How to dockerize an application?

- Programming - #Go #operations - 1113 words - 5min

As you could have seen in the previous post, docker is quite handy to make your application running EVERYWHERE. So now the problem you can have is, how do you actually dockerize your application. I will give you a guideline, feel free to contribute to it, I will surely be wrong or missing something.

Define the requirements of your application

It can sound a bit a cheap advice, but before to start everything you need to identify and isolate what your application needs. A database? A task queue? etc… You can plan to have a container for each.

But also internally… Your application needs to run inside the container so you can’t only ship your code and thinking be done with it. So for example let’s say I wrote a Django application. But it will work not by itself. I can install a uwsgi or running the default run command inside the container… The default run is not production ready, so you need to think about installing uwsgi… That’s the kind of things that you need to prepare before any dockerization…

Define a setting policy

A really mind fuck issue when you play with containers. Do I need to embed the configuration file? Should I use env variable? Yeap, we all started to wonder this questions. And sadly there’s no answer. Every case as their bad and good points. Let’s make a little list of it:

Configuration as environement variable:

Instead of using a config file as usual you will need to setup one variable for each settings you have to configure. It can be a big effort at the first sight but can be really usefull on the long therm when you don’t need to do anything dodgy to change the configuration that’s inside your container.

Pros

  • Easily configurable

Cons

  • Behavior can change in function of the environment as you might not use same value for the env. variables

Configuration embed in the container:

Pros

  • Static and sure that in dev or prod everything is working the same
  • Easy to scale

Cons

  • Nearly impossible to configure
  • Or if you want to configure it, you need to change the configuration before to build the container which can be dodgy sometimes even if you have some CI.

Configuration mounted as a volume:

Pros

  • Work as most software and can avoid you to loose time.
  • Easy to scale vertically

Cons

  • You need the configuration file somewhere in the host (So you might some tools to do that)
  • Annoying for vertical scale as the configuration needs to leave on all the docker hosts.

What to choose

Clearly no idea. I made 3 applications the last few months, they use the 3 ways and I am happy with none. I am tempted to say that the configuration with env variable is a the best as you can easily change this configuration and just depends on you to make sure that they are filled the same way across environments or host. You can even use a tool that will deploy your container on different hosts with the same env variables. I use ansible in my case but there’s lots, so feel free to use the one you want :)

It depends also on your case and your needs. Need a quick and dirty workaround? Config embed. No need to change the configuration? Config embed. Big application that lives magically with a setting file -> setting file as volume. Feeling like a boyscoot and ready to break things? -> env variable.

Install the minimum

When using docker, you knew that you would have to change some stuff. That’s where you will sweat a bit. Your application needs to be the tiniest possible. Tinier and isolated is your app and more gain you would have to use docker. Let’s imagine you have a big app containing a task queue and a webapp. And the first version is dockerized but both of it are in the same container. Well, you can’t scale (horizontally at least) your task queue as you want or your webapp, you always need to deploy both… So always have the strict minimum in your container.

To be clear, you will need to study every bit of your application to be able to run every single piece of software independently to be able to scale it properly.

Define a retention policy

A big point that might force you to rebuild your container is the way you will use docker volumes. It’s important to think about it before to continue in case your container gets bigger (and you’ll loose everything in it). Which it should not. So you need to detect any logging file, any folder or file that can grow or get updated and define them as volume in your dockerfile.

Prepare some performance tests

Ok, let’s imagine you finished with your dockerization but suddenly you realize that your application is getting really slow. To make sure that does not happen and be sure about it, is to create some performance tests or using a tool showing you the quality of your app (New Relic for example).

Example

Would be a shame to not give you a little example before you get back to your keyboard with a strange mood of dockerizing the world. I will add more examples when I will have experienced more docker deployement. Feel free to give me your example in the comments!

Django

As Django needs a settings file to survive, you might play first with a volume for this setting file. As well you might have configure the logging to log in a specific file… Then you need to add a volume in your dockerfile pointing to this/these file(s). Would you like to serve the statics on your application or with your proxy? If you choose to serve the statics file with django, you can use whitenoise. If not you need to define a new volume in your dockerfile pointing to your STATIC_ROOT AND call the python manage.py collectstatic. Your static files should magically appears in your volume :)

Also, You will need a webserver to run your django application. I invite you to use uwsgi which is extremely reliable.

And that’s it, you have all the tips to build a nice django app dockerized. You can have a look at the way that django-cookiecutter generate its docker env, it’s pretty cool.

Conclusion

There’s not much but that was the few points I would have liked to know before to dockerize an application. Feel free to add in the comment your guideline for your framework or your feedback of the article, it would be helpful for everyone! Sur ce, dockerizez bien! Ciao!