Docker for the absolute beginner
In this post I will try to cover what docker is, why you should use docker and more.
What is Docker?
Docker is a software platform for building applications based on containers — small and lightweight execution environments that make shared use of the operating system kernel but otherwise run in isolation from one another.
Why do you need Docker?
Let’s say you want to build an application using various technologies. For example, Nodejs and Express for the web server, MongoDB for the database and Redis for the messaging system. First of all, their compatibility with the underlying operating system (OS) is an issue. You have to make sure all the different services are compatible with the OS. Secondly, you have to make sure all the services are compatible with the libraries and dependencies of the OS. This compatibility matrix issue is referred to as “The Matrix from Hell”.
Read more about matrix from hell.
Another thing to mention is that every time a new developer is onboard, they will have to follow a large set of instructions and run hundreds of commands to setup their environment. All of this will make the development, building and shipping process really hard.
What does Docker do?
With docker you will be able to run each component in its own container with its own dependencies and libraries.
Once you’re done building the docker configuration file, anyone can get started with a docker run command irrespective of which operating they are on. They just have to install docker.
What are Docker images?
A Docker image is a read-only template that contains a set of instructions for creating a container. It provides a convenient way to package up applications and preconfigured server environments, which can be used for personal projects or shared publicly with other Docker users. Lots of programs have been dockerized already. You can find the images on docker hub.
Images can be created with a Dockerfile which provides the specifications for creating a Docker image.
What are containers?
A container is a running instance of an image. It is a completely isolated environment. It has its own services, processes, network interfaces… just like virtual machines except they share the same OS kernel.
On this post, we will be learning docker from the basics by creating a simple web application. You will also be able to deploy your own image to docker hub. So, follow along.
I am on a Linux machine. The OS details are listed below.
Let’s start by installing docker.
Go to https://docs.docker.com/engine/install and choose your OS. In my case, Ubuntu.
There are two ways to install docker. For now, we will use the convenience script to install Docker. Scroll down to the Install using the convenience script section and follow along the installation.
curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh
The 1st command downloads the get-docker.sh shell script and the 2nd one executes it.
Now, if we type sudo docker version in terminal, the following output will be shown.
Congrats, you are done installing docker. The next step is to create a Dockerfile.
Open a terminal and navigate to your desktop folder. Then clone the following github repository https://github.com/MyoniM/simple-web-app. Now navigate to the cloned folder. It should contain an app.py file.
git clone https://github.com/MyoniM/simple-web-app
Before creating the Dockerfile, let us start the app.
The first step is downloading package information from all configured sources from our Linux repository list. To do so, type the command
sudo apt-get update
and wait for it to finish. This command might defer between operating systems.
Once that finishes, we’ll install python3 and pip package manager. Run the command
sudo apt-get install -y python3 python3-pip
The next step is to install the python flask module, since the app uses flask as a server. Run
pip install flask or sudo apt install python3-flask
Once that finishes, run the app with
FLASK_APP=app.py flask run — host=0.0.0.0
You should see an output like this.
Open another terminal and check if the API is working by typing the command curl http://0.0.0.0:5000. If it outputs a Welcome! text on the terminal, that means the server is working correctly.
Now, let’s create a Dockerfile. First, create the file with a name Dockerfile by typing the command touch Dockerfile.
Open the Dockerfile with your preferred text editor. I will use vi. If you are also using vi, press I to go to insert mode.
The first line is FROM ubuntu. All Dockerfile must start from another image. In this case from the ubuntu image.
Then write what the image has to do when run. Write the commands as if you are setting up the server yourself. What was the first step when we set up the server? Run sudo apt-get update. We write these commands to the Dockerfile as shown below.
RUN apt-get update
RUN apt-get install -y python3 python3-pip
RUN pip install flask
Earlier, we run the app by using the command FLASK_APP=app.py flask run — host=0.0.0.0. This was possible because we were in the same directory as the app.py file. We are going to create an image from this Docker file. So, we need to copy the app.py file to a directory in the image when built. We do this by appending the following line,
COPY app.py /opt/app.py
to the Dockerfile. This command copies the app.py file to the /opt directory when the image is built.
Finally, we write what the image has to do when run. We can do this by appending the following command to the Dockerfile,
ENTRYPOINT FLASK_APP=/opt/app.py flask run — host=0.0.0.0
We are all set up. If you’re using VI editor, press the esc character on your keyboard, write :wq and press enter to save and exit the file. However, if you’re using nano or similar text editor, press ctrl + o to save and ctrl + x to exit. The final result should be like below.
Time to build the image.
To build an image using a Dockerfile, run this command
docker build . -t myonim/simple-web-app
myonim is my docker hub username, it is necessary if you want to deploy on docker hub repo. Go and sign up on https://hub.docker.com.
Type sudo docker images to check if the image is built and present. You are all set up, we can go and run our image by typing
sudo docker run myonim/simple-web-app
Perfect, we successfully run our image. It gave us a URL in which we can use to access the server at 172.17.0.2:5000.
Open another terminal and check if the API is working by typing curl http://172.17.0.2:5000 . If it says Welcome! that means it is working correctly.
You can see all running containers by typing
sudo docker ps
I you want to stop the container, type sudo docker stop id, provide only first few characters from the id or the container name. If it returns the id or the container name, it is successful.
But, if you run sudo docker ps -a, you still can the container. It is seating idle consuming resources. so you should remove it by typing
sudo docker rm id/name
Finally, we can now deploy our image to docker hub.
After creating account on docker hub, go to your terminal and login. It will ask you for credentials.
After authenticating, use sudo docker push imagename to push your image.
If you go to your docker hub account you will see the following screen.
Well, that’s it for now. I hope this helps. More advanced topics about Docker, Docker compose, Docker swarm… coming soon! Follow me not to miss them.
