DevOps vs Docker

Docker is just another tool available to DevOps Engineers, DevOps practitioners, or whatever you want to call them. What Docker does is it encapsulates code and code dependencies in a single unit (a container) that can be run anywhere where the Docker engine is installed. Why is this useful? For multiple reasons; but in terms of CI/CD it can help Engineers separate Configuration from Code, decrease the amount of time spent doing dependency management etc., can use it to scale (with the help of some other tools of course). The list goes on.

For example: If I had a single code repository, in my build script I could pull in environment specific dependencies to create a Container that functionally behaves the same in each environment, as I'm building from the same source repository, but it can contain a set of environment specific certificates and configuration files etc.

Another example: If you have multiple build servers, you can create a bunch of utility Docker containers that can be used in your CI/CD Pipeline to do a certain operation by pulling down a Container to do something during a stage. The only dependency on your build server now becomes Docker Engine. And you can change, add, modify, these utility containers independent of any other operation performed by another utility container.

Having said all of that, there really is a great deal you can do to utilize Docker in your CI/CD Pipelines. I think an understanding of what Docker is, and what Docker can do is more important that a "how to use Docker in your CI/CD" guide. While there are some common patterns out there, it all comes down to the problem(s) you are trying to solve, and certain patterns may not apply to a certain use case.

Docker facilitates the notion of "configuration as code". I can write a Dockerfile that specifies a particular base image that has all the frameworks I need, along with the custom configuration files that are checked into my repository. I can then build that image using the Dockerfile, push it to my docker registry, then tell my target host to pull the latest image, and then run the image. I can do all of this automatically, using target hosts that have nothing but Linux installed on them.

This is a simple scenario that illustrates how Docker can contribute to CI/CD.

Docker is also usefull for building your applications. If you have multiple applications with different dependencies you can avoid having a lot of dependencies and conflicts on your CI machine by building everything in docker containers that have the necessary dependencies. If you need to scale in the future all you need is another machine running your CI tool (like jenkins slave), and an installation of docker.

When using microservices this is very important. One applicatio can depend on an old version of a framework while another needs the new version. With containers thats not problem.