In this article, you will learn the importance of structuring Dockerfiles when containerising an application and the pitfalls of doing otherwise.
After reading this article, you will have a basic idea of what Docker is, how it can help you, and how it makes application development and deployment more streamlined.
What is Docker?
Docker is a containerisation platform capable of packaging up an application or service alongside all of its dependencies within a complete filesystem. This filesystem, typically labelled as a Docker image, guarantees that it will always run the same, regardless of the environment it is running on.
Everything the application needs to run is included: code, runtime, system libraries and anything else you would install on a server.
How is this different from Vagrant or any other Virtual Machine?
You may have heard of Vagrant, VirtualBox or any other virtualisation software that also allow you to isolate services. These tools have been very popular in the past within the development community but all differ majorly which makes the development to deployment process far less efficient.
Unlike Docker, each of the above methods requires that you install an entire guest operating system for each isolated instance. This is great if you require an operating system that differs from the one on your local machine, or you require a different operating system for each instance, but considering most services run on a linux platform these days this is an unlikely scenario.
Docker mitigates this by sharing the guest machine’s kernel where isolation is provided through cgroups and other linux kernel libraries and techniques. This means that you are not required to install an entire operating system for each service, potentially saving gigabytes of space.
But what does this mean to me?
It’s all great that I can save space and have linux handle all the dirty work, but what are the practical sides to using Docker? How does Docker help me write better software? If you’re looking to improve your productivity and not have to worry about maintaining multiple environments or tooling, you’ll appreciate the following 5 key benefits Docker offers:
1. Accelerate Developer Onboarding
Docker prevents the need for new developers having to waste hours setting up their environments and manually spinning up VM instance to get production code to run locally. This process can take all day or longer, and new developers can make mistakes.
With Docker all developers in your team can get your multi-service application running on their workstation in an automated, repeatable, and efficient way. You just run a few commands, and minutes later it all works.
2. Inspire Polyglotism
Sometimes relying on the language you know best can put a project at a disadvantage, but to switch to something else requires developers set up the new technology of choice. Since you can isolate an application in a Docker container, it becomes possible to broaden your horizons as a developer by experimenting with new languages and frameworks.
3. Infrastructure Agnostic
Docker allows you to encapsulate your application in such a way that you can easily move it between environments. It will work properly in all environments and on all machines capable of running Docker.
4. Collaboration as a First Principle
The Docker toolset allows developers and sysadmins to work together towards the common goal of deploying an application. You can track of version history and image updates across the organisation.
Docker acts as an abstraction. You can share, distribute and manage your application within Docker Hub, and members of another team can link to or test against your application without having to learn or worry about how it works.
5. Ship It! Quicker
Without having to spend time setting up a production server, you already have a working environment contained within your Docker image. Every time an image is updated it can be very quickly deployed to a production server without any hassle. This quick turn around provides increased value to end users.
Docker containers can spin up and down in a matter of seconds making it easy to scale up and down applications when resources are required or no longer needed.
What is a Dockerfile?
To build a Docker image the Docker daemon needs to know the steps required to setup the environment under which your application or service will be run. This is where the Dockerfile comes in. A Dockerfile is a simple series of instructions that tells the Docker daemon what it must do to build your image. This includes what base image to use, how to grab and install any dependencies and finally how to run the container created from the image.
How they work?
It is important how the Dockerfile is structured because having steps in the wrong order can make the build process inefficient and time consuming. In order to understand why this is you need to understand how Docker works.
Think of a Docker image as a git repository. Just like git the Docker image is built up from a series of “commits”. Each commit represents changes that have happened since the last; such that replaying these commits in the correct order will reproduce the exact same Docker image.
Every instruction that is performed in the Dockerfile causes Docker to create a new commit. Therefore a Dockerfile that has 10 instructions will produce a image that is built up of 10 commits.
If you have ever performed a rebase in git then you’ll understand what happens
if you were to alter one of these commits. Every commit that follows the
changed commit will be altered and recomputed (a new hash will be calculated).
Think of this now in terms of Docker. If you build a Docker image containing a
Ruby application then it is likely that part of the Dockerfile requires
install to be run to fetch the application dependencies. Now during
development you add another dependency to your Gemfile and rebuild the Docker
image. Since there has been a change to the Gemfile the step in the Dockerfile
bundle install needs to be recomputed. Since the hash for this
commit has altered then each subsequent commit needs to be recomputed also.
The problem with the above is that if the Gemfile is changed frequently then this step, and all that follow it, need to be computed every time the image is built. If this step is towards the beginning of the Dockerfile then every step will need to be computed. This is incredibly inefficient but can be remedied by moving the step further toward the end of the Dockerfile.
How to write them efficiently
As we have discovered, the order in which instructions are performed in a Dockerfile can incur significant efficiency penalties if not considered. Therefore the most efficient way to construct a Dockerfile is to move steps that change most often towards the end of the Dockerfile. So steps that are the least likely to change go first and those most likely go last. This is not a hard and fast rule and should be viewed as a guide.
As an example I have included the Dockerfile I use to build a Ruby on Rails application:
Breaking that Down
The first couple of instructions define the base image and the maintainer of the image. These will never change and therefore go first.
Following this are some software dependencies gathered by
apk. These very
rarely change whilst I am developing a Ruby on Rails application.
Next the Gemfile is added and
bundle install is run. This can change from
time to time during development and as such has been placed after those steps
which change less frequently. Now when the Gemfile is changed and the image is
built each step before
bundle install will be taken from the Docker cache and
not have to be recomputed.
Last the current working directory (or the project directory) is shared with the image. The project code changes very frequently during development and has therefore been put last. Now when the image it built only steps that follow this a recomputed.
Following this procedure makes our Dockerfiles far more efficient and easier to reason with.