Whats Docker? And how to start with it

Cover image by Tim Easley on Unsplash

During the last couple of years Docker has became a thing in software developer world. With this post I would like to explain what problem does it solve, how to install it and test on a simple examples. Apart from the technical topics I also would like to elaborate a little bit on the containerization concept of a software.

Container technology. What problem does it solve?

Imagine this scenario. You, as a developer, has finished a new amazing app. Everything was tested and it could be a next big thing. Now your boss is asking you to move it from your development machine (usually local PC) to companys server (production).

Would it be a simple copy-pasting task? In some cases it might, but what if on this server runs couple of other applications? It might be some conflicts when for instance, a new software you need to have the 8 Java version, but other applications are using older versions. And number of such conflicts may skyrocket with a complexity of installed application on a server. So instead of quickly moving new solution to a production we would need to consume a lot of time to make sure that our new application would run and also make sure that old ones wont crash after these changes. So here we have the old problem some software works only on my machine.

Source quick meme

Luckily the containers concept was introduced! Instead of moving a finished app from one environment to another we need first to wrap it in a container that handle for us all required dependencies and libraries, so it runs reliably on every machine (Windows, Linux, Mac, cloud, and others)!

But is a the only benefit of the container?

Not at all! Apart from that now every software is portable, their development could be more faster than it was. Especially when we develop a microservices , where each service is a separate part of a bigger solution. With this approach we can split large software into smaller parts and each of it could be written in a different technology (Java, Python, R), so we are able to choose the best tool for a particular problem.

Moreover the microservice architecture helps with agile approach. For instance if we have an e-commerce website and one of payment method is PayPal, but suddenly customers wants to replace it with another method. Using this approach you wont need to create a new version of entire app, just the tiny part responsible for payments, which is far less time-consuming.

And finally, containers technology enables some of the cloud features (like scalability, self-healing), it reduce solutions time to market, reduce IT infrastructure or issue time resolving.

What is a role of Docker?

Ok, so now, what is a role of the Docker in container technology? Basically its a tool that allows us to develop, deploy and run software in a container. With Docker we can package up the application with all libraries and dependencies, and unlike virtual machine they dont require so much resources, so they are faster and more easy to use.

Lets move on to show you how Docker really works, but first we must install it on your local machine.

How to install Docker?

Depending on the system you have the installation might be a slightly different. If youre using a Linux, youll be able to install Docker in a terminal, but if youre using Windows or MacOS you will need to install a Docker Desktop app which is a lightweight Linux virtual machine.

Here are the instructions how to install Docker on Ubuntu (Linux), Windows and MacOS.

Running first, hello-world, Docker container

Everything is set up, so open a terminal (on Windows and Macs, first make sure that Docker is up and running) and type following command:

> docker --versionDocker version 18.09.2, build 6247962

If you get similar output as I have its installed correctly .

Now lets try with a first container, to do that run the following:

> docker run hello-worldUnable to find image 'hello-world:latest' locallylatest: Pulling from library/hello-world1b930d010525: Pull complete Digest: sha256:2557e3c07ed1e38f26e389462d03ed943586f744621577a99efb77324b0fe535Status: Downloaded newer image for hello-world:latestHello 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/](https://hub.docker.com/)    For more examples and ideas, visit:     [https://docs.docker.com/get-started/](https://docs.docker.com/get-started/)

So what happens here? First of all, weve pulled a hello-world image from an official Docker image repository Docker Hub. A Docker image can be compared to a recipe for a container, its an executable file that has all information needed to run an application. Docker Hub is a an official repository, where some of the images are stored and are available for us.

In above print out you could found two other terms Docker client and Docker daemon. First one is a terminal, command line application that were using to communicate with Docker daemon, which is a local background service which is responsible for managing the containers.

Now, after typing docker info we should get some info about current status of container and images.

> docker infoContainers: 1Running: 0Paused: 0Stopped: 1Images: 1....

Another command would be docker images which print for us list of locally saved images

> docker imagesREPOSITORY    TAG        IMAGE ID        CREATED             SIZEhello-world   latest    fce289e99eb9    2 months ago         1.84kB

Lets try something different and run another container:

> docker run busybox echo "Hello Docker funs!"Hello Docker funs!

What happened here? A first part is very similar to what we already had docker run busybox command pulled an image from Ducker Hub and build it to the container. The second part echo "hello Docker funs!" is a command that was ran inside the container!

Now lets check out if containers are running.

> docker ps -aCONTAINER ID  IMAGE      COMMAND          CREATED        STATUS               5411cd0e5873  busybox    "echo 'Hello " 3 minutes ago Exited (0)... abac886c2a2d  busybox    "sh"            4 minutes ago Exited (0)...                      9a1437750643  hello-world "/hello"       40 minutes ago Exited (0)...

Column STATUS is showing that all our images are stopped.

But what if we want to run multiple commands? Just add -i flag to run method:

> docker run -i busybox    > # ls    bin   dev   etc   home  proc  root  sys   tmp   usr   var    > # cd home    >  /home # echo "Hello World!"    Hello World!

Finally lets do something more complex and run a container with a Tomcat server.

> docker run -i --name tomcat-server --rm -p 8888:8080 tomcat:8.0Status: Downloaded newer image for tomcat:8.0Using CATALINA_BASE:   /usr/local/tomcatUsing CATALINA_HOME:   /usr/local/tomcatUsing CATALINA_TMPDIR: /usr/local/tomcat/tempUsing JRE_HOME:        /docker-java-home/jre...19-Mar-2019 05:40:43.322 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["http-apr-8080"]19-Mar-2019 05:40:43.336 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["ajp-apr-8009"]19-Mar-2019 05:40:43.348 INFO [main] org.apache.catalina.startup.Catalina.start Server startup in 974 ms

Using above command with parameters we did certain task:

• -i it keeps the container up and running,

• --name tomcat-server in this way we specify the alias (tomcat-server) of the container,

• --rm it tells Docker to automatically remove container when it exits

• -p 8888:8080 it maps the inside port of the container (8080) to the host (outside) port, so when you type *http://localhost:8888/ *on your local you should get something like this:

Tomact GUI so it confirms that youve got a running application server on your machine without even installing a thing!

Before we end our work with Docker for today, its a good practice to check whether none of the containers is running on the background. To check it use one of already introduces command:

> docker ps -aCONTAINER ID    IMAGE        CREATED             STATUS              91b2db85e50d  tomcat:8.0   8 minutes ago       Up 8 minutes        e00541868e30  busybox      16 minutes ago      Up 16 minutes  

As you can see on the STATUS print out two containers are still up and running. To stop both of them use following command:

> docker stop $(docker ps -a -q)91b2db85e50de00541868e30

And now when you check again none of them are running:

> docker ps -CONTAINER ID   IMAGE      CREATED                STATUS                      e00541868e30  busybox  21 minutes ago    Exited (137) 6 seconds ago    

You may wonder why you dont see the tomcat-server container. Its because weve added a -rm option when it was ran, so it automatically deletes the container to free space.

And its everything for today! If youre want more information about the Docker, please be patient. New stories are coming really soon !

If youre interested in Docker topic you can check my other blog posts:

• Database in a Docker container how to start and whats it about

• How to put your Java application into Docker container

• Build and run Angular application in a Docker container

References

• Get Started, Part 1: Orientation and setupon docs.docker.com
• What is a Container? | Docker on docker.com
• Learn Enough Docker to be Useful on towardsdatascience.com
• Why Docker? | Docker on docker.com