services:
  redis-server:
    image: 'redis'
  node-app:
    build: .
    ports:
      - "4000:8080"

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Docker Compose Commands

• Run & Build:

docker-compose up         # For image creation & run
docker-compose up --build # Build & run

• Run in Background:

docker-compose up -d
docker-compose ps
docker-compose down
docker-compose logs

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Docker Compose Container Maintenance

Think of it like PM2 for managing processes.

Restart Policies:

1. "no" → Never restart.
2. "always" → Always restart.
3. "on-failure" → Restart on failure.
4. "unless-stopped" → Restart unless manually stopped.

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Running Dockerfile with Custom File Name

Command:

docker build -f ./Dockerfile.dev .

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Docker Volumes

Example Command:

docker run -v /app/node-modules:/app <image-name>

Diagram illustrating Docker Volumes:

graph LR
A[Local Folder] --> B[Docker Container]
A[Frontend]
A --> C[/src]
A --> D[/public]
A --> E[/node_modules]
B[Backend]
B --> F[/src]
B --> G[/public]
B --> H[/node_modules]

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Using Docker Compose for Volumes

volumes:
  - /app/node-modules
  - ./app

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Docker Bind Mounts vs Volumes

• Volumes:
  COPY also works but it's not efficient → COPY also works, but it is less efficient.
  Volumes are used to link a directory from inside the nginx container.

Example Command:

docker run -d -p 8080:80 -v ~/projects/docker/html:/usr/share/nginx/html --name custom-nginx nginx

Use Cases:

1. Share data from host machine to nginx container.
2. Share data from nginx container to host machine.

Example for logs:

-v ~/projects/docker/logs:/var/log/nginx

This will transfer the nginx container files to:

~/projects/docker/logs

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Building New Docker Images

• A Dockerfile is a text document that contains all the commands you could normally execute manually in order to build a Docker image.

Steps:

1. Build the image using the following command:

docker build -t mynginx-image .

2. Push the image to Docker Hub:

docker login
docker tag [image] [username/repository:tag]
docker push [username/repository]

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Docker Services

• A web app contains a DB, Redis server, and much more to coordinate.
• In Docker, we can orchestrate such "services" by combining multiple images.
• This can be done using a Dockerfile, but docker-compose comes to the rescue.

Example:

For a ToDo Application, we need:

1. REST API
2. MongoDB Server

Dockerignore:

A .dockerignore file is like .gitignore.

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Example Dockerfile for a Node.js Application:

FROM node
WORKDIR /usr/src/app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 4000
CMD npm start

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Steps for Docker Compose:

Step 1:

Add docker-compose.yml informing about services:

1. Node App
2. MongoDB

Step 2:

Run the following command:

docker-compose up

This will run both services, and bing bong done.

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References & Related Topics:

• Docker Documentation
• Related topics: Kubernetes, Containerization, Virtualization.