Understanding Docker for Beginners – the Container Technology

Get an idea of the Docker Container.

In today’s technology space, “it works on my system” is a very common phrase used by developers. But there are many cases where the application running on a developer system does not run on a tester’s machine or on a production machine, which leads to chaos.

Docker was introduced to solve such problems. 💯

Why Docker?

Let me tell you about my personal experience of why Docker is so essential.

I was working on a full-stack project with my team, where I had to create an application that used Angular in the frontend, Node.js in the backend, and MySQL as a database. We got stuck into several issues while working on this application using multiple technologies.

• The first issue was the operating system compatibility. Since I was using multiple technologies, I had to make sure that all the technologies are compatible with the OS I am using. There were cases where the version of angular was not working with the OS version I was using, so I had to change the OS version multiple times.
• Since the architecture of the application, I was building changed over time, I got into situations where the software and libraries’ versions and dependencies had to be upgraded. I faced a lot of compatibility issues before arriving at a stable version of all the technologies that I was using for my application. Upgrading even a single tool used in my application was a risky business.
• Later, I had more developers working on my application. Since the technology stack was heavy, they had to run multiple commands for hours to set up their environment, which was compatible with the application. I had to check and evaluate every developer’s environment to make sure there is no mismatch from the technology version I was using.
• Finally, if any of the developers was not comfortable with the OS I was using to build my application, it was challenging for him/her to work on the application.

All of this made life difficult in developing, building, and shipping the application on time. This is where Docker came into the picture. It solves all the issues I just mentioned.

What is Docker?

Docker is a containerization platform that packages the application and its dependencies together inside a container so that the application works seamlessly in any environment, be it Development, Staging, or Production.

It is a tool designed to make it easier to create, deploy, and run applications by using containers. Docker containers are lightweight alternatives to virtual machines, and it uses the host operating system. You do not have to pre-allocate any RAM in containers as you do in virtual machines.

It is an open-source platform for developers, sysadmins, and enterprises to build, ship, and run distributed applications on the fly.

To know more about how docker works internally, check out the Docker architecture.

Docker Images, Containers, Dockerfile

Docker Image

Docker image is a template needed to run an application. It consists of application code, libraries, tools, dependencies, etc.

Docker images are read-only immutable files, which are sometimes also called snapshots. You do not start or run a docker image; you build a container through it. When a container is created from a Docker image, it creates a container layer on top of the image layer.

There has to be a base docker image that is used to create multiple other docker images with some modifications to the base image.

Docker Hub is a repository of Docker images for almost all technology stacks. You can pull a docker image from DockerHub and start building containers.

Docker Containers

Docker containers are executable software package that includes all dependencies required to execute an application. With Docker containers, applications can work efficiently in different computer environments.

Below are the Docker Containers Features:

• Lightweight
• Minimal overhead (CPU/IO/Network)
• Faster deployments
• Easily scalable
• Decrease storage consumption
• Portable, run it everywhere.
• Minimal base OS
• Application Isolation

Dockerfile

Dockerfile is a shell script that defines all the tasks that need to be executed. From a docker file, you create a docker image; from the docker image, you create a docker container.

Installing Docker

Docker can be easily installed on various Linux platforms, Windows, or macOS. Check out this post on How to Install Docker on Ubuntu, CentOS, Debian, and Windows.

Also, Docker Desktop is available to Windows and Mac machines. It is an executable application that is very easy to install and helps to build and containerize applications on Windows and Mac environments.

Docker Editions and Pricing

There are two editions of Docker:

• Community Edition (CE)
• Enterprise Edition (EE)

Docker community edition is open source and free to use. Docker CE aims at “do it yourself” approach where DevOps engineers can containerize their applications on their own.

Docker enterprise edition comes with three versions – basic, standard, and advanced. The basic edition comes with a docker platform, support, and certification, whereas standard and advanced editions come with container management and docker security scanning features added to the basic edition features.

Mostly techies use Docker for free. But if you need advanced features, you can use the paid version of Docker. The pricing depends on features such as repository management, CI/CD features, user management, developer tools, and support options.

Below are the plan details of Docker:

Now that you know the fundamentals of Docker let me show you a very simple docker example.

Docker Hello World Container

I am going to pull a hello-world docker image from DockerHub and build the image to create a docker container that will run the application. This image contains a simple application to print a hello message from Docker.

Firstly, I need to pull the hello-world docker image on my Ubuntu machine.

Note: this assume Docker is already installed. If not yet, check out the installation guide.

geekflare@geekflare:~$ docker pull hello-world
Using default tag: latest
latest: Pulling from library/hello-world
0e03bdcc26d7: Pull complete
Digest: sha256:49a1c8800c94df04e9658809b006fd8a686cab8028d33cfba2cc049724254202
Status: Downloaded newer image for hello-world:latest

Check if the docker image got pulled.

geekflare@geekflare:~$ docker images
REPOSITORY                     TAG                 IMAGE ID            CREATED             SIZE
hello-world                    latest              bf756fb1ae65        7 months ago        13.3kB
kubernetesui/dashboard         v2.0.0-beta8        eb51a3597525        8 months ago        90.8MB
geekflare_mongodb              latest              095d17727ca0        12 months ago       325MB

Run the command below to create a container and execute the application.

geekflare@geekflare:~$ docker run hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
1. The Docker client contacted the Docker daemon.
2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
(amd64)
3. The Docker daemon created a new container from that image which runs the
executable that produces the output you are currently reading.
4. The Docker daemon streamed that output to the Docker client, which sent it
to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
$ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
https://hub.docker.com/

For more examples and ideas, visit:
https://docs.docker.com/get-started/

Now, list all the docker containers. You can see the hello-world container ran 1 minute ago.

geekflare@geekflare:~$ docker container ls -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                       PORTS               NAMES
1a1056caffc4        hello-world         "/hello"                 1 minutes ago       Exited (0) 1 minutes ago                         zealous_bose
b0095c1e5536        geekflare_mongodb   "/bin/sh -c usr/bin/…"   12 months ago       Exited (0) 12 months ago                         mongo_container

Start exploring Docker; try out these fundamental docker commands.

Running Nginx Inside a Docker Container

Here, I will show you how to deploy Nginx inside a container.

The command below looks for an Nginx image locally. Since there is no local image available, it pulls the Nginx docker image from the docker hub. It then creates a container with the name nginx_geekflare using the Nginx docker image and runs Nginx on port 80.

geekflare@geekflare:~$ docker run --name nginx_geekflare -p 80:80 -d nginx
Unable to find image 'nginx:latest' locally
latest: Pulling from library/nginx
bf5952930446: Pull complete 
ba755a256dfe: Pull complete 
c57dd87d0b93: Pull complete 
d7fbf29df889: Pull complete 
1f1070938ccd: Pull complete 
Digest: sha256:36b74457bccb56fbf8b05f79c85569501b721d4db813b684391d63e02287c0b2
Status: Downloaded newer image for nginx:latest
fdad18387b27b48b8f2e14f8641791cc4d5edf667cc664198b67579cd76a5162

When you list the docker images available on your system, you can see the Nginx Docker image was pulled.

geekflare@geekflare:~$ docker images
REPOSITORY                     TAG                 IMAGE ID            CREATED             SIZE
nginx                          latest              08393e824c32        4 days ago          132MB
hello-world                    latest              bf756fb1ae65        7 months ago        13.3kB
kubernetesui/dashboard         v2.0.0-beta8        eb51a3597525        8 months ago        90.8MB
geekflare_mongodb              latest              095d17727ca0        12 months ago       325MB

Run the command below to list all the containers running. You can see the container nginx_geekflare is running at 0.0.0.0:80

geekflare@geekflare:~$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                         PORTS                NAMES
fdad18387b27        nginx               "/docker-entrypoint.…"   58 seconds ago      Up 32 seconds                   0.0.0.0:80->80/tcp   nginx_geekflare
1a1056caffc4        hello-world         "/hello"                 About an hour ago   Exited (0) About an hour ago                        zealous_bose
b0095c1e5536        geekflare_mongodb   "/bin/sh -c usr/bin/…"   12 months ago       Exited (0) 12 months ago                            mongo_container

Go to your browser and open 0.0.0.0:80, you can see nginx is up and running inside a container.

Common Docker Use Cases

Configuration Simplification: Docker can run on any platform with the help of its configuration without the actual overhead of a virtual machine.

It allows you to put the configuration file into the code and pass environment variables to cater to different environments. So that one docker image could be used in a different environment.

Code Management: The code travels through a different environment in its journey from development to production.

Each environment is having a slight variation than the other. Docker eliminates this difference by providing a consistent environment, making the development and coding so much more comfortable.

Docker images being immutable, they come with the advantage of having zero change in application environment from dev to production.

Improved Development Productivity: The two essential objectives in the development ecosystem, is to have the development environment replicate as close as possible to the production environment and next goal to get quality code delivered as soon as possible.

Docker allows the code to run in a container that reflects the production environment, and unlike VM, docker has lesser overhead memory capacity wise, which helps several services to run.

The other goal is achieved as we use the Docker’s shared volume for the application code to be available to the container from the host. This allows the developer to edit the source code from his platform and editor, which will reflect on the running environment inside the Docker.

Isolation of Applications: There are cases where application isolation may be needed, for example, API servers that require different apache and a different set of dependencies.

Running API servers under different containers is a much better way out.

Debugging Capabilities: Docker provides numerous tools that work well with containers, with the ability to insert checkpoints within containers and also different containers, which are quite essential while testing applications.

Rapid Deployment: Docker containers can be created quite quickly, which achieved as containers are not booting up an OS but just running the application.

Once set, they give you peace that once the code has worked, it will work in all environments.

Conclusion

Docker, with its vast benefits, is becoming a valuable addition to the IT infrastructure. I hope the above gives you an idea about it. If you are interested in taking online courses, I would recommend taking Docker Mastery online course.