Everybody is talking about Docker nowadays. What it is about? Do you remember Solaris Zones or Containers? It is more or less the same although development of Docker during the last years made Linux Containers the de-facto standard for deploying applications in a standardized and isolated way. Docker is build in a classical client server model: There is the docker server (or daemon) which servers the requests of docker clients. The client is the one you’ll use to tell the server what you want to do. The main difference from the classical client/server model is that docker uses the same binary for the server as well as for the client. It is just a matter of how you invoke the docker binary that makes it a server or client application. In contrast to the Solaris Zones Docker containers are stateless by default, that means: When you shutdown a docker container you’ll lose everything that was done when the container started to what happened when container got destroyed (Although there are ways to avoid that). This is important to remember.
When you start a docker container on a host the host’s resources are shared with the container (Although you can limit that). It is not like when you fire up a virtual machine (which brings up an instance of a whole operating system) but more like a process that shares resources with the host it is running on. This might be as simple as running a “wget” command but it might be as complicated as bringing up a whole infrastructure that serves your service desk. Docker containers should be lightweight.
So what does make docker unique then? It is the concept of a layered filesystem. We’ll come to that soon. Lets start by installing everything we need to run a docker daemon. As always we’ll start with as CentOS 7 minimal installation:
The easiest way to get docker installed is to add the official docker yum repository (for CentOS in this case):
Working directly as root never is a good idea so lets create a user for that and let this user do everything via sudo ( not a good practice, I know 
):
Ready to install:
This will install the docker engine and these additional packages:
Enable the service:
Start the service:
And we’re done. Lets check if docker is working as expected:
What happened here is that we already executed our first docker image: “hello-world”. The “–rm” flag tells docker to automatically remove the image once it exits. As the image was not available on our host it was automatically downloaded from the docker hub:
You can browse the docker hub for many, many other images using your favorite browser or you can use the command line:
The first one is the official PostgreSQL image. How do I run it?
And ready. As with the “hello:world” image docker had to download the image as it was not available locally. Once that was done the image was started and new PostgreSQL instance was created automatically. Here you can see what the layered filesystem is about:
Each of this lines represents a layered/stacked filesystem on top of the previous one. This is an important concept because when you change things only the layer that contains the change needs to be rebuild, but not the layers below. In other words you could build an image based on a CentOS 7 image and then deploy your changes on top of that. You deliver that image and some time later you need to make some modifications: The only thing you need to deliver are the modifications you did because the layers below did not change.
You will notice that you cannot type any command when the image was started. As soon as you enter “CRTL-C” the container will shutdown (this is because of the “-it” switch, which is “interactive” and “pseudo terminal”):
Everything what happened inside the container is now gone. The correct way to launch it is:
The “-d” switch tells docker to detach, so we get back our shell. The magic string dockers returns is the container id:
When you want to know what images you have available locally you can ask docker for that:
How do you now connect to the PostgreSQL image?
Or to get bash:
Ok, this PostgreSQL image is based on Debian 8. Lets say this is not what I like because I want my PostgreSQL image based on CentOS. This is the topic for the next post: We’ll build our own CentOS image and get deeper in what the stacked filesystem is about. Once we’ll have that available we’ll use that image to build a PostgreSQL image on top of that.
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