Can you explain Docker with a practical example/case?

Question

I have read this and the intro docs on Docker.io and I like the concept it presents. But, can you help me understand it a little better? Can you give me some practical examples and/or case studies on how Docker is used and when it makes sense to actually use it?

Just a side note, I have recently started using Vagrant to distribute a preconfigured DEV box to our development team (so we all use the same base system). I have even seen examples where Docker is used inside Vagrant and whatnot but I don't get what are the benefits to do this in a practical sense; meaning that I understand the difference between VMs and containers and the logical separation the latter provide, but when should I use the one instead of the other and when Docker inside Vagrant? (this is a more specific question but I am mostly interested in the bigger picture as outlined in the first question above).

Answer 1

There is a nice docker hack day use case:

Auto-deployment of a java stack with Git and Jenkins. You push your code into your contenairezied git repository. It will trigger a Jenkins build so your webapp will be packaged into a Container Docker and will be run by Docker.

https://www.youtube.com/watch?v=Q1l-WoJ7I7M

Answer 2

I participate in an effort to make software for doing science analysis more available to the research community. Often, the software in question is written by one individual or just a few without sufficient planning for reuse, such as one person on their own computer writing a Python script or a Matlab module. If the software works well, often others would like to try it themselves...but it can be a real challenge in some cases to successfully replicate an environment that's undocumented or difficult to reimplement.

Docker is a great tool to help others reuse software like this, since it is an even lower barrier of entry that writing a Vagrant script to install software in an environment. If I give a person a Docker container, she can do whatever she wants inside of it (write code, install libraries, set up environment, etc. When it's "done", she can save an image of it and publish the image in a Docker repository and tell another researcher, "here it is, just start it up and run this..."

We are also considering using containers as our own configuration management strategy for delivering and archiving production software...at least the server-side components.

We have also done some work with writing scripts in Python and shell to run data processing workflows of multiple Docker containers. One demo that we concocted was to run OpenCV on an image to extract faces of people, then ImageMagick to crop out the faces, and finally ImageMagick again to make a collage of all of the faces. We built a container for OpenCV and a container for ImageMagick, then wrote a Python script to execute a "docker run ..." on each of the containers with the requisite parameters. The Python scripting was accomplished using the docker-py project which worked well for what we needed from it.

Answer 3

Have a look at "how and why Spotify uses Docker" for a case study.

To answer your last question :

I have even seen examples where Docker is used inside Vagrant and whatnot but I don't get what are the benefits to do this in a practical sense; meaning that I understand the difference between VMs and containers and the logical separation the latter provide, but when should I use the one instead of the other and when Docker inside Vagrant?

Docker are frequently used inside Vagrant because it doesn't currenlty run on Mac OSX (see Kernel Requirements), which is very commonly used by developers.

Then to have your dev-team working on the same containers, builds and tests products on a laptop and later on "running at scale, in production, on VMs, bare metal, OpenStack clusters, public clouds and more", you need Vagrant on their Mac OSX.

That said, here you can see another awesome case study.