What is Docker and how to use it?

Docker

 Docker is a software framework used for packaging, running, deploying and managing multiple apps on a server. Docker delivers OS-level virtualization to package a software called a container. It's an open-source and the most popular containerization platform.

With docker, we can package our application for a specific environment. For example, our app needs:

• Node 16.15
• MongoDB  5.0.1
• App

Docker got you covered. You can package your app and run it on any machine that can run docker.

Why docker is required ?

Docker was developed to solve a very common problem in a software development environment "It work's on my machine" there are a lot of cases a developer would say:

Docker Image

Image is a read-only template/snapshot for docker that contains instructions to run the docker container. Docker image also includes server environment configuration already packed. We can also build our custom image on the top of Base-OS Image. Alpine, Ubuntu, NodeJS and Python are a few examples of Base Images.

Docker Image is then hosted on a docker registry. One of them is DockerHub (Which is docker’s official registry hosting provider). Other registry providers are Amazon Elastic Container Registry (ECR)

Any developer can then pull the image (the app) with docker pull and run that in a docker container, or build another image on top of it. It is important to remember that 1 docker image can be used or run in multiple Docker containers.

Example

Let’s run a sample docker image hello-world hosted on a Dockerhub:

docker run hello-world

The response would be something like this

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world

2db29710123e: Pull complete

Digest: sha256:80f31da1ac7b312ba29d65080fddf797dd76acfb870e677f390d5acba9741b17
Status: Downloaded newer image for hello-world:latest

Hello from Docker!

This message shows that your installation appears to be working correctly.

Sample pre-build docker image.

Redis

docker run -d -p 6379:6379 --name redis redis

Postgres

docker run --rm -it -p 5432:5432 -e POSTGRES_PASSWORD=mydb_secret_password -e POSTGRES_USER=postgres -e POSTGRES_DB=InitialDB postgres

Docker container flags

• -p flag is mapping host port with docker port for port forwarding like -p host:container
• -e flag is specifying environment variables and their values like -e key=value

Docker commands:

To view a list of images downloaded on your local machine, docker image ls would help with a response like:

docker image ls

REPOSITORY   TAG       IMAGE ID       CREATED        SIZE
node         latest    4c60c47ec5e4   1 hours ago    996MB
nginx        latest    0e901e68141f   7 hours ago    142MB
memcached    latest    7665ebc98caa   8 hours ago    89.2MB
postgres     latest    dd21862d2f49   10 days ago    376MB
redis        latest    87c26977fd90   2 months ago   113MB

To remove a downloaded image with image id.

docker rmi 4c60c47ec5e4

Docker Container

Docker Container is like a process running on Operating System which uses a docker image. With Docker Container, you can run hundreds of apps with different versions in a completely isolated environment on the same machine without the need for any hypervisor or a guest OS.

For example:

Imagine, there are two NodeJS apps

1. appA works with Node 14 and requires a database of Postgres 12
2. appB is developed for Node 16 and requires Postgres 14 version respectively.

Docker Container Architecture

Difference between Docker Container & virtualized OS

It is also crucial to know that: when you stop the container, any state or data changed or created inside the container will be lost and reset to the image's default state. There can situations where you are required to store the data persistently on a local machine, which can be shared by multiple containers. Here comes Docker Volume to rescue!

Docker Volumes

Docker volume is a dedicated shared folder on a host machine. It is used as persistent storage for Docker Container apart from the container itself, which could be managed and maintained by the Docker engine.

As we discussed, a dockerized container is a special kind of process and the data created inside the container vanishes when the container turns off. But, we can attach a volume with the container that acts as persistent data storage for the container and files created inside a container could stick around.

Commands

To list of all of the docker volumes in your local machine.

docker volume ls

DRIVER    VOLUME NAME
local     app-volume
local     another-app-volume

To view more information on a specific volume, we would type:

docker volume inspect app-volume

The response would be something like this:

[
    {
        "CreatedAt": "2022-05-17T12:00:51Z",
        "Driver": "local",
        "Labels": null,
        "Mountpoint": "/var/lib/docker/volumes/app-volume/_data",
        "Name": "app-volume",
        "Options": null,
        "Scope": "local"
    }
]

Docker CLI

• Docker Desktop
  Docker Desktop is available for almost all the popular OS (macOS, Linux and Windows)

Download Docker Desktop | Docker

Docker Desktop is available to download for free on Mac, Windows, or Linux operating systems. Get started with Docker today!

Docker

Dockerize an App with Dockerfile

The process of packaging an app using a file name Dockerfile is known as Dockerizing.

A Dockerfile is like a blueprint for building docker image.

FROM alpine:latest

RUN apk add --update htop && rm -rf /var/cache/apk/*

CMD ["htop"]

Dockerfile Commands

Ignore a file with .dockerignore

File named .dockerignore works exactly the same as .gitignore or .npmignore

node_modules/      # node_module build

.env               # virtual environment variables
.venv

venv/              # virtual environment of python app
build/             # python build 

.git/              # current project git repository information

logs/              # Any logs
*.log

*.md               # Markdown readme file 

Build docker image

It is recommended that you setup a username on Dockerhub before name a build image.

docker build -t [username]/[image-name]:[tag] .

docker build -t saqe/sample-docker:1.0 .

By default the tag of the docker image would be :latest

To view that image.

docker image ls

REPOSITORY           TAG       IMAGE ID       CREATED         SIZE
saqe/sample-docker   1.0       c225b3a4c6ed   7 minutes ago   6.43MB

To start the saqe/sample-docker:1.0 into a container.

docker run -t saqe/sample-docker:1.0

Docker Compose

A docker-compose.yml will look like this:

services:
  web:
    build: .
    ports:
      - "8000:5000"
    volumes:
      - .:/code
      - logvolume01:/var/log
    links:
      - redis
  redis:
    image: redis
volumes:
  logvolume01: {}

docker-compose up

Docker CLI Commands

• docker ps
  Docker process
• docker run
  Create and start the container
• docker create
  Create container
• docker exec
  Run commands in container for once
• docker volume
  Create a docker volume.
• docker network
  Create a docker network.
• docker rm
  Remove container.
• docker images
  List the images.
• docker rmi
  Remove image.
• docker build
  Build a new image from Dockerfile.
• docker push
  Push your image to docker repo.
• docker pull
  Download an image from docker repo.
• docker commit
  Create an image from container.

Learning Docker

https://labs.play-with-docker.com

Kubernetes

Kubernetes also know as "K8s" is a container orchestration application to manage cluster of hosts/containers. Kubernetes automates software deployment, scaling and management.
Kubernetes was originally developed by Google, but now actively maintained by Cloud Native Computing Foundation.