Should I use separate Docker containers for my web app?

Question

Do I need use separate Docker container for my complex web application or I can put all required services in one container? Could anyone explain me why I should divide my app to many containers (for example php-fpm container, mysql container, mongo container) when I have ability to install and launch all stuff in one container?

Answer 1

Something to think about when working with Docker is how it works inside. Docker replaces your PID 1 with the command you specify in the CMD (and ENTRYPOINT, which is slightly more complex) directive in your Dockerfile. PID 1 is normally where your init system lives (sysvinit, runit, systemd, whatever). Your container lives and dies by whatever process is started there. When the process dies, your container dies. Stdout and stderr for that process in the container is what you are given on the host machine when you type docker logs myContainer. Incidentally, this is why you need to jump through hoops to start services and run cronjobs (things normally done by your init system). This is very important in understanding the motivation for doing things a certain way.

Now, you can do whatever you want. There are many opinions about the "right" way to do this, but you can throw all that away and do what you want. So you COULD figure out how to run all of those services in one container. But now that you know how docker replaces PID 1 with whatever command you specify in CMD (and ENTRYPOINT) in your Dockerfiles, you might think it prudent to try and keep your apps running each in their own containers, and let them work with each other via container linking. (Update -- 27 April 2017: Container linking has been deprecated in favor of regular ole container networking, which is much more robust, the idea being that you simply join your separate application containers to the same network so they can talk to one another).

If you want a little help deciding, I can tell you from my own experience that it ends up being much cleaner and easier to maintain when you separate your apps into individual containers and then link them together. Just now I am building a Wordpress installation from HHVM, and I am installing Nginx and HHVM/php-fpm with the Wordpress installation in one container, and the MariaDB stuff in another container. In the future, this will let me drop in a replacement Wordpress installation directly in front of my MariaDB data with almost no hassle. It is worth it to containerize per app. Good luck!

Answer 2

When you divide your web application to many containers, you don't need to restart all the services when you deploy your application. Like traditionally you don't restart your mysql server when you update your web layer.

Also if you want to scale your application, it is easier if your application is divided separate containers. Then you can just scale those parts of your application that are needed to solve your bottlenecks.