Should I use separate Docker containers for my web app?

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!

Some will tell you that you should run only 1 process per container. Others will say 1 application per container. Those advices are based on principles of microservices.

I don't believe microservices is the right solution for all cases, so I would not follow those advices blindly just for that reason. If it makes sense to have multiples processes in one container for your case, then do so. (See Supervisor and Phusion baseimage for that matter)

But there is also another reason to separate containers: In most cases, it is less work for you to do.

On the Docker Hub, there are plenty of ready to use Docker images. Just pull the ones you need.

What's remaining for you to do is then:

• read the doc for those docker images (what environnement variable to set, etc)
• create a docker-compose.yml file to ease operating those containers

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.

It is probably better to have your webapp in a single container and your supporting services like databases etc. in a separate containers. By doing this if you need to do rolling updates or restarts you can keep your database online while your application nodes are doing individual restarts so you wont experience downtime. If you have caching with something like Redis etc this is also useful for the same reason. It will also allow you to more easily add nodes to scale in a loosely coupled fashion. It will also allow you to manage the containers in a manner more suitable to a specific purpose. For the type of application you are describing I see very few arguments for running all services on a single container.