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  • Docker 101
  • Install Docker in Ubuntu 16.04
  • Play with Busybox
  • Build and deploy your distributed applications easily to the cloud with docker

Docker 101

What is Docker?

Wikipedia defines Docker as:

  • an open-source project that automates the deployment of software applications inside containers by providing an additional layer of abstraction and automation of OS-level virtualization on Linux.

Wow! That's a mouthful. In simpler words, Docker is a tool that allows developers, sys-admins etc. to easily deploy their applications in a sandbox (called containers) to run on the host operating system i.e. Linux. The key benefit of Docker is that it allows users to package an application with all of its dependencies into a standardized unit for software development. Unlike virtual machines, containers do not have the high overhead and hence enable more efficient usage of the underlying system and resources.

What are containers?

The industry standard today is to use Virtual Machines (VMs) to run software applications. VMs run applications inside a guest Operating System, which runs on virtual hardware powered by the server’s host OS.

VMs are great at providing full process isolation for applications: there are very few ways a problem in the host operating system can affect the software running in the guest operating system, and vice-versa. But this isolation comes at great cost — the computational overhead spent virtualizing hardware for a guest OS to use is substantial.

Containers take a different approach: by leveraging the low-level mechanics of the host operating system, containers provide most of the isolation of virtual machines at a fraction of the computing power.

Why should I use it?

Docker's rise has been nothing short of meteoric. Although containers by themselves are not a new technology, it was not until Docker arrived that they started to get mainstream attention. By providing standard APIs that made containers easy to use and creating a way for the community to collaborate around libraries of containers, Docker has radically changed the face of the technology landscape. In an article published by The Register in mid-2014, it was claimed that Google runs over two billion containers per week.

In addition to Docker's continual growth, Docker, Inc., the developer behind Docker has been valued at over a billion dollars! Due to its benefits of efficiency and portability, Docker has been gaining mind share rapidly, and is now leading the Containerization movement. As developers going out into the world, it is important that we understand this trend and see how we can benefit from it.

Install Docker in Ubuntu 16.04

Preface

Getting all the tooling setup on your computer can be a daunting task, but thankfully as Docker has become stable, getting Docker up and running on your favorite OS has become very easy. First, we'll install Docker.

Until a few releases ago, running Docker on OSX and Windows was quite a hassle. Lately however, Docker has invested significantly into improving the on-boarding experience for its users on these OSes, thus running Docker now is a cakewalk. The getting started guide on Docker has detailed instructions for setting up Docker on Mac, Linux and Windows.

We will give an example to install Docker in Ubuntu 16.04

Install using the repository Before you install Docker CE for the first time on a new host machine, you need to set up the Docker repository. Afterward, you can install and update Docker from the repository.

1). Set up the repository

Install packages to allow apt to use a repository over HTTPS:

ubuntu@master:~$ sudo apt-get update
ubuntu@master:~$ sudo apt-get install apt-transport-https ca-certificates curl
ubuntu@master:~$ sudo apt-get install gnupg2 software-properties-common

Add and verify Docker's official GPG key:

ubuntu@master:~$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
ubuntu@master:~$ sudo apt-key fingerprint 0EBFCD88

Add the stable repository:

ubuntu@master:~$ sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) \
   stable"

2). Install Docker CE 

ubuntu@master:~$ sudo apt-get update
ubuntu@master:~$ sudo apt-get install docker-ce

3). Test Docker installation

ubuntu@master:~$ sudo docker run hello-world

Play with Busybox

Now that we have everything setup, it's time to get our hands dirty. In this section, we are going to run a Busybox container on our system and get a taste of the docker run command.

To get started, let's run the following in our terminal:

ubuntu@master:~$ sudo docker pull busybox
  • Note: Depending on how you've installed docker on your system, you might see a permission denied error after running the above command. If you're on a Mac, make sure the Docker engine is running. If you're on Linux, then prefix your docker commands with sudo. Alternatively you can create a docker group to get rid of this issue.

The pull command fetches the busybox image from the Docker registry and saves it to our system. You can use the docker images command to see a list of all images on your system.

ubuntu@master:~$ sudo docker images

1). Docker Run

Great! Let's now run a Docker container based on this image. To do that we are going to use the almighty docker run command.

ubuntu@master:~$ sudo docker run busybox

Wait, nothing happened! Is that a bug? Well, no. Behind the scenes, a lot of stuff happened. When you call run, the Docker client finds the image (busybox in this case), loads up the container and then runs a command in that container. When we run docker run busybox, we didn't provide a command, so the container booted up, ran an empty command and then exited. Well, yeah - kind of a bummer. Let's try something more exciting.

ubuntu@master:~$ sudo docker run busybox echo "hello from busybox"

Nice - finally we see some output. In this case, the Docker client dutifully ran the echo command in our busybox container and then exited it. If you've noticed, all of that happened pretty quickly. Imagine booting up a virtual machine, running a command and then killing it. Now you know why they say containers are fast! Ok, now it's time to see the docker ps command. The docker ps command shows you all containers that are currently running.

ubuntu@master:~$ sudo docker ps

Since no containers are running, we see a blank line. Let's try a more useful variant: docker ps -a 

ubuntu@master:~$ sudo docker ps -a

So what we see above is a list of all containers that we ran. Do notice that the STATUS column shows that these containers exited a few minutes ago.

You're probably wondering if there is a way to run more than just one command in a container. Let's try that now:

ubuntu@master:~$ sudo docker run -it busybox sh

Running the run command with the -it flags attaches us to an interactive tty in the container. Now we can run as many commands in the container as we want. Take some time to run your favorite commands.

  • Danger Zone: If you're feeling particularly adventurous you can try rm -rf bin in the container. Make sure you run this command in the container and not in your laptop. Doing this will not make any other commands like ls, echo work. Once everything stops working, you can exit the container (type exit and press Enter) and then start it up again with the docker run -it busybox sh command. Since Docker creates a new container every time, everything should start working again.

That concludes a whirlwind tour of the mighty docker run command, which would most likely be the command you'll use most often. It makes sense to spend some time getting comfortable with it. To find out more about run, use docker run --help to see a list of all flags it supports. As we proceed further, we'll see a few more variants of docker run.

Delete container: On deletion, you should see the IDs echoed back to you.

ubuntu@master:~$ docker rm $(docker ps -a -q -f status=exited)

This command deletes all containers that have a status of exited. In case you're wondering, the -q flag, only returns the numeric IDs and -f filters output based on conditions provided. One last thing that'll be useful is the --rm flag that can be passed to docker run which automatically deletes the container once it's exited from. For one off docker runs, --rm flag is very useful.

Lastly, you can also delete images that you no longer need by running docker rmi.

2). Terminology

In the last section, we used a lot of Docker-specific jargon which might be confusing to some. So before we go further, let me clarify some terminology that is used frequently in the Docker ecosystem.

  • Images - The blueprints of our application which form the basis of containers. In the demo above, we used the docker pull command to download the busybox image.

  • Containers - Created from Docker images and run the actual application. We create a container using docker run which we did using the busybox image that we downloaded. A list of running containers can be seen using the docker ps command.

  • Docker Daemon - The background service running on the host that manages building, running and distributing Docker containers. The daemon is the process that runs in the operation system to which clients talk to.

  • Docker Client - The command line tool that allows the user to interact with the daemon. More generally, there can be other forms of clients too - such as Kitematic which provide a GUI to the users.

  • Docker Hub - A registry of Docker images. You can think of the registry as a directory of all available Docker images. If required, one can host their own Docker registries and can use them for pulling images.

Build and deploy your distributed applications easily to the cloud with docker

Cont'd

References